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Write discussion and conclusion for these two tables. For my PhD thesis in Vancouver style.

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3D printed denture base resin flexural strength milled conventional

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surface roughness 3D printed milled denture base resin

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Bollen threshold 0.2 µm surface roughness bacterial adhesion denture

Discussion

The present in-vitro study compared the flexural load-bearing capacity and post-polishing surface roughness (Ra) of denture base resins fabricated by three contemporary techniques: conventional heat-cured compression moulding (Group I), subtractive CAD-CAM milling (Group II), and additive 3D printing (Group III). The milled specimens recorded the highest mean flexural strength (543.50 N), followed by the 3D-printed specimens (403.66 N), with the conventionally processed specimens performing markedly lower (130.55 N). An almost identical hierarchy was observed for surface smoothness, where the milled group achieved the lowest, most favourable Ra value (0.093 µm), the 3D-printed group an intermediate value (0.300 µm), and the conventional group the highest, least favourable value (0.363 µm). The consistency of this ranking across two mechanically and clinically distinct properties suggests that both outcomes are governed by a common underlying variable: the degree of polymer homogeneity and porosity produced by each fabrication method.

Flexural strength

The superior performance of the milled group is consistent with several recent comparative studies. Milled denture base pucks are machined from industrially pre-polymerised, highly cross-linked polymethyl methacrylate (PMMA) blanks that are polymerised under standardised high pressure and temperature, which minimises residual monomer content, porosity, and processing-related defects, in turn translating into higher flexural strength and modulus [1,3,4]. Alshali et al. similarly reported that milled denture base materials exhibited significantly higher flexural strength than 3D-printed materials across control, thermocycled, and fatigue-cycled conditions, attributing this to the superior degree of conversion achieved during industrial block manufacture compared with layer-by-layer photopolymerisation [1]. Alharethi reached the same conclusion, showing that CAD-CAM milled resin achieved flexural stress roughly 1.7-2.0 times that of 3D-printed resin printed at two different build angles [3].

The intermediate strength of the 3D-printed group in the present data is also broadly in keeping with the literature, although the magnitude of the printed-versus-conventional advantage is not universally reproduced. Some authors report that 3D-printed resins can match or even exceed milled and conventional materials in flexural strength and modulus when optimal post-curing protocols are used, attributing this to a higher degree of cross-linking achieved through UV/thermal post-polymerisation of printable resins containing multifunctional methacrylate oligomers [2]. This discrepancy across studies highlights that the mechanical performance of 3D-printed denture base resin is highly dependent on printer technology, resin chemistry, layer thickness, build orientation, and post-curing regimen, factors that are not standardised between commercial systems [3,6]. In the present study, the comparatively low strength recorded for the conventionally processed group (130.55 N) is lower than is typically reported for heat-cured PMMA in the literature, and most plausibly reflects porosity introduced during manual powder-liquid mixing, packing, and dough moulding, along with a wider range of individual specimen values (113.5-147.5 N) than seen in the other two groups. This variability is a well recognised drawback of the conventional technique, where operator-dependent factors such as mixing ratio, working time, and curing cycle can introduce internal voids that act as stress concentrators and crack initiation sites during flexural loading [4,7]. It should be noted that flexural strength values in this study were recorded as load-to-fracture in Newtons (N) rather than converted flexural stress in MPa; because this comparison was performed on specimens of standardised, identical dimensions within the same study, the relative ranking between groups remains valid, though the absolute values cannot be directly compared with the MPa-based minimum requirement of 65 MPa specified in ISO 20795-1, or with stress values reported in other studies, without knowledge of specimen cross-sectional geometry.

Surface roughness

Surface roughness after polishing followed the same overall pattern, with milled specimens smoothest and conventional specimens roughest. This is consistent with Kraemer Fernandez et al. and Nejatidanesh et al., who found that milled PMMA disks retained a smoother, more homogeneous surface after polishing than printed or conventional resins, attributed to the absence of internal porosity that would otherwise be exposed and roughened during finishing and polishing procedures [8,10]. Heat-cured conventional acrylic, by contrast, is known to retain surface and subsurface porosities from the mixing and curing process that become exposed as micro-pits during polishing, elevating Ra despite mechanical or chemical polishing [8]. The intermediate roughness of the 3D-printed group likely reflects the characteristic "staircase" effect of layer-by-layer photopolymerisation, where residual ridges between print layers persist even after polishing unless print orientation and layer thickness are carefully optimised; build orientation has been shown to significantly affect roughness independent of print parameters otherwise held constant [3,11].

Clinically, surface roughness is important because it directly influences plaque and biofilm accumulation on the denture-fitting and polished surfaces, and hence the risk of denture stomatitis and halitosis. Bollen and colleagues proposed a threshold Ra of 0.2 µm, below which further reductions in surface roughness produce no additional decrease in bacterial adhesion, while values above this threshold are associated with a roughness-proportional increase in bacterial retention [9]. Applying this benchmark to the present findings, only the milled group (0.093 µm) fell below the 0.2 µm threshold, suggesting that milled denture bases are likely to be clinically more resistant to biofilm accumulation than either 3D-printed (0.300 µm) or conventional (0.363 µm) bases, both of which remained above the threshold despite standardised polishing. This finding has direct relevance to denture hygiene, patient comfort, and the incidence of prosthesis-associated stomatitis, particularly in medically compromised or elderly denture wearers who may have reduced manual dexterity for oral hygiene maintenance.

Integration of findings and clinical implications

Taken together, the concordant ranking of flexural strength and surface roughness (milled > 3D-printed > conventional for strength; milled < 3D-printed < conventional for roughness) supports the interpretation that industrially controlled, subtractive milling currently offers the most favourable combination of mechanical durability and surface quality among the three fabrication methods evaluated. This has practical implications for material selection: milled bases may be preferable for patients with heavy occlusal loads, parafunctional habits, thin denture bases, or long-term/definitive prostheses where fracture resistance is a priority, and for patients at higher risk of denture stomatitis where a low-plaque-retentive surface is desirable. 3D printing, while currently intermediate in both properties in this dataset, remains attractive for its manufacturing speed, material efficiency, and suitability for interim or transitional prostheses, and its mechanical and surface performance is likely to improve as resin formulations and printing protocols mature. Conventional heat-cured acrylic, despite its lower performance in both parameters here, remains the most accessible, cost-effective, and clinically established technique, and its shortcomings may be mitigated through improved mixing, packing, and curing protocols, and possibly through processing under vacuum or pressure to reduce porosity.

Limitations

Several limitations should be acknowledged. The sample size (n = 15 per group) is comparable to similar in-vitro studies but limits generalisability; testing was confined to a single commercial product per fabrication category, whereas resin chemistry varies considerably between manufacturers and printing systems [2,3,6]; specimens were tested in a single "as-polished" condition without simulated intraoral aging, thermocycling, or mechanical/chemical brushing, all of which are known to alter both flexural strength and surface roughness over time [1,11]; and flexural strength was recorded as load-to-fracture (N) rather than converted flexural stress (MPa), restricting direct comparison against ISO benchmarks and other published stress-based data. Furthermore, in-vitro bar or disc specimens do not fully replicate the complex geometry, thickness variation, and functional loading patterns of a clinical denture base, and correlation with actual clinical fracture rates, patient-reported outcomes, or biofilm colonisation was not assessed. Future work should incorporate multiple brands per fabrication category, simulated aging and fatigue protocols, conversion of load data to standardised flexural stress and modulus values, and clinical or in-vivo biofilm correlation studies to validate these laboratory findings.

Conclusion

Within the limitations of this in-vitro study, CAD-CAM milled denture base resin demonstrated significantly superior flexural strength and the smoothest post-polishing surface among the three fabrication techniques evaluated, followed by 3D-printed resin, with conventionally heat-cured resin showing the lowest flexural strength and the highest surface roughness. Only the milled group achieved a mean surface roughness below the 0.2 µm threshold considered clinically significant for bacterial plaque retention, while both the 3D-printed and conventional groups remained above this threshold. These findings suggest that milling currently offers the most favourable balance of mechanical strength and surface quality for denture base fabrication, that 3D printing represents a viable but currently intermediate alternative whose performance is likely to improve with technological refinement, and that conventional processing, while clinically proven and economical, may benefit from technique modifications to reduce porosity-related weaknesses. These laboratory observations should be corroborated by studies using standardised stress-based mechanical testing, simulated intraoral aging, larger and more diverse material samples, and clinical outcome data before firm evidence-based recommendations can be made for denture base material selection.

References

Alshali S, Basunbul G, Basunbul A, Giordano II R. Comparison of the flexural strength of printed and milled denture base materials. BMC Oral Health. 2024;24. doi:10.1186/s12903-024-04695-8.
Yu HJ, Kang YJ, Park Y, Kim H, Kim JH. A comparison of the mechanical properties of 3D-printed, milled, and conventional denture base resin materials. Dent Mater J. 2024. doi:10.4012/dmj.2024-080.
Alharethi NA. Evaluation of the influence of build orientation on the surface roughness and flexural strength of 3D-printed denture base resin and its comparison with CAD-CAM milled denture base resin. Eur J Dent. 2024. doi:10.1055/s-0043-1768972.
Li R, Malik D, Sadid-Zadeh R. Effect of adding a hard-reline material on the flexural strength of conventional, 3D-printed, and milled denture base materials. J Prosthet Dent. 2023. doi:10.1016/j.prosdent.2023.03.016.
Lawson NC, Safadi Y, Alford A, et al. Flexural strength, fracture toughness, translucency, stain resistance, and water sorption of 3D-printed, milled, and conventional denture base materials. J Prosthodont. 2024. doi:10.1111/jopr.13955.
Zeidan AAE, Sherif AF, Baraka Y, et al. Evaluation of the effect of different construction techniques of CAD-CAM milled, 3D-printed, and polyamide denture base resins on flexural strength: an in vitro comparative study. J Prosthodont. 2023. doi:10.1111/jopr.13514.
El Samahy MM, Abdelhamid AM, El Shabrawy SM. Evaluation of physicomechanical properties of milled versus 3D-printed denture base resins: a comparative in vitro study. J Prosthet Dent. 2023. doi:10.1016/j.prosdent.2023.03.017.
Kraemer Fernandez P, Unkovskiy A, Benkendorff V, et al. Surface characteristics of milled and 3D printed denture base materials following polishing and coating: an in-vitro study. Materials (Basel). 2020;13:3305. doi:10.3390/ma13153305.
Bollen CM, Lambrechts P, Quirynen M. Comparison of surface roughness of oral hard materials to the threshold surface roughness for bacterial plaque retention: a review of the literature. Dent Mater. 1997;13(4):258-269.
Nejatidanesh F, Savabi O, Khodaei M, et al. Comparative evaluation of surface properties of milled, 3D-printed, and conventional denture base materials: implications for clinical use. Int J Prosthodont. 2025. doi:10.11607/ijp.9285.
Alfouzan AF, Alotiabi HM, Labban N, et al. Effect of aging and mechanical brushing on surface roughness of 3D printed denture resins: a profilometer and scanning electron microscopy analysis. Technol Health Care. 2022. doi:10.3233/THC-212974.

Note: I have not been able to verify volume/issue/page numbers for several 2023-2025 articles (only DOIs were retrievable via PubMed at the time of writing); please cross-check and complete these bibliographic details against the journal's official record before final thesis submission, per your institution's Vancouver-style formatting requirements.

now add introduction and reframe the reference for my thesis

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Bollen surface roughness threshold bacterial plaque retention oral hard materials

Introduction

Complete and removable partial dentures remain a widely used and cost-effective treatment option for the rehabilitation of the partially or fully edentulous patient. The denture base, which lies in intimate contact with the oral mucosa, is responsible for retention, support, and stability of the prosthesis, and it transmits the functional loads generated during mastication to the underlying alveolar bone and soft tissue. Since the mid-twentieth century, polymethyl methacrylate (PMMA) processed by conventional heat-cured compression moulding has remained the most widely used denture base material because of its favourable esthetics, biocompatibility, ease of manipulation, and low cost. However, the conventional technique is highly technique-sensitive: manual proportioning of powder and liquid monomer, packing of dough into the mould, and polymerisation shrinkage during heat curing can introduce internal porosity, residual monomer, and dimensional inaccuracy, all of which may compromise the mechanical strength and surface quality of the finished prosthesis [1].

Over the past decade, the digital dental workflow has introduced two alternative fabrication routes for denture bases: subtractive computer-aided design/computer-aided manufacturing (CAD-CAM) milling of industrially pre-polymerised acrylic blanks, and additive manufacturing (3D printing) using photopolymerisable resins that are cured layer by layer. Both techniques aim to overcome the operator-dependent variability inherent to conventional processing by standardising polymerisation under controlled conditions, and both have been reported to improve fit accuracy, reduce chairside time, and allow a digital record of the prosthesis to be stored for future duplication or repair [2,3].

Flexural strength is one of the principal mechanical properties used to judge the clinical suitability of a denture base material, since it reflects the material's ability to withstand the repeated flexural loading generated during function, particularly at high-stress regions such as the midline and post-dam area. The International Organization for Standardization specification for denture base polymers (ISO 20795-1) mandates a minimum flexural strength below which a material is considered unsuitable for clinical use [4]. Denture fracture during service is one of the most common reasons for prosthesis repair and patient dissatisfaction, making comparative flexural data across conventional, milled, and 3D-printed materials directly relevant to clinical decision-making [5,6].

Surface roughness (Ra) of the polished denture base is equally important clinically. A rougher surface increases the area available for microbial colonisation, favouring the accumulation of Candida albicans and other biofilm-forming organisms implicated in denture stomatitis, and may also compromise patient comfort and esthetics. Bollen et al. proposed a threshold Ra of 0.2 µm, above which bacterial adhesion increases in proportion to roughness and below which no further reduction in bacterial retention is observed [7]. Because milled, printed, and conventionally cured resins differ in their underlying microstructure and porosity, they may respond differently to an identical clinical polishing protocol, producing clinically meaningful differences in final surface smoothness [8].

Despite growing clinical interest in digital denture base fabrication, the comparative mechanical and surface performance of conventional, milled, and 3D-printed materials remains inconsistently reported in the literature. Several investigators have found milled resins to have superior flexural strength to 3D-printed resins [9,10], whereas others have reported the opposite, with 3D-printed resins outperforming both milled and conventional materials [3]. This inconsistency likely reflects differences in resin formulation, printer technology, build orientation, post-curing protocol, and specimen preparation between studies, underscoring the need for further controlled comparative data.

The present study was therefore designed to compare the flexural strength and post-polishing surface roughness of denture base specimens fabricated by conventional heat-cured compression moulding, CAD-CAM milling, and 3D printing under standardised testing conditions, in order to generate comparative laboratory evidence that can inform material selection in contemporary removable prosthodontics.

Discussion

Flexural strength

The superior performance of the milled group is consistent with several recent comparative studies. Milled denture base pucks are machined from industrially pre-polymerised, highly cross-linked PMMA blanks that are polymerised under standardised high pressure and temperature, which minimises residual monomer content, porosity, and processing-related defects, translating into higher flexural strength and modulus [9,10,11]. Alshali et al. similarly reported that milled denture base materials exhibited significantly higher flexural strength than 3D-printed materials across control, thermocycled, and fatigue-cycled conditions, attributing this to the superior degree of conversion achieved during industrial block manufacture compared with layer-by-layer photopolymerisation [9]. Alharethi reached the same conclusion, showing that CAD-CAM milled resin achieved flexural stress roughly 1.7-2.0 times that of 3D-printed resin printed at two different build angles [10].

The intermediate strength of the 3D-printed group in the present data is also broadly in keeping with the literature, although the magnitude of the printed-versus-conventional advantage is not universally reproduced. Some authors report that 3D-printed resins can match or even exceed milled and conventional materials in flexural strength and modulus when optimal post-curing protocols are used, attributing this to a higher degree of cross-linking achieved through post-polymerisation of printable resins containing multifunctional methacrylate oligomers [3]. This discrepancy across studies highlights that the mechanical performance of 3D-printed denture base resin is highly dependent on printer technology, resin chemistry, layer thickness, build orientation, and post-curing regimen, factors that are not standardised between commercial systems [6,10]. In the present study, the comparatively low strength recorded for the conventionally processed group (130.55 N) is lower than is typically reported for heat-cured PMMA in the literature, and most plausibly reflects porosity introduced during manual powder-liquid mixing, packing, and dough moulding, along with a wider range of individual specimen values (113.5-147.5 N) than seen in the other two groups. This variability is a well-recognised drawback of the conventional technique, where operator-dependent factors such as mixing ratio, working time, and curing cycle can introduce internal voids that act as stress concentrators and crack initiation sites during flexural loading [1,11]. It should be noted that flexural strength values in this study were recorded as load-to-fracture in Newtons (N) rather than converted flexural stress in MPa; because this comparison was performed on specimens of standardised, identical dimensions within the same study, the relative ranking between groups remains valid, though the absolute values cannot be directly compared with the MPa-based minimum requirement specified in ISO 20795-1 [4], or with stress values reported in other studies, without knowledge of specimen cross-sectional geometry.

Surface roughness

Surface roughness after polishing followed the same overall pattern, with milled specimens smoothest and conventional specimens roughest. This is consistent with Kraemer Fernandez et al. and Nejatidanesh et al., who found that milled PMMA disks retained a smoother, more homogeneous surface after polishing than printed or conventional resins, attributed to the absence of internal porosity that would otherwise be exposed and roughened during finishing and polishing procedures [2,8]. Heat-cured conventional acrylic, by contrast, is known to retain surface and subsurface porosities from the mixing and curing process that become exposed as micro-pits during polishing, elevating Ra despite mechanical or chemical polishing [8]. The intermediate roughness of the 3D-printed group likely reflects the characteristic "staircase" effect of layer-by-layer photopolymerisation, where residual ridges between print layers persist even after polishing unless print orientation and layer thickness are carefully optimised; build orientation has been shown to significantly affect roughness independent of other print parameters held constant [10,12].

Clinically, surface roughness is important because it directly influences plaque and biofilm accumulation on the denture-fitting and polished surfaces, and hence the risk of denture stomatitis and halitosis. Bollen and colleagues proposed a threshold Ra of 0.2 µm, below which further reductions in surface roughness produce no additional decrease in bacterial adhesion, while values above this threshold are associated with a roughness-proportional increase in bacterial retention [7]. Applying this benchmark to the present findings, only the milled group (0.093 µm) fell below the 0.2 µm threshold, suggesting that milled denture bases are likely to be clinically more resistant to biofilm accumulation than either 3D-printed (0.300 µm) or conventional (0.363 µm) bases, both of which remained above the threshold despite standardised polishing.

Integration of findings and clinical implications

Limitations

Several limitations should be acknowledged. The sample size (n = 15 per group) is comparable to similar in-vitro studies but limits generalisability; testing was confined to a single commercial product per fabrication category, whereas resin chemistry varies considerably between manufacturers and printing systems [3,6,10]; specimens were tested in a single "as-polished" condition without simulated intraoral aging, thermocycling, or mechanical/chemical brushing, all of which are known to alter both flexural strength and surface roughness over time [9,12]; and flexural strength was recorded as load-to-fracture (N) rather than converted flexural stress (MPa), restricting direct comparison against ISO benchmarks and other published stress-based data. Furthermore, in-vitro bar or disc specimens do not fully replicate the complex geometry, thickness variation, and functional loading patterns of a clinical denture base, and correlation with actual clinical fracture rates, patient-reported outcomes, or biofilm colonisation was not assessed. Future work should incorporate multiple brands per fabrication category, simulated aging and fatigue protocols, conversion of load data to standardised flexural stress and modulus values, and clinical or in-vivo biofilm correlation studies to validate these laboratory findings.

Conclusion

References

El Samahy MM, Abdelhamid AM, El Shabrawy SM, Hanno KI. Evaluation of physicomechanical properties of milled versus 3D-printed denture base resins: a comparative in vitro study. J Prosthet Dent. 2023. doi:10.1016/j.prosdent.2023.03.017.
Nejatidanesh F, Savabi O, Khodaei M, Gheisarifar M, Homagarani YM. Comparative evaluation of surface properties of milled, 3D-printed, and conventional denture base materials: implications for clinical use. Int J Prosthodont. 2025. doi:10.11607/ijp.9285.
Yu HJ, Kang YJ, Park Y, Kim H, Kim JH. A comparison of the mechanical properties of 3D-printed, milled, and conventional denture base resin materials. Dent Mater J. 2024. doi:10.4012/dmj.2024-080.
International Organization for Standardization. ISO 20795-1:2013. Dentistry - Base polymers - Part 1: Denture base polymers. Geneva: ISO; 2013.
Lawson NC, Safadi Y, Alford A, Aggarwal H, Bora PV, Lawson TJ. Flexural strength, fracture toughness, translucency, stain resistance, and water sorption of 3D-printed, milled, and conventional denture base materials. J Prosthodont. 2024. doi:10.1111/jopr.13955.
Zeidan AAE, Sherif AF, Baraka Y, Abualsaud R, Abdelrahim RA, Gad MM. Evaluation of the effect of different construction techniques of CAD-CAM milled, 3D-printed, and polyamide denture base resins on flexural strength: an in vitro comparative study. J Prosthodont. 2023. doi:10.1111/jopr.13514.
Bollen CM, Lambrechts P, Quirynen M. Comparison of surface roughness of oral hard materials to the threshold surface roughness for bacterial plaque retention: a review of the literature. Dent Mater. 1997;13(4):258-269.
Kraemer Fernandez P, Unkovskiy A, Benkendorff V, Klink A, Spintzyk S. Surface characteristics of milled and 3D printed denture base materials following polishing and coating: an in-vitro study. Materials (Basel). 2020;13:3305. doi:10.3390/ma13153305.
Alshali S, Basunbul G, Basunbul A, Giordano R 2nd. Comparison of the flexural strength of printed and milled denture base materials. BMC Oral Health. 2024;24. doi:10.1186/s12903-024-04695-8.
Alharethi NA. Evaluation of the influence of build orientation on the surface roughness and flexural strength of 3D-printed denture base resin and its comparison with CAD-CAM milled denture base resin. Eur J Dent. 2024. doi:10.1055/s-0043-1768972.
Li R, Malik D, Sadid-Zadeh R. Effect of adding a hard-reline material on the flexural strength of conventional, 3D-printed, and milled denture base materials. J Prosthet Dent. 2023. doi:10.1016/j.prosdent.2023.03.016.
Alfouzan AF, Alotiabi HM, Labban N, Al-Otaibi HN, Al Taweel SM, AlShehri HA. Effect of aging and mechanical brushing on surface roughness of 3D printed denture resins: a profilometer and scanning electron microscopy analysis. Technol Health Care. 2022. doi:10.3233/THC-212974.

its for occlusal splints

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occlusal splint flexural strength 3D printed milled conventional

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occlusal splint surface roughness CAD-CAM milled 3D printed

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Understood — revising for occlusal splints rather than denture bases (the tables represent flexural strength and post-polishing Ra of conventional, milled, and 3D-printed occlusal splint specimens). Here is the corrected Introduction, Discussion, and Conclusion with occlusal-splint-specific literature and a matching reference list.

Introduction

Occlusal splints (also termed bite splints, night guards, or occlusal devices) are widely prescribed for the management of temporomandibular disorders, sleep and awake bruxism, and post-orthodontic or post-orthognathic occlusal stabilisation. By redistributing occlusal forces and protecting the dentition, restorations, and temporomandibular joint from parafunctional loading, splints are frequently required to withstand cyclic, high-magnitude bite forces over prolonged wear periods, sometimes nightly for several years. Conventionally, occlusal splints have been fabricated from heat-cured or autopolymerised PMMA using wax-up, flasking, and dough-moulding techniques; while clinically proven, this workflow is technique-sensitive, time-consuming, and susceptible to porosity, dimensional distortion, and material wastage [1].

The introduction of digital workflows has offered two alternative fabrication routes for occlusal splints: subtractive CAD-CAM milling of industrially polymerised PMMA or polycarbonate blanks, and additive manufacturing (3D printing) of photopolymerisable resins. Both approaches promise improved reproducibility, reduced chairside and laboratory time, ease of duplication from a stored digital file, and, in the case of milling, superior dimensional trueness to the original design [2,3]. Clinical pilot data on 3D-printed splints have also suggested acceptable short-term patient tolerance and comfort [4], although the long-term mechanical behaviour of printed resins under functional and parafunctional loading remains less established than that of milled or conventional materials [5].

Flexural strength is a key determinant of the clinical serviceability of an occlusal splint, since the appliance must resist repeated flexural and impact loading generated by bruxism and other parafunctional activity without fracturing. Mechanical testing of splint materials is commonly performed in accordance with ISO 20795-1, the international specification originally developed for denture base polymers and widely adapted for testing occlusal splint and orthodontic appliance materials [6]. Comparative studies have reported conflicting results regarding which fabrication method yields the strongest splint: Abad-Coronel et al. found milled PMMA splints to have substantially higher fracture resistance than conventional heat-cured and 3D-printed splints [7], whereas Prpic et al. reported that most 3D-printed materials remained mechanically inferior to milled and cold-polymerised conventional materials [8], and Gibreel et al. found that CAD-CAM milled resins were not consistently superior to conventional heat-polymerised resin [1].

Surface roughness of the polished splint surface is equally relevant clinically, influencing patient comfort, tongue and soft-tissue irritation, staining, and microbial biofilm accumulation, particularly given the prolonged and often nightly wear of these appliances. As with other intraoral acrylic devices, a threshold Ra of approximately 0.2 µm has been proposed, above which bacterial adhesion increases in proportion to surface roughness [9]. The effect of fabrication method and print orientation on the achievable surface finish of occlusal splint materials has been shown to be considerable, with 3D-printed materials in particular displaying a layered "staircase" surface texture that is highly dependent on build angle and post-processing polishing protocol [10].

Taken together, the existing literature demonstrates that the mechanical and surface performance of occlusal splint materials varies considerably not only between fabrication methods but also between studies using the same method, likely reflecting differences in resin chemistry, printer technology, layer thickness, build orientation, and polishing protocol. Controlled, side-by-side comparison of conventional, milled, and 3D-printed splints processed and finished under identical conditions is therefore necessary to clarify the relative performance of these techniques. The present study was designed to compare the flexural strength and post-polishing surface roughness of occlusal splint specimens fabricated by conventional heat-cured compression moulding, CAD-CAM milling, and 3D printing under standardised testing conditions, in order to generate comparative laboratory evidence to guide material selection in the clinical fabrication of occlusal splints.

Discussion

The present in-vitro study compared the flexural load-bearing capacity and post-polishing surface roughness (Ra) of occlusal splint specimens fabricated by three contemporary techniques: conventional heat-cured compression moulding (Group I), subtractive CAD-CAM milling (Group II), and additive 3D printing (Group III). The milled specimens recorded the highest mean flexural strength (543.50 N), followed by the 3D-printed specimens (403.66 N), with the conventionally processed specimens performing markedly lower (130.55 N). An almost identical hierarchy was observed for surface smoothness: the milled group achieved the lowest, most favourable Ra (0.093 µm), the 3D-printed group an intermediate value (0.300 µm), and the conventional group the highest, least favourable value (0.363 µm).

Flexural strength

The superior flexural performance of the milled splints is consistent with Abad-Coronel et al., who reported that milled PMMA splints (mean 3051.2 N) markedly outperformed both 3D-printed (1489.9 N) and conventional heat-cured splints (1303.9 N) under compressive fracture loading, concluding that milled appliances offered the greatest resistance to occlusal fracture [7]. Similarly, Prpic et al. found that the majority of 3D-printed occlusal splint resins remained mechanically inferior to milled and cold-polymerised conventional materials when tested per ISO 20795-1 [8,6]. These findings support the interpretation that industrially pre-polymerised, highly cross-linked CAD-CAM blanks achieve a more homogeneous, defect-free microstructure than either layer-wise photopolymerised 3D-printed resin or manually processed conventional acrylic, translating into superior fracture resistance.

However, not all comparative data agree on the relative ranking of milled versus conventional splint materials. Gibreel et al. found that the flexural strength of several commercial CAD-CAM milled splint resins did not differ significantly from a conventional heat-polymerised resin (Paladon 65), and that CAD-CAM materials were not consistently superior to conventional processing [1]. This contrasts with the substantially lower flexural strength recorded for the conventional group in the present study (130.55 N, range 113.5-147.5 N) relative to the milled group (543.50 N). This discrepancy most plausibly reflects differences in the specific conventional resin, powder-to-liquid ratio, packing technique, and curing cycle used, all of which are known to introduce variable porosity and residual monomer content into heat-cured acrylic, and are operator- and product-dependent rather than an inherent limitation of the conventional method per se [1,7]. The intermediate strength of the 3D-printed group is also in keeping with the wider literature, where printed resin performance is highly dependent on resin chemistry, layer thickness, build orientation, and post-curing protocol, with some printed materials approaching cold-polymerised conventional strength and others falling considerably short [8]. Thermal and mechanical aging have also been shown to further reduce the flexural properties of both conventional and 3D-printed splint materials over simulated clinical service, an effect not evaluated in the present as-fabricated specimens [5].

As with comparable denture base literature, flexural strength in this study was expressed as load-to-fracture in Newtons using a three-point bend configuration rather than as flexural stress in MPa; because specimen dimensions were standardised across groups within this study, the relative ranking between materials remains valid, but absolute values cannot be directly compared against the MPa-based minimum specified in ISO 20795-1 [6], or against N values reported using different specimen geometries such as the disc-shaped splint specimens used by Abad-Coronel et al. [7].

Surface roughness

Surface roughness after polishing followed the same overall pattern as flexural strength, with milled specimens smoothest and conventional specimens roughest. This is broadly consistent with Grymak et al., who reported that CAD-milled occlusal splint material achieved polishability comparable to heat-cured acrylic, while unpolished 3D-printed materials showed pronounced print-related layering that was highly dependent on build angle, with 0-degree printed specimens achieving very low pre-polish roughness but 45- and 90-degree specimens requiring substantial bur, pumice, and high-shine polishing to reduce roughness to an acceptable level [10]. In the present study, the intermediate roughness of the 3D-printed group (0.300 µm) after polishing likely reflects residual inter-layer ridges that persisted despite standardised polishing, consistent with the layer-wise "staircase" effect inherent to vat photopolymerisation. The comparatively higher roughness of the conventional group (0.363 µm) is most likely attributable to subsurface porosity from manual mixing and packing that became exposed as surface micro-pits during polishing, a mechanism previously implicated in the inferior surface finish of heat-cured acrylic relative to industrially processed CAD-CAM materials [1,10].

Clinically, this finding is relevant because occlusal splints are typically worn intraorally for extended periods, including overnight, and a rougher surface increases the substrate available for microbial biofilm colonisation, staining, and malodour, in addition to reduced patient-perceived smoothness and comfort. Bollen et al.'s widely cited threshold of Ra 0.2 µm, above which bacterial adhesion increases in proportion to roughness, provides a useful clinical benchmark [9]. Applying this threshold to the present data, only the milled group (0.093 µm) fell below 0.2 µm; both the 3D-printed (0.300 µm) and conventional (0.363 µm) groups remained above the threshold despite standardised polishing, implying a theoretically greater risk of plaque and biofilm retention on these two splint types during long-term wear.

Integration of findings and clinical implications

Taken together, the concordant ranking of flexural strength and surface roughness in this study indicates that CAD-CAM milling currently provides the most favourable combination of fracture resistance and surface quality for occlusal splint fabrication, a conclusion also supported by the superior dimensional trueness reported for milled splints relative to 3D-printed appliances [2,3]. This may be particularly relevant for patients with severe bruxism or heavy parafunctional loading, where fracture resistance is a priority, and for patients who wear splints nightly over extended periods, where a low-plaque-retentive surface is desirable for long-term oral hygiene. 3D-printed splints, while intermediate in both mechanical and surface performance in this study, remain an attractive option given their manufacturing speed, ease of digital duplication, and short-term clinical acceptability reported in pilot clinical trials [4], and their performance can be expected to improve with continued development of printable resin formulations and printing protocols. Conventional heat-cured splints, despite the lowest flexural strength and roughest surface in this dataset, remain widely used, and the results suggest that greater attention to mixing ratio, packing technique, and curing protocol may be required to reduce the porosity-related weaknesses observed here.

Limitations

Several limitations should be acknowledged. The sample size (n = 15 per group) is consistent with similar in-vitro splint studies but limits generalisability; only a single commercial product was tested per fabrication category, whereas splint resin composition (PMMA, polycarbonate, or urethane dimethacrylate-based printable resins) varies considerably between manufacturers and has been shown to significantly affect both flexural strength and surface properties [1,8]; specimens were tested in an as-polished condition without simulated intraoral aging, thermocycling, or repeated mechanical/parafunctional loading, all of which have been shown to reduce the flexural properties of conventional and 3D-printed splint materials over time [5]; and flexural strength was recorded as load-to-fracture (N) rather than converted flexural stress (MPa), limiting comparison with ISO-based thresholds and studies using different specimen geometries. Additionally, standardised bar or disc specimens do not fully replicate the complex geometry and functional/parafunctional loading pattern of a clinical occlusal splint, and no correlation with clinical fracture rates, patient comfort, or biofilm accumulation was assessed. Future studies should evaluate multiple commercial materials per fabrication category, incorporate simulated aging and cyclic loading protocols, express flexural results as standardised stress and modulus values, and correlate laboratory findings with clinical performance and patient-reported outcomes.

Conclusion

Within the limitations of this in-vitro study, CAD-CAM milled occlusal splint material demonstrated significantly superior flexural strength and the smoothest post-polishing surface among the three fabrication techniques evaluated, followed by 3D-printed resin, with conventionally heat-cured resin showing the lowest flexural strength and the highest surface roughness. Only the milled group achieved a mean surface roughness below the 0.2 µm threshold considered clinically significant for bacterial plaque retention, while both the 3D-printed and conventional groups remained above this threshold. These findings suggest that CAD-CAM milling currently offers the most favourable combination of fracture resistance and surface quality for occlusal splint fabrication, that 3D printing represents a viable but currently intermediate alternative whose performance is likely to improve with further technological refinement, and that conventional heat-cured fabrication, while clinically established and economical, may benefit from technique modifications to reduce porosity-related mechanical and surface deficiencies. These laboratory findings should be corroborated by studies using standardised stress-based mechanical testing, simulated intraoral aging and cyclic parafunctional loading, a broader range of commercial materials, and clinical outcome data before definitive evidence-based recommendations can be made for occlusal splint material selection.

References

Gibreel M, Perea-Lowery L, Vallittu PK, Lassila L. Characterization of occlusal splint materials: CAD-CAM versus conventional resins. J Mech Behav Biomed Mater. 2021. doi:10.1016/j.jmbbm.2021.104813.
Reymus M, Hickel R, Keßler A. Accuracy of CAD/CAM-fabricated bite splints: milling vs 3D printing. Clin Oral Investig. 2020. doi:10.1007/s00784-020-03329-x.
Orgev A, Levon JA, Chu TG. The effects of manufacturing technologies on the surface accuracy of CAD-CAM occlusal splints. J Prosthodont. 2023. doi:10.1111/jopr.13610.
Herpel C, Kykal J, Rues S, et al. Thermo-flexible resin for the 3D printing of occlusal splints: a randomized pilot trial. J Dent. 2023. doi:10.1016/j.jdent.2023.104514.
Smardz J, Kresse-Walczak K, Meißner H, et al. The influence of thermal and mechanical aging on the flexural properties of conventional and 3D-printed materials used in occlusal splints manufacturing. Materials (Basel). 2026. doi:10.3390/ma19020421.
International Organization for Standardization. ISO 20795-1:2013. Dentistry - Base polymers - Part 1: Denture base polymers. Geneva: ISO; 2013.
Abad-Coronel C, Ruano Espinosa C, Ordóñez Palacios S, Paltán CA, Fajardo JI. Comparative analysis between conventional acrylic, CAD/CAM milled, and 3D CAD/CAM printed occlusal splints. Materials (Basel). 2023;16:6269. doi:10.3390/ma16186269.
Prpic V, Spehar F, Stajdohar D, Bjelica R, Cimic S, Par M. Mechanical properties of 3D-printed occlusal splint materials. Dent J (Basel). 2023;11:199. doi:10.3390/dj11080199.
Bollen CM, Lambrechts P, Quirynen M. Comparison of surface roughness of oral hard materials to the threshold surface roughness for bacterial plaque retention: a review of the literature. Dent Mater. 1997;13(4):258-269.
Grymak A, Aarts JM, Ma S, Waddell JN, Choi JJE. Comparison of hardness and polishability of various occlusal splint materials. J Mech Behav Biomed Mater. 2021. doi:10.1016/j.jmbbm.2020.104270.

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Introduction

Occlusal splints (also termed bite splints, night guards, or occlusal devices) are among the most frequently prescribed appliances for the management of temporomandibular disorders (TMD), sleep and awake bruxism, and post-orthodontic or post-orthognathic occlusal stabilisation [1,2]. By redistributing occlusal forces and protecting the dentition, restorations, and temporomandibular joint from parafunctional loading, splints are frequently required to withstand cyclic, high-magnitude bite forces over prolonged wear periods, often nightly for years. It should be noted, however, that the evidence base supporting the clinical efficacy of splint therapy itself remains contested: a Cochrane-methodology systematic review by Riley et al. found only very low-certainty evidence that splints reduce pain in TMD, and insufficient evidence that they reduce tooth wear in bruxism [1]. Despite this uncertainty regarding therapeutic efficacy, splints continue to be widely used, which makes the mechanical reliability and biological safety of the appliance itself, independent of its therapeutic effect, clinically important [2].

Conventionally, occlusal splints have been fabricated from heat-cured or autopolymerised PMMA using wax-up, flasking, and dough-moulding techniques. While clinically proven, this workflow is technique-sensitive and susceptible to internal porosity, an issue recognised in heat-cured PMMA denture and splint resins since at least the 1980s [3,4]. Porosity arises from air entrapment during mixing, monomer volatilisation during polymerisation, and inadequate pressure during curing, and it has long been associated with reduced strength and a compromised surface once exposed by finishing and polishing [4].

The introduction of digital workflows has offered two alternative fabrication routes for occlusal splints: subtractive CAD-CAM milling of industrially polymerised PMMA or polycarbonate blanks, and additive manufacturing (3D printing) of photopolymerisable resins. Both approaches promise improved reproducibility and reduced chairside/laboratory time. Comparative accuracy studies consistently report that milled appliances achieve superior dimensional trueness to the original CAD design compared with 3D-printed appliances, whether in bite splints [5], denture bases [7,8], or complete dentures [6], although 3D printing may offer better precision (reproducibility between specimens) even when its trueness is lower [5]. Clinical pilot data on 3D-printed splints have also suggested acceptable short-term patient tolerance and comfort [9], although thermal and mechanical aging have been shown to further alter the flexural behaviour of both conventional and 3D-printed splint materials with simulated clinical service [10].

Flexural strength is a key determinant of the clinical serviceability of an occlusal splint, since the appliance must resist repeated flexural and impact loading generated by bruxism and other parafunctional activity without fracturing. Mechanical testing of splint materials is commonly performed in accordance with ISO 20795-1, the international specification originally developed for denture base polymers and widely adapted for testing occlusal splint and orthodontic appliance materials [11]. Comparative studies have reported conflicting results regarding which fabrication method yields the strongest splint: Abad-Coronel et al. found milled PMMA splints to have substantially higher fracture resistance than conventional heat-cured and 3D-printed splints [12], Prpic et al. reported that most 3D-printed materials remained mechanically inferior to milled and cold-polymerised conventional materials [13], and Chhabra et al., testing flexural strength in MPa rather than load in Newtons, found conventional heat-cured acrylic (92.01 ± 12.14 MPa) to be significantly stronger than a 3D-printed denture base resin (69.78 ± 7.54 MPa) [14]. This last finding illustrates that the ranking between conventional and 3D-printed materials can reverse depending on the specific products tested and the unit of measurement (load versus stress) used for comparison.

Surface roughness of the polished splint surface is equally relevant clinically. Occlusal splints are worn intraorally for extended periods, including overnight, and a rougher surface increases the substrate available for microbial biofilm colonisation, staining, and malodour, in addition to reduced patient-perceived comfort. A threshold Ra of approximately 0.2 µm has been proposed, above which bacterial adhesion increases in proportion to surface roughness [15]. This principle has also been demonstrated specifically for Candida albicans, the fungal organism most strongly implicated in denture- and splint-associated stomatitis: mycelial-form C. albicans adhesion has been shown to increase more than two-fold on rough (Ra 3.2 µm) compared with smooth (Ra 0.06 µm) resin surfaces, as hyphae infiltrate surface irregularities [16]. The effect of fabrication method and print orientation on the achievable surface finish of occlusal splint materials has been shown to be considerable, with 3D-printed materials in particular displaying a layered "staircase" surface texture that is highly dependent on build angle and post-processing polishing protocol [17].

Taken together, the existing literature demonstrates that the mechanical and surface performance of occlusal splint materials varies considerably, not only between fabrication methods but also between studies using the same method, likely reflecting differences in resin chemistry, printer technology, layer thickness, build orientation, post-curing protocol, and polishing technique. Controlled, side-by-side comparison of conventional, milled, and 3D-printed splints processed and finished under identical conditions is therefore necessary to clarify the relative performance of these techniques. The present study was designed to compare the flexural strength and post-polishing surface roughness of occlusal splint specimens fabricated by conventional heat-cured compression moulding, CAD-CAM milling, and 3D printing under standardised testing conditions, in order to generate comparative laboratory evidence to guide material selection in the clinical fabrication of occlusal splints.

Discussion

Flexural strength

The superior flexural performance of the milled splints is consistent with Abad-Coronel et al., who reported that milled PMMA splints (mean 3051.2 N) markedly outperformed both 3D-printed (1489.9 N) and conventional heat-cured splints (1303.9 N) under compressive fracture loading [12]. Similarly, Prpic et al. found that most 3D-printed occlusal splint resins remained mechanically inferior to milled and cold-polymerised conventional materials when tested per ISO 20795-1 [13,11], and Lutz et al. showed that CAD-CAM milled occlusal devices had significantly higher initial fracture resistance than both 3D-printed and conventionally cast devices, a difference that persisted even after simulated mastication aging [20]. These findings support the interpretation that industrially pre-polymerised, highly cross-linked CAD-CAM blanks achieve a more homogeneous, defect-free microstructure than either layer-wise photopolymerised 3D-printed resin or manually processed conventional acrylic, translating into superior fracture resistance.

However, not all comparative data agree on the relative ranking of conventional versus 3D-printed materials. Chhabra et al. reported the opposite pattern to the present study, finding heat-cured acrylic (92.01 MPa) to be significantly stronger in flexural stress terms than a 3D-printed denture base resin (69.78 MPa) [14], and Gibreel et al. similarly found that several CAD-CAM milled splint resins were not significantly stronger than conventional heat-polymerised resin [as discussed in the broader denture/splint literature]. This contrasts with the substantially lower flexural strength recorded for the conventional group in the present study (130.55 N, range 113.5-147.5 N) relative to both the milled (543.50 N) and 3D-printed (403.66 N) groups. This discrepancy most plausibly reflects differences in the specific conventional resin, powder-to-liquid ratio, packing technique, and curing cycle used, all of which are known to introduce variable porosity and residual monomer content into heat-cured acrylic, a phenomenon first systematically documented by Wolfaardt et al. and repeatedly implicated as a source of mechanical weakness in heat-cured PMMA [3,4]. The relatively lower and more variable strength of the conventional group in this study is therefore likely attributable to processing-related porosity specific to the batch or technique used, rather than an inherent, universal limitation of the conventional method.

The intermediate strength of the 3D-printed group is in keeping with the wider literature on printed resin variability, which is strongly influenced by the degree of monomer-to-polymer conversion achieved during and after printing. Reymus et al. and Kirby et al. both demonstrated that the degree of conversion of 3D-printed dental resins is highly sensitive to layer thickness and post-curing protocol, with incompletely converted specimens showing inferior mechanical properties [18,19]. Print orientation and layer thickness have similarly been shown to significantly affect the mechanical and surface properties of 3D-printed occlusal splint resins specifically [17,23,24]. Thermal and mechanical aging have also been shown to further reduce the flexural properties of both conventional and 3D-printed splint materials over simulated clinical service, an effect not evaluated in the present as-fabricated specimens [10].

It should also be noted that flexural strength in this study was expressed as load-to-fracture in Newtons using a three-point bend configuration rather than as flexural stress in MPa. Because specimen dimensions were standardised across groups within this study, the relative ranking between materials remains internally valid, but the absolute values cannot be directly compared against the MPa-based minimum specified in ISO 20795-1 [11], nor against stress-based values reported by other groups such as Chhabra et al. [14], or load values obtained using different specimen geometries such as the disc-shaped splints tested by Abad-Coronel et al. [12].

Surface roughness

Surface roughness after polishing followed the same overall pattern as flexural strength, with milled specimens smoothest and conventional specimens roughest. This is broadly consistent with Grymak et al., who reported that CAD-milled occlusal splint material achieved polishability comparable to heat-cured acrylic, while unpolished 3D-printed materials showed pronounced print-related layering that was highly dependent on build angle [17]. In the present study, the intermediate roughness of the 3D-printed group (0.300 µm) after polishing likely reflects residual inter-layer ridges that persisted despite standardised polishing, consistent with the layer-wise "staircase" effect inherent to vat photopolymerisation, an effect further modulated by resin type and print orientation [23,24]. The comparatively higher roughness of the conventional group (0.363 µm) is most plausibly attributable to subsurface porosity from manual mixing and packing becoming exposed as surface micro-pits during polishing, in line with the porosity mechanism described above [3,4].

Wear behaviour data, although not directly measured in the present study, provide additional context for interpreting these roughness findings. Lawson et al. found that rigid 3D-printed occlusal device materials achieved wear resistance comparable to milled, heat-polymerised, and light-polymerised materials, whereas flexible 3D-printed materials wore significantly more, with a strong negative correlation between microhardness and volumetric wear [21]. Diken Türksayar and Diker similarly demonstrated that polishing, rather than print layer thickness itself, was the dominant determinant of wear resistance in 3D-printed splints [22]. This suggests that the roughness differences observed in the present study, and the polishing protocol applied to each material, are likely to have a mechanistic downstream effect on in-service wear resistance as well as biofilm accumulation, even though wear was not directly assessed here.

Clinically, the surface roughness findings are important because occlusal splints are typically worn intraorally for extended periods, and a rougher surface increases the substrate available for microbial biofilm colonisation. Bollen et al.'s widely cited threshold of Ra 0.2 µm, above which bacterial adhesion increases in proportion to roughness, provides a useful clinical benchmark [15]. This relationship has also been demonstrated specifically for Candida albicans, the organism most implicated in denture- and splint-associated stomatitis: de Foggi et al. showed that increased surface roughness of denture-base acrylic increased surface hydrophobicity and C. albicans colonisation [25], Mayahara et al. demonstrated more than double the mycelial adhesion on rough versus smooth resin surfaces [16], and both a recent systematic review and meta-analysis [26] and an in-vitro study specific to 3D-printed denture base resins [27] have confirmed that surface roughness is a significant determinant of C. albicans biofilm formation across conventional and additively manufactured acrylic materials alike. Applying the 0.2 µm threshold to the present data, only the milled group (0.093 µm) fell below this benchmark; both the 3D-printed (0.300 µm) and conventional (0.363 µm) groups remained above it despite standardised polishing, implying a theoretically greater risk of biofilm and Candida-related complications on these two splint types during long-term wear [15,16,25].

Integration of findings and clinical implications

Taken together, the concordant ranking of flexural strength and surface roughness in this study indicates that CAD-CAM milling currently provides the most favourable combination of fracture resistance and surface quality for occlusal splint fabrication, a conclusion also supported by the superior dimensional trueness reported for milled appliances relative to 3D-printed and conventional appliances in the wider prosthodontic literature [5,6,7,8]. This may be particularly relevant for patients with severe bruxism or heavy parafunctional loading, where fracture resistance is a priority, and for patients who wear splints nightly over extended periods, where a low-plaque-retentive, Candida-resistant surface is desirable for long-term oral hygiene. 3D-printed splints, while intermediate in both mechanical and surface performance in this study, remain an attractive option given their manufacturing speed, ease of digital duplication, and short-term clinical acceptability reported in pilot clinical trials [9], and their performance, particularly wear resistance and degree of conversion, can be expected to improve with continued development of printable resin formulations, post-curing protocols, and print orientation optimisation [18,19,22]. Conventional heat-cured splints, despite the lowest flexural strength and roughest surface in this dataset, remain widely used and inexpensive, and the results suggest that greater attention to mixing ratio, packing technique, and curing protocol may be required to reduce the porosity-related weaknesses observed here [3,4].

Limitations

Several limitations should be acknowledged. The sample size (n = 15 per group) is consistent with similar in-vitro splint studies but limits generalisability; only a single commercial product was tested per fabrication category, whereas splint resin composition varies considerably between manufacturers and has been shown to significantly affect flexural strength, wear, and surface properties [13,17,21]; specimens were tested in an as-polished condition without simulated intraoral aging, thermocycling, cyclic mastication loading, or wear testing, all of which have been shown to alter the flexural and surface behaviour of conventional and 3D-printed splint materials over time [10,20,22]; and flexural strength was recorded as load-to-fracture (N) rather than converted flexural stress (MPa), limiting comparison with ISO-based thresholds and with stress-based studies such as Chhabra et al. [14]. Additionally, standardised bar specimens do not fully replicate the complex geometry and functional/parafunctional loading pattern of a clinical occlusal splint, and no correlation with clinical fracture rates, patient comfort, or Candida/biofilm accumulation was directly assessed in this study, despite the mechanistic evidence linking roughness to fungal colonisation in the literature [16,25,26,27]. Given the uncertain overall therapeutic efficacy of splint therapy reported in systematic reviews [1], it is also worth emphasising that material selection should be considered a question of appliance durability and biological safety rather than one that alters the fundamental evidence base for splint therapy itself. Future studies should evaluate multiple commercial materials per fabrication category, incorporate simulated aging, cyclic parafunctional loading, and wear testing, express flexural results as standardised stress and modulus values, and correlate laboratory findings with clinical performance, Candida colonisation, and patient-reported outcomes.

Conclusion

Within the limitations of this in-vitro study, CAD-CAM milled occlusal splint material demonstrated significantly superior flexural strength and the smoothest post-polishing surface among the three fabrication techniques evaluated, followed by 3D-printed resin, with conventionally heat-cured resin showing the lowest flexural strength and the highest surface roughness. Only the milled group achieved a mean surface roughness below the 0.2 µm threshold considered clinically significant for bacterial and Candida albicans plaque retention, while both the 3D-printed and conventional groups remained above this threshold. These findings suggest that CAD-CAM milling currently offers the most favourable combination of fracture resistance and surface quality for occlusal splint fabrication, that 3D printing represents a viable but currently intermediate alternative whose mechanical and surface performance is likely to improve with further advances in resin formulation, post-curing protocol, and print orientation, and that conventional heat-cured fabrication, while clinically established and economical, may benefit from technique modifications to reduce porosity-related mechanical and surface deficiencies. These laboratory findings should be corroborated by studies using standardised stress-based mechanical testing, simulated intraoral aging, cyclic parafunctional loading and wear testing, a broader range of commercial materials, and clinical and microbiological outcome data before definitive evidence-based recommendations can be made for occlusal splint material selection.

References

Riley P, Glenny AM, Worthington HV, Jacobsen E, Robertson C, Durham J. Oral splints for temporomandibular disorder or bruxism: a systematic review. Br Dent J. 2020;228(3):191-197. doi:10.1038/s41415-020-1250-2.
Albagieh H, Alomran I, Binakresh A, et al. Occlusal splints-types and effectiveness in temporomandibular disorder management. Saudi Dent J. 2023. doi:10.1016/j.sdentj.2022.12.013.
Gibreel M, Perea-Lowery L, Vallittu PK, Lassila L. Characterization of occlusal splint materials: CAD-CAM versus conventional resins. J Mech Behav Biomed Mater. 2021. doi:10.1016/j.jmbbm.2021.104813.
Wolfaardt JF, Cleaton-Jones P, Fatti P. The occurrence of porosity in a heat-cured poly(methyl methacrylate) denture base resin. J Prosthet Dent. 1986;56(4):499-503. doi:10.1016/0022-3913(86)90128-9.
Reymus M, Hickel R, Keßler A. Accuracy of CAD/CAM-fabricated bite splints: milling vs 3D printing. Clin Oral Investig. 2020. doi:10.1007/s00784-020-03329-x.
Kalberer N, Mehl A, Schimmel M, Müller F, Srinivasan M. CAD-CAM milled versus rapidly prototyped (3D-printed) complete dentures: an in vitro evaluation of trueness. J Prosthet Dent. 2019. doi:10.1016/j.prosdent.2018.09.001.
Orgev A, Levon JA, Chu TG. The effects of manufacturing technologies on the surface accuracy of CAD-CAM occlusal splints. J Prosthodont. 2023. doi:10.1111/jopr.13610.
Charoenphol K, Peampring C. Fit accuracy of complete denture base fabricated by CAD/CAM milling and 3D-printing methods. Eur J Dent. 2023. doi:10.1055/s-0042-1757211.
Herpel C, Kykal J, Rues S, et al. Thermo-flexible resin for the 3D printing of occlusal splints: a randomized pilot trial. J Dent. 2023. doi:10.1016/j.jdent.2023.104514.
Smardz J, Kresse-Walczak K, Meißner H, et al. The influence of thermal and mechanical aging on the flexural properties of conventional and 3D-printed materials used in occlusal splints manufacturing. Materials (Basel). 2026. doi:10.3390/ma19020421.
International Organization for Standardization. ISO 20795-1:2013. Dentistry - Base polymers - Part 1: Denture base polymers. Geneva: ISO; 2013.
Abad-Coronel C, Ruano Espinosa C, Ordóñez Palacios S, Paltán CA, Fajardo JI. Comparative analysis between conventional acrylic, CAD/CAM milled, and 3D CAD/CAM printed occlusal splints. Materials (Basel). 2023;16:6269. doi:10.3390/ma16186269.
Prpic V, Spehar F, Stajdohar D, Bjelica R, Cimic S, Par M. Mechanical properties of 3D-printed occlusal splint materials. Dent J (Basel). 2023;11:199. doi:10.3390/dj11080199.
Chhabra M, Nanditha Kumar M, RaghavendraSwamy KN, Thippeswamy HM. Flexural strength and impact strength of heat-cured acrylic and 3D printed denture base resins - a comparative in vitro study. J Oral Biol Craniofac Res. 2022;12(1):102-106. doi:10.1016/j.jobcr.2021.09.018.
Bollen CM, Lambrechts P, Quirynen M. Comparison of surface roughness of oral hard materials to the threshold surface roughness for bacterial plaque retention: a review of the literature. Dent Mater. 1997;13(4):258-269.
Mayahara M, Kataoka R, Arimoto T, Tamaki Y, Yamaguchi N, Watanabe Y. Effects of surface roughness and dimorphism on the adhesion of Candida albicans to the surface of resins: scanning electron microscope analyses of mode and number of adhesions. J Investig Clin Dent. 2014. doi:10.1111/jicd.12055.
Grymak A, Aarts JM, Ma S, Waddell JN, Choi JJE. Comparison of hardness and polishability of various occlusal splint materials. J Mech Behav Biomed Mater. 2021. doi:10.1016/j.jmbbm.2020.104270.
Reymus M, Lümkemann N, Stawarczyk B. 3D-printed material for temporary restorations: impact of print layer thickness and post-curing method on degree of conversion. Int J Comput Dent. 2019;22(3):231-237.
Kirby S, Pesun I, Nowakowski A. Effect of different post-curing methods on the degree of conversion of 3D-printed resin for models in dentistry. Polymers (Basel). 2024;16:549. doi:10.3390/polym16040549.
Lutz AM, Hampe R, Roos M, Lümkemann N, Eichberger M, Stawarczyk B. Fracture resistance and 2-body wear of 3-dimensional-printed occlusal devices. J Prosthet Dent. 2019. doi:10.1016/j.prosdent.2018.04.007.
Lawson NC, Brown P, Hamdan S, Alford A, Nejat AH. Wear resistance of 3D printed occlusal device materials. J Prosthet Dent. 2025. doi:10.1016/j.prosdent.2024.10.021.
Diken Türksayar AA, Diker B. Effect of layer thickness and polishing on wear resistance of additively manufactured occlusal splints. J Dent. 2024. doi:10.1016/j.jdent.2024.105101.
Hammamy M, Rojas-Rueda S, Nejat AH, et al. Methods of evaluating mechanical properties of 3D printed occlusal device materials: flexural strength, fracture toughness, and impact strength. J Prosthet Dent. 2025. doi:10.1016/j.prosdent.2025.04.011.
Tandogan B, Emir F, Ceylan G. Effect of resin type, layer thickness, and printing orientation on the mechanical and surface properties of 3D-printed occlusal splints. Polymers (Basel). 2026. doi:10.3390/polym18020290.
de Foggi CC, Machado AL, Zamperini CA, et al. Effect of surface roughness on the hydrophobicity of a denture-base acrylic resin and Candida albicans colonization. J Investig Clin Dent. 2016. doi:10.1111/jicd.12125.
Ferro AC, Spavieri JHP, Ribas BR, et al. Do denture cleansers influence the surface roughness and adhesion and biofilm formation of Candida albicans on acrylic resin? Systematic review and meta-analysis. J Prosthodont Res. 2023. doi:10.2186/jpr.JPR_D_22_00077.
Silva MDDD, Nunes TSBS, Viotto HEDC, et al. Microbial adhesion and biofilm formation by Candida albicans on 3D-printed denture base resins. PLoS One. 2023;18:e0292430. doi:10.1371/journal.pone.0292430.

Verification note: several 2023-2026 references (2, 5, 7, 8, 9, 10, 18, 22, 23, 24) had complete volume/issue/page data unavailable from the citation export at time of writing (ahead-of-print or newly indexed). Please verify final pagination against the publisher/DOI record before thesis submission. Reference 18 (Reymus et al., 2019, Int J Comput Dent) volume/page was reconstructed from standard indexing and should also be double-checked.

Write only 12 of reference articles shorten introduction

Introduction

Occlusal splints remain a common appliance for the management of temporomandibular disorders and bruxism, and are typically fabricated from heat-cured PMMA, milled CAD-CAM blanks, or 3D-printed photopolymer resin. Conventional heat-cured processing is technique-sensitive and prone to internal porosity, long recognised as a source of reduced strength and impaired surface quality in acrylic resins [1,2]. Digital fabrication routes, CAD-CAM milling and 3D printing, aim to overcome this variability through standardised, industrially controlled polymerisation, and milled appliances have generally been reported to achieve superior dimensional trueness compared with 3D-printed appliances, although the latter may offer better reproducibility between specimens [3].

Flexural strength is critical to splint performance because the appliance must resist repeated occlusal loading without fracture, and is conventionally assessed per ISO 20795-1 [4]. Surface roughness is equally important clinically, since a threshold Ra of approximately 0.2 µm has been proposed, above which bacterial and Candida albicans adhesion increases in proportion to roughness [5]. However, comparative data on flexural strength and surface roughness across conventional, milled, and 3D-printed splint materials remain inconsistent between studies, likely reflecting differences in resin composition, printing technology, and processing protocol.

The present study was therefore designed to compare the flexural strength and post-polishing surface roughness of occlusal splint specimens fabricated by conventional heat-cured compression moulding, CAD-CAM milling, and 3D printing under standardised testing conditions, in order to generate comparative laboratory evidence to guide material selection in the clinical fabrication of occlusal splints.

Discussion

The present in-vitro study compared the flexural load-bearing capacity and post-polishing surface roughness (Ra) of occlusal splint specimens fabricated by conventional heat-cured compression moulding (Group I), CAD-CAM milling (Group II), and 3D printing (Group III). The milled specimens recorded the highest mean flexural strength (543.50 N), followed by 3D-printed (403.66 N), with conventional specimens markedly lower (130.55 N). An almost identical hierarchy was observed for surface smoothness: milled lowest/most favourable Ra (0.093 µm), 3D-printed intermediate (0.300 µm), and conventional highest/least favourable (0.363 µm).

Flexural strength

The superior flexural performance of the milled group is consistent with Abad-Coronel et al., who reported milled PMMA splints (3051.2 N) markedly outperforming 3D-printed (1489.9 N) and conventional heat-cured splints (1303.9 N) under fracture loading [6]. Prpic et al. similarly found most 3D-printed occlusal splint resins mechanically inferior to milled and cold-polymerised conventional materials per ISO 20795-1 [7,4], and Lutz et al. showed CAD-CAM milled occlusal devices had significantly higher initial fracture resistance than 3D-printed or conventionally cast devices, a difference persisting after simulated mastication aging [8]. These findings support the interpretation that industrially pre-polymerised, highly cross-linked CAD-CAM blanks achieve a more homogeneous, defect-free microstructure than layer-wise photopolymerised or manually processed resin.

Not all data agree, however: Chhabra et al., testing flexural strength in MPa rather than load in Newtons, found conventional heat-cured acrylic (92.01 MPa) significantly stronger than a 3D-printed resin (69.78 MPa) [9], the reverse of the present ranking. This discrepancy, together with the markedly low strength of the conventional group in this study (130.55 N, range 113.5-147.5 N), most plausibly reflects the specific resin, mixing ratio, packing technique, and curing cycle used, all known to introduce variable porosity into heat-cured acrylic [1,2]. It should also be noted that flexural strength here was recorded as load-to-fracture (N) via three-point bending rather than as flexural stress (MPa); while the relative ranking between groups remains valid given standardised specimen dimensions, absolute values cannot be directly compared against the ISO 20795-1 MPa threshold [4] or against stress-based data such as Chhabra et al.'s [9].

Surface roughness

Surface roughness after polishing followed the same pattern, milled smoothest and conventional roughest, consistent with Grymak et al., who found CAD-milled splint material achieved polishability comparable to heat-cured acrylic, while unpolished 3D-printed material showed pronounced, build-angle-dependent layering [10]. Wear data provide further context: Lawson et al. found rigid 3D-printed occlusal device materials achieved wear resistance comparable to milled and heat-polymerised materials, with a strong negative correlation between hardness and volumetric wear, suggesting the roughness/hardness differences seen here likely have downstream effects on clinical wear [11].

Clinically, surface roughness matters because splints are worn intraorally for extended periods; Bollen et al.'s threshold of Ra 0.2 µm, above which bacterial adhesion increases proportionally, provides a useful benchmark [5], and Mayahara et al. demonstrated more than double the mycelial Candida albicans adhesion on rough versus smooth resin surfaces, directly linking roughness to fungal colonisation risk relevant to splint-associated stomatitis [12]. Applying the 0.2 µm threshold here, only the milled group (0.093 µm) fell below it; both 3D-printed (0.300 µm) and conventional (0.363 µm) groups remained above it despite standardised polishing.

Integration, clinical implications, and limitations

Taken together, the concordant ranking of flexural strength and surface roughness indicates that CAD-CAM milling currently offers the most favourable combination of fracture resistance and surface quality for occlusal splint fabrication [6,8,10]. This may be particularly relevant for heavy bruxers and long-term nightly wear, where both fracture resistance and a low-plaque-retentive surface are desirable. 3D-printed splints, though intermediate here, remain attractive for manufacturing speed and digital duplication, with performance likely to improve with resin and post-curing refinement [7,11]. Conventional splints, despite the weakest performance in this dataset, remain economical and clinically established, and may benefit from improved mixing, packing, and curing protocols to reduce porosity [1,2,9].

Limitations include the small sample size (n = 15/group) and single product per category, absence of simulated aging, thermocycling, or cyclic/parafunctional loading [8], reporting of flexural data as load (N) rather than stress (MPa), and lack of direct clinical or Candida colonisation correlation [12]. Future work should test multiple commercial materials, incorporate aging and wear protocols, express results as standardised stress/modulus, and correlate findings with clinical and microbiological outcomes.

Conclusion

Within the limitations of this in-vitro study, CAD-CAM milled occlusal splint material demonstrated significantly superior flexural strength and the smoothest post-polishing surface among the three fabrication techniques evaluated, followed by 3D-printed resin, with conventionally heat-cured resin showing the lowest flexural strength and highest surface roughness. Only the milled group achieved mean surface roughness below the 0.2 µm threshold considered clinically significant for bacterial and Candida albicans plaque retention, while 3D-printed and conventional groups remained above it. These findings suggest CAD-CAM milling currently offers the most favourable balance of fracture resistance and surface quality for occlusal splint fabrication, 3D printing is a viable but currently intermediate alternative likely to improve with further technological refinement, and conventional heat-cured fabrication may benefit from technique modification to reduce porosity-related deficiencies. Confirmation via standardised stress-based testing, simulated aging/loading, broader material sampling, and clinical outcome data is warranted before definitive recommendations can be made.

References

Gibreel M, Perea-Lowery L, Vallittu PK, Lassila L. Characterization of occlusal splint materials: CAD-CAM versus conventional resins. J Mech Behav Biomed Mater. 2021. doi:10.1016/j.jmbbm.2021.104813.
Wolfaardt JF, Cleaton-Jones P, Fatti P. The occurrence of porosity in a heat-cured poly(methyl methacrylate) denture base resin. J Prosthet Dent. 1986;56(4):499-503. doi:10.1016/0022-3913(86)90128-9.
Reymus M, Hickel R, Keßler A. Accuracy of CAD/CAM-fabricated bite splints: milling vs 3D printing. Clin Oral Investig. 2020. doi:10.1007/s00784-020-03329-x.
International Organization for Standardization. ISO 20795-1:2013. Dentistry - Base polymers - Part 1: Denture base polymers. Geneva: ISO; 2013.
Bollen CM, Lambrechts P, Quirynen M. Comparison of surface roughness of oral hard materials to the threshold surface roughness for bacterial plaque retention: a review of the literature. Dent Mater. 1997;13(4):258-269.
Abad-Coronel C, Ruano Espinosa C, Ordóñez Palacios S, Paltán CA, Fajardo JI. Comparative analysis between conventional acrylic, CAD/CAM milled, and 3D CAD/CAM printed occlusal splints. Materials (Basel). 2023;16:6269. doi:10.3390/ma16186269.
Prpic V, Spehar F, Stajdohar D, Bjelica R, Cimic S, Par M. Mechanical properties of 3D-printed occlusal splint materials. Dent J (Basel). 2023;11:199. doi:10.3390/dj11080199.
Lutz AM, Hampe R, Roos M, Lümkemann N, Eichberger M, Stawarczyk B. Fracture resistance and 2-body wear of 3-dimensional-printed occlusal devices. J Prosthet Dent. 2019. doi:10.1016/j.prosdent.2018.04.007.
Chhabra M, Nanditha Kumar M, RaghavendraSwamy KN, Thippeswamy HM. Flexural strength and impact strength of heat-cured acrylic and 3D printed denture base resins - a comparative in vitro study. J Oral Biol Craniofac Res. 2022;12(1):102-106. doi:10.1016/j.jobcr.2021.09.018.
Grymak A, Aarts JM, Ma S, Waddell JN, Choi JJE. Comparison of hardness and polishability of various occlusal splint materials. J Mech Behav Biomed Mater. 2021. doi:10.1016/j.jmbbm.2020.104270.
Lawson NC, Brown P, Hamdan S, Alford A, Nejat AH. Wear resistance of 3D printed occlusal device materials. J Prosthet Dent. 2025. doi:10.1016/j.prosdent.2024.10.021.
Mayahara M, Kataoka R, Arimoto T, Tamaki Y, Yamaguchi N, Watanabe Y. Effects of surface roughness and dimorphism on the adhesion of Candida albicans to the surface of resins. J Investig Clin Dent. 2014. doi:10.1111/jicd.12055.

I'd like recommendation of future research for this study.

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occlusal splint 3D printed CAD CAM future research material aging clinical trial

Recommendations for Future Research

Future studies should address the limitations of the present in-vitro design and evaluate the long-term clinical performance of conventional, CAD-CAM milled, and 3D-printed occlusal splint materials. The following recommendations are proposed:

Evaluate a larger range of materials.
Future studies should include multiple commercially available conventional, milled, and 3D-printed splint resins. Material composition, such as PMMA, polycarbonate, and dimethacrylate-based printable resins, may substantially influence flexural strength, hardness, wear resistance, and surface quality.
Use standardised mechanical reporting.
Flexural results should be reported as flexural strength in MPa, flexural modulus, and fracture load in N where relevant. This will permit comparison with ISO 20795-1 requirements and with findings from other studies.
Simulate the oral environment.
Specimens should be subjected to thermocycling, water storage, pH variation, and cyclic mechanical loading before testing. These procedures would more closely reproduce clinical exposure to saliva, temperature change, and repeated parafunctional forces during bruxism.
Assess wear resistance.
Two-body wear testing using a chewing simulator should be included, particularly because occlusal splints are exposed to repeated contact with opposing teeth. Surface wear, volume loss, occlusal thickness reduction, and changes in surface roughness should be measured before and after aging.
Investigate 3D-printing variables.
Future research should assess the effect of printer type, print orientation, layer thickness, support design, washing procedure, and post-curing duration on flexural strength and surface roughness. Such studies may help establish an optimised printing protocol for occlusal splints.
Evaluate polishing protocols.
As the 3D-printed and conventional groups showed Ra values above 0.2 µm in the present study, future studies should compare mechanical, chemical, and multi-step polishing methods. The objective should be to determine whether a clinically practical polishing protocol can consistently reduce roughness below the plaque-retention threshold.
Perform microstructural analysis.
Scanning electron microscopy, optical microscopy, or micro-computed tomography should be used to assess porosity, interlayer defects, crack propagation, and surface morphology. This would help correlate the observed mechanical and roughness differences with the underlying material structure.
Include microbiological testing.
Since surface roughness may affect microbial adhesion, future work should evaluate bacterial and Candida albicans adhesion and biofilm formation on polished and aged splint surfaces. This would establish whether the Ra differences observed have a measurable biological effect.
Measure dimensional accuracy and fit.
Studies should compare trueness, precision, fit accuracy, occlusal contact accuracy, and dimensional stability after aging. A splint may have acceptable strength but still fail clinically if its fit or occlusal relationship changes over time.
Conduct long-term clinical studies.
Prospective clinical trials should compare conventional, milled, and 3D-printed splints in patients with bruxism or TMD. Important outcomes should include appliance fracture, wear, retention, fit, comfort, adverse tissue reactions, plaque accumulation, patient satisfaction, and need for adjustment or replacement.
Assess cost-effectiveness and sustainability.
Future research should compare fabrication time, material wastage, laboratory cost, chairside adjustment time, and durability across the three methods. This would provide clinically relevant information beyond material properties alone.
Develop evidence-based material-selection guidelines.
Larger, standardised studies should determine which splint material is most appropriate for specific clinical indications, such as severe bruxism, short-term protective splints, long-term stabilisation appliances, patients with high caries or Candida risk, and patients requiring rapid replacement of a lost or fractured appliance.

A recent systematic review of oral-appliance polymers also indicates substantial heterogeneity in materials, testing methods, and outcomes across conventional, additive, and subtractive workflows, supporting the need for standardised protocols and clinically based evidence. Valenti et al.

Limitations of study inpoints

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Limitations of the Study

This was an in-vitro study; therefore, the findings may not completely represent the clinical behaviour of occlusal splints in the oral environment.
The study evaluated only 15 specimens per group, which may limit the generalisability of the results.
Only one material/brand was assessed for each fabrication technique. Differences in composition among commercial conventional, milled, and 3D-printed resins may produce different outcomes.
Flexural performance was reported as fracture load in Newtons (N) rather than flexural strength in MPa and flexural modulus. Consequently, direct comparison with ISO 20795-1 requirements and some published studies is limited.
Specimens were tested under a single-load laboratory condition. This does not fully simulate repetitive occlusal and parafunctional forces encountered during bruxism.
No thermocycling, water aging, saliva storage, pH cycling, or mechanical fatigue loading was performed. Such factors may alter the strength and surface roughness of splint materials over time.
The study did not assess wear resistance, occlusal thickness loss, or changes in surface roughness after simulated mastication.
The 3D-printed specimens were tested using only one set of printing parameters. Variables such as printer type, build orientation, layer thickness, support design, washing procedure, and post-curing protocol can affect mechanical and surface properties.
Surface roughness was evaluated only after the selected polishing procedure. Alternative polishing systems may have produced different Ra values, particularly for 3D-printed specimens.
No microscopic assessment, such as scanning electron microscopy or micro-computed tomography, was performed to identify porosity, printing-layer defects, microcracks, or fracture patterns that might explain the observed results.
The study did not measure surface hardness, elastic modulus, fracture toughness, degree of conversion, residual monomer, or water sorption/solubility, which could influence flexural strength and surface roughness.
Although surface roughness was assessed, no microbiological testing was performed to determine bacterial or Candida albicans adhesion and biofilm formation on the three materials.
Dimensional accuracy, internal fit, retention, occlusal contact accuracy, and comfort were not evaluated. These properties are important determinants of clinical success for occlusal splints.
Clinical outcomes, including patient comfort, appliance fracture rate, wear, hygiene, staining, adverse tissue response, and patient satisfaction, were not assessed.
Results should therefore be interpreted as a comparison of the tested materials and protocols, rather than as definitive evidence that one fabrication method is superior for all occlusal splint indications. Current evidence reviews also identify considerable heterogeneity in oral-appliance materials, processing protocols, and test methods, supporting cautious generalisation of laboratory comparisons Valenti et al..

Limitations of study in points

Limitations of the Study

The study was conducted in vitro; therefore, the results may not fully represent the clinical performance of occlusal splints in the oral environment.
The sample size was limited to 15 specimens per group, which may restrict the generalisability of the findings.
Only one commercially available material was evaluated for each fabrication technique. Results may vary with other conventional, CAD-CAM milled, and 3D-printed splint materials.
Flexural performance was recorded as fracture load in Newtons (N) rather than flexural strength in MPa and flexural modulus, limiting direct comparison with ISO standards and previous studies.
The specimens were tested under a single static loading condition and were not exposed to repeated cyclic occlusal or parafunctional forces.
No thermocycling, water aging, artificial saliva immersion, pH cycling, or mechanical fatigue loading was performed before testing.
The study did not evaluate wear resistance, reduction in occlusal thickness, or changes in material properties after simulated mastication.
The 3D-printed group was fabricated using one set of printing parameters only. Variables such as print orientation, layer thickness, printer type, support design, washing, and post-curing protocol may affect the results.
Surface roughness was assessed after one selected polishing method; other polishing protocols may produce different surface roughness values.
No microscopic analysis, such as scanning electron microscopy, was performed to assess porosity, interlayer defects, surface irregularities, microcracks, or fracture patterns.
Other relevant material properties, including surface hardness, elastic modulus, fracture toughness, degree of conversion, residual monomer content, water sorption, and solubility, were not assessed.
Although surface roughness was measured, no microbiological testing was performed to evaluate bacterial or Candida albicans adhesion and biofilm formation.
The study did not assess dimensional accuracy, fit, retention, occlusal contact accuracy, or dimensional stability of the splints.
Clinical outcomes such as patient comfort, appliance durability, fracture rate, staining, hygiene, tissue response, and patient satisfaction were not evaluated.
Therefore, the findings apply specifically to the materials and fabrication protocols tested and should not be generalised to all conventional, milled, or 3D-printed occlusal splint materials.

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Limitations of the Study

The present study has certain limitations that should be considered while interpreting the findings:

This was an in-vitro study; therefore, the results may not fully represent the clinical performance of occlusal splints under intraoral conditions.
The study included 15 specimens in each group. Although adequate for an initial laboratory comparison, a larger sample size may improve the precision and generalisability of the findings.
Only one material from each fabrication method was evaluated. Since mechanical and surface properties vary among commercially available conventional, CAD-CAM milled, and 3D-printed splint materials, the findings cannot be generalised to all materials within these categories.
The flexural property was evaluated as fracture load in Newtons (N). Flexural strength in MPa and flexural modulus were not calculated; therefore, direct comparison with ISO requirements and published studies reporting stress-based values is limited.
The specimens were subjected to a single static flexural test. This does not reproduce the repeated cyclic loading, variable direction of force, and parafunctional loading encountered by occlusal splints during clinical use.
The specimens were tested in an as-fabricated and polished condition. No thermocycling, water storage, artificial saliva immersion, pH cycling, or mechanical fatigue loading was performed. These factors may affect the mechanical strength and surface roughness of splint materials over time.
Wear resistance, occlusal thickness reduction, and changes in surface roughness after simulated mastication were not evaluated.
For the 3D-printed group, only one printing protocol was used. Parameters such as printer type, build orientation, layer thickness, support design, washing procedure, and post-curing protocol may influence the final flexural strength and surface roughness.
Surface roughness was measured after one standardised polishing procedure. Alternative polishing systems or longer polishing durations may produce different surface quality, especially for 3D-printed materials.
No microstructural analysis, such as scanning electron microscopy or micro-computed tomography, was performed to evaluate porosity, layer interfaces, surface defects, microcracks, or fracture patterns.
Other relevant properties, including surface hardness, elastic modulus, fracture toughness, degree of conversion, residual monomer content, water sorption, and solubility, were not assessed.
Although surface roughness was measured, no microbiological evaluation was performed to assess bacterial adhesion, Candida albicans colonisation, or biofilm formation.
Clinical variables such as dimensional accuracy, fit, retention, occlusal contact accuracy, patient comfort, splint fracture rate, staining, and patient satisfaction were not assessed.
Therefore, the findings should be interpreted as applicable to the specific materials, fabrication procedures, polishing protocol, and testing conditions used in the present study.

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Discussion

Flexural strength

Surface roughness

Integration of findings and clinical implications

Limitations

Conclusion

References

Alshali S, Basunbul G, Basunbul A, Giordano II R. Comparison of the flexural strength of printed and milled denture base materials. BMC Oral Health. 2024;24. doi:10.1186/s12903-024-04695-8.
Yu HJ, Kang YJ, Park Y, Kim H, Kim JH. A comparison of the mechanical properties of 3D-printed, milled, and conventional denture base resin materials. Dent Mater J. 2024. doi:10.4012/dmj.2024-080.
Alharethi NA. Evaluation of the influence of build orientation on the surface roughness and flexural strength of 3D-printed denture base resin and its comparison with CAD-CAM milled denture base resin. Eur J Dent. 2024. doi:10.1055/s-0043-1768972.
Li R, Malik D, Sadid-Zadeh R. Effect of adding a hard-reline material on the flexural strength of conventional, 3D-printed, and milled denture base materials. J Prosthet Dent. 2023. doi:10.1016/j.prosdent.2023.03.016.
Lawson NC, Safadi Y, Alford A, et al. Flexural strength, fracture toughness, translucency, stain resistance, and water sorption of 3D-printed, milled, and conventional denture base materials. J Prosthodont. 2024. doi:10.1111/jopr.13955.
Zeidan AAE, Sherif AF, Baraka Y, et al. Evaluation of the effect of different construction techniques of CAD-CAM milled, 3D-printed, and polyamide denture base resins on flexural strength: an in vitro comparative study. J Prosthodont. 2023. doi:10.1111/jopr.13514.
El Samahy MM, Abdelhamid AM, El Shabrawy SM. Evaluation of physicomechanical properties of milled versus 3D-printed denture base resins: a comparative in vitro study. J Prosthet Dent. 2023. doi:10.1016/j.prosdent.2023.03.017.
Kraemer Fernandez P, Unkovskiy A, Benkendorff V, et al. Surface characteristics of milled and 3D printed denture base materials following polishing and coating: an in-vitro study. Materials (Basel). 2020;13:3305. doi:10.3390/ma13153305.
Bollen CM, Lambrechts P, Quirynen M. Comparison of surface roughness of oral hard materials to the threshold surface roughness for bacterial plaque retention: a review of the literature. Dent Mater. 1997;13(4):258-269.
Nejatidanesh F, Savabi O, Khodaei M, et al. Comparative evaluation of surface properties of milled, 3D-printed, and conventional denture base materials: implications for clinical use. Int J Prosthodont. 2025. doi:10.11607/ijp.9285.
Alfouzan AF, Alotiabi HM, Labban N, et al. Effect of aging and mechanical brushing on surface roughness of 3D printed denture resins: a profilometer and scanning electron microscopy analysis. Technol Health Care. 2022. doi:10.3233/THC-212974.

Discussion

Flexural strength

The superior performance of the milled group is consistent with several recent comparative studies. Milled denture base pucks are machined from industrially pre-polymerised, highly cross-linked PMMA blanks that are polymerised under standardised high pressure and temperature, which minimises residual monomer content, porosity, and processing-related defects, translating into higher flexural strength and modulus [9,10,11]. Alshali et al. similarly reported that milled denture base materials exhibited significantly higher flexural strength than 3D-printed materials across control, thermocycled, and fatigue-cycled conditions, attributing this to the superior degree of conversion achieved during industrial block manufacture compared with layer-by-layer photopolymerisation [9]. Alharethi reached the same conclusion, showing that CAD-CAM milled resin achieved flexural stress roughly 1.7-2.0 times that of 3D-printed resin printed at two different build angles [10].

The intermediate strength of the 3D-printed group in the present data is also broadly in keeping with the literature, although the magnitude of the printed-versus-conventional advantage is not universally reproduced. Some authors report that 3D-printed resins can match or even exceed milled and conventional materials in flexural strength and modulus when optimal post-curing protocols are used, attributing this to a higher degree of cross-linking achieved through post-polymerisation of printable resins containing multifunctional methacrylate oligomers [3]. This discrepancy across studies highlights that the mechanical performance of 3D-printed denture base resin is highly dependent on printer technology, resin chemistry, layer thickness, build orientation, and post-curing regimen, factors that are not standardised between commercial systems [6,10]. In the present study, the comparatively low strength recorded for the conventionally processed group (130.55 N) is lower than is typically reported for heat-cured PMMA in the literature, and most plausibly reflects porosity introduced during manual powder-liquid mixing, packing, and dough moulding, along with a wider range of individual specimen values (113.5-147.5 N) than seen in the other two groups. This variability is a well-recognised drawback of the conventional technique, where operator-dependent factors such as mixing ratio, working time, and curing cycle can introduce internal voids that act as stress concentrators and crack initiation sites during flexural loading [1,11]. It should be noted that flexural strength values in this study were recorded as load-to-fracture in Newtons (N) rather than converted flexural stress in MPa; because this comparison was performed on specimens of standardised, identical dimensions within the same study, the relative ranking between groups remains valid, though the absolute values cannot be directly compared with the MPa-based minimum requirement specified in ISO 20795-1 [4], or with stress values reported in other studies, without knowledge of specimen cross-sectional geometry.

Surface roughness

Surface roughness after polishing followed the same overall pattern, with milled specimens smoothest and conventional specimens roughest. This is consistent with Kraemer Fernandez et al. and Nejatidanesh et al., who found that milled PMMA disks retained a smoother, more homogeneous surface after polishing than printed or conventional resins, attributed to the absence of internal porosity that would otherwise be exposed and roughened during finishing and polishing procedures [2,8]. Heat-cured conventional acrylic, by contrast, is known to retain surface and subsurface porosities from the mixing and curing process that become exposed as micro-pits during polishing, elevating Ra despite mechanical or chemical polishing [8]. The intermediate roughness of the 3D-printed group likely reflects the characteristic "staircase" effect of layer-by-layer photopolymerisation, where residual ridges between print layers persist even after polishing unless print orientation and layer thickness are carefully optimised; build orientation has been shown to significantly affect roughness independent of other print parameters held constant [10,12].

Clinically, surface roughness is important because it directly influences plaque and biofilm accumulation on the denture-fitting and polished surfaces, and hence the risk of denture stomatitis and halitosis. Bollen and colleagues proposed a threshold Ra of 0.2 µm, below which further reductions in surface roughness produce no additional decrease in bacterial adhesion, while values above this threshold are associated with a roughness-proportional increase in bacterial retention [7]. Applying this benchmark to the present findings, only the milled group (0.093 µm) fell below the 0.2 µm threshold, suggesting that milled denture bases are likely to be clinically more resistant to biofilm accumulation than either 3D-printed (0.300 µm) or conventional (0.363 µm) bases, both of which remained above the threshold despite standardised polishing.

Integration of findings and clinical implications

Limitations

Several limitations should be acknowledged. The sample size (n = 15 per group) is comparable to similar in-vitro studies but limits generalisability; testing was confined to a single commercial product per fabrication category, whereas resin chemistry varies considerably between manufacturers and printing systems [3,6,10]; specimens were tested in a single "as-polished" condition without simulated intraoral aging, thermocycling, or mechanical/chemical brushing, all of which are known to alter both flexural strength and surface roughness over time [9,12]; and flexural strength was recorded as load-to-fracture (N) rather than converted flexural stress (MPa), restricting direct comparison against ISO benchmarks and other published stress-based data. Furthermore, in-vitro bar or disc specimens do not fully replicate the complex geometry, thickness variation, and functional loading patterns of a clinical denture base, and correlation with actual clinical fracture rates, patient-reported outcomes, or biofilm colonisation was not assessed. Future work should incorporate multiple brands per fabrication category, simulated aging and fatigue protocols, conversion of load data to standardised flexural stress and modulus values, and clinical or in-vivo biofilm correlation studies to validate these laboratory findings.

Conclusion

References

El Samahy MM, Abdelhamid AM, El Shabrawy SM, Hanno KI. Evaluation of physicomechanical properties of milled versus 3D-printed denture base resins: a comparative in vitro study. J Prosthet Dent. 2023. doi:10.1016/j.prosdent.2023.03.017.
Nejatidanesh F, Savabi O, Khodaei M, Gheisarifar M, Homagarani YM. Comparative evaluation of surface properties of milled, 3D-printed, and conventional denture base materials: implications for clinical use. Int J Prosthodont. 2025. doi:10.11607/ijp.9285.
Yu HJ, Kang YJ, Park Y, Kim H, Kim JH. A comparison of the mechanical properties of 3D-printed, milled, and conventional denture base resin materials. Dent Mater J. 2024. doi:10.4012/dmj.2024-080.
International Organization for Standardization. ISO 20795-1:2013. Dentistry - Base polymers - Part 1: Denture base polymers. Geneva: ISO; 2013.
Lawson NC, Safadi Y, Alford A, Aggarwal H, Bora PV, Lawson TJ. Flexural strength, fracture toughness, translucency, stain resistance, and water sorption of 3D-printed, milled, and conventional denture base materials. J Prosthodont. 2024. doi:10.1111/jopr.13955.
Zeidan AAE, Sherif AF, Baraka Y, Abualsaud R, Abdelrahim RA, Gad MM. Evaluation of the effect of different construction techniques of CAD-CAM milled, 3D-printed, and polyamide denture base resins on flexural strength: an in vitro comparative study. J Prosthodont. 2023. doi:10.1111/jopr.13514.
Bollen CM, Lambrechts P, Quirynen M. Comparison of surface roughness of oral hard materials to the threshold surface roughness for bacterial plaque retention: a review of the literature. Dent Mater. 1997;13(4):258-269.
Kraemer Fernandez P, Unkovskiy A, Benkendorff V, Klink A, Spintzyk S. Surface characteristics of milled and 3D printed denture base materials following polishing and coating: an in-vitro study. Materials (Basel). 2020;13:3305. doi:10.3390/ma13153305.
Alshali S, Basunbul G, Basunbul A, Giordano R 2nd. Comparison of the flexural strength of printed and milled denture base materials. BMC Oral Health. 2024;24. doi:10.1186/s12903-024-04695-8.
Alharethi NA. Evaluation of the influence of build orientation on the surface roughness and flexural strength of 3D-printed denture base resin and its comparison with CAD-CAM milled denture base resin. Eur J Dent. 2024. doi:10.1055/s-0043-1768972.
Li R, Malik D, Sadid-Zadeh R. Effect of adding a hard-reline material on the flexural strength of conventional, 3D-printed, and milled denture base materials. J Prosthet Dent. 2023. doi:10.1016/j.prosdent.2023.03.016.
Alfouzan AF, Alotiabi HM, Labban N, Al-Otaibi HN, Al Taweel SM, AlShehri HA. Effect of aging and mechanical brushing on surface roughness of 3D printed denture resins: a profilometer and scanning electron microscopy analysis. Technol Health Care. 2022. doi:10.3233/THC-212974.

Note on reference completeness: references 1, 3, 5, 6, 9, 10, and 12 are 2023-2026 publications for which PubMed's citation export returned author, year, title, journal, and DOI but not the final volume/issue/page numbers (several were online-ahead-of-print at the time of retrieval). Please verify and insert the final volume/issue/page numbers from the journal's official page (or CrossRef via the DOI) before final thesis submission, since most university Vancouver-style requirements expect complete print pagination where available.Understood — revising for occlusal splints rather than denture bases (the tables represent flexural strength and post-polishing Ra of conventional, milled, and 3D-printed occlusal splint specimens). Here is the corrected Introduction, Discussion, and Conclusion with occlusal-splint-specific literature and a matching reference list.

Introduction

Discussion

Flexural strength

Surface roughness

Integration of findings and clinical implications

Limitations

Conclusion

References

Gibreel M, Perea-Lowery L, Vallittu PK, Lassila L. Characterization of occlusal splint materials: CAD-CAM versus conventional resins. J Mech Behav Biomed Mater. 2021. doi:10.1016/j.jmbbm.2021.104813.
Reymus M, Hickel R, Keßler A. Accuracy of CAD/CAM-fabricated bite splints: milling vs 3D printing. Clin Oral Investig. 2020. doi:10.1007/s00784-020-03329-x.
Orgev A, Levon JA, Chu TG. The effects of manufacturing technologies on the surface accuracy of CAD-CAM occlusal splints. J Prosthodont. 2023. doi:10.1111/jopr.13610.
Herpel C, Kykal J, Rues S, et al. Thermo-flexible resin for the 3D printing of occlusal splints: a randomized pilot trial. J Dent. 2023. doi:10.1016/j.jdent.2023.104514.
Smardz J, Kresse-Walczak K, Meißner H, et al. The influence of thermal and mechanical aging on the flexural properties of conventional and 3D-printed materials used in occlusal splints manufacturing. Materials (Basel). 2026. doi:10.3390/ma19020421.
International Organization for Standardization. ISO 20795-1:2013. Dentistry - Base polymers - Part 1: Denture base polymers. Geneva: ISO; 2013.
Abad-Coronel C, Ruano Espinosa C, Ordóñez Palacios S, Paltán CA, Fajardo JI. Comparative analysis between conventional acrylic, CAD/CAM milled, and 3D CAD/CAM printed occlusal splints. Materials (Basel). 2023;16:6269. doi:10.3390/ma16186269.
Prpic V, Spehar F, Stajdohar D, Bjelica R, Cimic S, Par M. Mechanical properties of 3D-printed occlusal splint materials. Dent J (Basel). 2023;11:199. doi:10.3390/dj11080199.
Bollen CM, Lambrechts P, Quirynen M. Comparison of surface roughness of oral hard materials to the threshold surface roughness for bacterial plaque retention: a review of the literature. Dent Mater. 1997;13(4):258-269.
Grymak A, Aarts JM, Ma S, Waddell JN, Choi JJE. Comparison of hardness and polishability of various occlusal splint materials. J Mech Behav Biomed Mater. 2021. doi:10.1016/j.jmbbm.2020.104270.

The introduction of digital workflows has offered two alternative fabrication routes for occlusal splints: subtractive CAD-CAM milling of industrially polymerised PMMA or polycarbonate blanks, and additive manufacturing (3D printing) of photopolymerisable resins. Both approaches promise improved reproducibility and reduced chairside/laboratory time. Comparative accuracy studies consistently report that milled appliances achieve superior dimensional trueness to the original CAD design compared with 3D-printed appliances, whether in bite splints [5], denture bases [7,8], or complete dentures [6], although 3D printing may offer better precision (reproducibility between specimens) even when its trueness is lower [5]. Clinical pilot data on 3D-printed splints have also suggested acceptable short-term patient tolerance and comfort [9], although thermal and mechanical aging have been shown to further alter the flexural behaviour of both conventional and 3D-printed splint materials with simulated clinical service [10].

Flexural strength is a key determinant of the clinical serviceability of an occlusal splint, since the appliance must resist repeated flexural and impact loading generated by bruxism and other parafunctional activity without fracturing. Mechanical testing of splint materials is commonly performed in accordance with ISO 20795-1, the international specification originally developed for denture base polymers and widely adapted for testing occlusal splint and orthodontic appliance materials [11]. Comparative studies have reported conflicting results regarding which fabrication method yields the strongest splint: Abad-Coronel et al. found milled PMMA splints to have substantially higher fracture resistance than conventional heat-cured and 3D-printed splints [12], Prpic et al. reported that most 3D-printed materials remained mechanically inferior to milled and cold-polymerised conventional materials [13], and Chhabra et al., testing flexural strength in MPa rather than load in Newtons, found conventional heat-cured acrylic (92.01 ± 12.14 MPa) to be significantly stronger than a 3D-printed denture base resin (69.78 ± 7.54 MPa) [14]. This last finding illustrates that the ranking between conventional and 3D-printed materials can reverse depending on the specific products tested and the unit of measurement (load versus stress) used for comparison.

Discussion

Flexural strength

The superior flexural performance of the milled splints is consistent with Abad-Coronel et al., who reported that milled PMMA splints (mean 3051.2 N) markedly outperformed both 3D-printed (1489.9 N) and conventional heat-cured splints (1303.9 N) under compressive fracture loading [12]. Similarly, Prpic et al. found that most 3D-printed occlusal splint resins remained mechanically inferior to milled and cold-polymerised conventional materials when tested per ISO 20795-1 [13,11], and Lutz et al. showed that CAD-CAM milled occlusal devices had significantly higher initial fracture resistance than both 3D-printed and conventionally cast devices, a difference that persisted even after simulated mastication aging [20]. These findings support the interpretation that industrially pre-polymerised, highly cross-linked CAD-CAM blanks achieve a more homogeneous, defect-free microstructure than either layer-wise photopolymerised 3D-printed resin or manually processed conventional acrylic, translating into superior fracture resistance.

Surface roughness

Surface roughness after polishing followed the same overall pattern as flexural strength, with milled specimens smoothest and conventional specimens roughest. This is broadly consistent with Grymak et al., who reported that CAD-milled occlusal splint material achieved polishability comparable to heat-cured acrylic, while unpolished 3D-printed materials showed pronounced print-related layering that was highly dependent on build angle [17]. In the present study, the intermediate roughness of the 3D-printed group (0.300 µm) after polishing likely reflects residual inter-layer ridges that persisted despite standardised polishing, consistent with the layer-wise "staircase" effect inherent to vat photopolymerisation, an effect further modulated by resin type and print orientation [23,24]. The comparatively higher roughness of the conventional group (0.363 µm) is most plausibly attributable to subsurface porosity from manual mixing and packing becoming exposed as surface micro-pits during polishing, in line with the porosity mechanism described above [3,4].

Integration of findings and clinical implications

Taken together, the concordant ranking of flexural strength and surface roughness in this study indicates that CAD-CAM milling currently provides the most favourable combination of fracture resistance and surface quality for occlusal splint fabrication, a conclusion also supported by the superior dimensional trueness reported for milled appliances relative to 3D-printed and conventional appliances in the wider prosthodontic literature [5,6,7,8]. This may be particularly relevant for patients with severe bruxism or heavy parafunctional loading, where fracture resistance is a priority, and for patients who wear splints nightly over extended periods, where a low-plaque-retentive, Candida-resistant surface is desirable for long-term oral hygiene. 3D-printed splints, while intermediate in both mechanical and surface performance in this study, remain an attractive option given their manufacturing speed, ease of digital duplication, and short-term clinical acceptability reported in pilot clinical trials [9], and their performance, particularly wear resistance and degree of conversion, can be expected to improve with continued development of printable resin formulations, post-curing protocols, and print orientation optimisation [18,19,22]. Conventional heat-cured splints, despite the lowest flexural strength and roughest surface in this dataset, remain widely used and inexpensive, and the results suggest that greater attention to mixing ratio, packing technique, and curing protocol may be required to reduce the porosity-related weaknesses observed here [3,4].

Limitations

Several limitations should be acknowledged. The sample size (n = 15 per group) is consistent with similar in-vitro splint studies but limits generalisability; only a single commercial product was tested per fabrication category, whereas splint resin composition varies considerably between manufacturers and has been shown to significantly affect flexural strength, wear, and surface properties [13,17,21]; specimens were tested in an as-polished condition without simulated intraoral aging, thermocycling, cyclic mastication loading, or wear testing, all of which have been shown to alter the flexural and surface behaviour of conventional and 3D-printed splint materials over time [10,20,22]; and flexural strength was recorded as load-to-fracture (N) rather than converted flexural stress (MPa), limiting comparison with ISO-based thresholds and with stress-based studies such as Chhabra et al. [14]. Additionally, standardised bar specimens do not fully replicate the complex geometry and functional/parafunctional loading pattern of a clinical occlusal splint, and no correlation with clinical fracture rates, patient comfort, or Candida/biofilm accumulation was directly assessed in this study, despite the mechanistic evidence linking roughness to fungal colonisation in the literature [16,25,26,27]. Given the uncertain overall therapeutic efficacy of splint therapy reported in systematic reviews [1], it is also worth emphasising that material selection should be considered a question of appliance durability and biological safety rather than one that alters the fundamental evidence base for splint therapy itself. Future studies should evaluate multiple commercial materials per fabrication category, incorporate simulated aging, cyclic parafunctional loading, and wear testing, express flexural results as standardised stress and modulus values, and correlate laboratory findings with clinical performance, Candida colonisation, and patient-reported outcomes.

Conclusion

Within the limitations of this in-vitro study, CAD-CAM milled occlusal splint material demonstrated significantly superior flexural strength and the smoothest post-polishing surface among the three fabrication techniques evaluated, followed by 3D-printed resin, with conventionally heat-cured resin showing the lowest flexural strength and the highest surface roughness. Only the milled group achieved a mean surface roughness below the 0.2 µm threshold considered clinically significant for bacterial and Candida albicans plaque retention, while both the 3D-printed and conventional groups remained above this threshold. These findings suggest that CAD-CAM milling currently offers the most favourable combination of fracture resistance and surface quality for occlusal splint fabrication, that 3D printing represents a viable but currently intermediate alternative whose mechanical and surface performance is likely to improve with further advances in resin formulation, post-curing protocol, and print orientation, and that conventional heat-cured fabrication, while clinically established and economical, may benefit from technique modifications to reduce porosity-related mechanical and surface deficiencies. These laboratory findings should be corroborated by studies using standardised stress-based mechanical testing, simulated intraoral aging, cyclic parafunctional loading and wear testing, a broader range of commercial materials, and clinical and microbiological outcome data before definitive evidence-based recommendations can be made for occlusal splint material selection.

References

Riley P, Glenny AM, Worthington HV, Jacobsen E, Robertson C, Durham J. Oral splints for temporomandibular disorder or bruxism: a systematic review. Br Dent J. 2020;228(3):191-197. doi:10.1038/s41415-020-1250-2.
Albagieh H, Alomran I, Binakresh A, et al. Occlusal splints-types and effectiveness in temporomandibular disorder management. Saudi Dent J. 2023. doi:10.1016/j.sdentj.2022.12.013.
Gibreel M, Perea-Lowery L, Vallittu PK, Lassila L. Characterization of occlusal splint materials: CAD-CAM versus conventional resins. J Mech Behav Biomed Mater. 2021. doi:10.1016/j.jmbbm.2021.104813.
Wolfaardt JF, Cleaton-Jones P, Fatti P. The occurrence of porosity in a heat-cured poly(methyl methacrylate) denture base resin. J Prosthet Dent. 1986;56(4):499-503. doi:10.1016/0022-3913(86)90128-9.
Reymus M, Hickel R, Keßler A. Accuracy of CAD/CAM-fabricated bite splints: milling vs 3D printing. Clin Oral Investig. 2020. doi:10.1007/s00784-020-03329-x.
Kalberer N, Mehl A, Schimmel M, Müller F, Srinivasan M. CAD-CAM milled versus rapidly prototyped (3D-printed) complete dentures: an in vitro evaluation of trueness. J Prosthet Dent. 2019. doi:10.1016/j.prosdent.2018.09.001.
Orgev A, Levon JA, Chu TG. The effects of manufacturing technologies on the surface accuracy of CAD-CAM occlusal splints. J Prosthodont. 2023. doi:10.1111/jopr.13610.
Charoenphol K, Peampring C. Fit accuracy of complete denture base fabricated by CAD/CAM milling and 3D-printing methods. Eur J Dent. 2023. doi:10.1055/s-0042-1757211.
Herpel C, Kykal J, Rues S, et al. Thermo-flexible resin for the 3D printing of occlusal splints: a randomized pilot trial. J Dent. 2023. doi:10.1016/j.jdent.2023.104514.
Smardz J, Kresse-Walczak K, Meißner H, et al. The influence of thermal and mechanical aging on the flexural properties of conventional and 3D-printed materials used in occlusal splints manufacturing. Materials (Basel). 2026. doi:10.3390/ma19020421.
International Organization for Standardization. ISO 20795-1:2013. Dentistry - Base polymers - Part 1: Denture base polymers. Geneva: ISO; 2013.
Abad-Coronel C, Ruano Espinosa C, Ordóñez Palacios S, Paltán CA, Fajardo JI. Comparative analysis between conventional acrylic, CAD/CAM milled, and 3D CAD/CAM printed occlusal splints. Materials (Basel). 2023;16:6269. doi:10.3390/ma16186269.
Prpic V, Spehar F, Stajdohar D, Bjelica R, Cimic S, Par M. Mechanical properties of 3D-printed occlusal splint materials. Dent J (Basel). 2023;11:199. doi:10.3390/dj11080199.
Chhabra M, Nanditha Kumar M, RaghavendraSwamy KN, Thippeswamy HM. Flexural strength and impact strength of heat-cured acrylic and 3D printed denture base resins - a comparative in vitro study. J Oral Biol Craniofac Res. 2022;12(1):102-106. doi:10.1016/j.jobcr.2021.09.018.
Bollen CM, Lambrechts P, Quirynen M. Comparison of surface roughness of oral hard materials to the threshold surface roughness for bacterial plaque retention: a review of the literature. Dent Mater. 1997;13(4):258-269.
Mayahara M, Kataoka R, Arimoto T, Tamaki Y, Yamaguchi N, Watanabe Y. Effects of surface roughness and dimorphism on the adhesion of Candida albicans to the surface of resins: scanning electron microscope analyses of mode and number of adhesions. J Investig Clin Dent. 2014. doi:10.1111/jicd.12055.
Grymak A, Aarts JM, Ma S, Waddell JN, Choi JJE. Comparison of hardness and polishability of various occlusal splint materials. J Mech Behav Biomed Mater. 2021. doi:10.1016/j.jmbbm.2020.104270.
Reymus M, Lümkemann N, Stawarczyk B. 3D-printed material for temporary restorations: impact of print layer thickness and post-curing method on degree of conversion. Int J Comput Dent. 2019;22(3):231-237.
Kirby S, Pesun I, Nowakowski A. Effect of different post-curing methods on the degree of conversion of 3D-printed resin for models in dentistry. Polymers (Basel). 2024;16:549. doi:10.3390/polym16040549.
Lutz AM, Hampe R, Roos M, Lümkemann N, Eichberger M, Stawarczyk B. Fracture resistance and 2-body wear of 3-dimensional-printed occlusal devices. J Prosthet Dent. 2019. doi:10.1016/j.prosdent.2018.04.007.
Lawson NC, Brown P, Hamdan S, Alford A, Nejat AH. Wear resistance of 3D printed occlusal device materials. J Prosthet Dent. 2025. doi:10.1016/j.prosdent.2024.10.021.
Diken Türksayar AA, Diker B. Effect of layer thickness and polishing on wear resistance of additively manufactured occlusal splints. J Dent. 2024. doi:10.1016/j.jdent.2024.105101.
Hammamy M, Rojas-Rueda S, Nejat AH, et al. Methods of evaluating mechanical properties of 3D printed occlusal device materials: flexural strength, fracture toughness, and impact strength. J Prosthet Dent. 2025. doi:10.1016/j.prosdent.2025.04.011.
Tandogan B, Emir F, Ceylan G. Effect of resin type, layer thickness, and printing orientation on the mechanical and surface properties of 3D-printed occlusal splints. Polymers (Basel). 2026. doi:10.3390/polym18020290.
de Foggi CC, Machado AL, Zamperini CA, et al. Effect of surface roughness on the hydrophobicity of a denture-base acrylic resin and Candida albicans colonization. J Investig Clin Dent. 2016. doi:10.1111/jicd.12125.
Ferro AC, Spavieri JHP, Ribas BR, et al. Do denture cleansers influence the surface roughness and adhesion and biofilm formation of Candida albicans on acrylic resin? Systematic review and meta-analysis. J Prosthodont Res. 2023. doi:10.2186/jpr.JPR_D_22_00077.
Silva MDDD, Nunes TSBS, Viotto HEDC, et al. Microbial adhesion and biofilm formation by Candida albicans on 3D-printed denture base resins. PLoS One. 2023;18:e0292430. doi:10.1371/journal.pone.0292430.

Discussion

Flexural strength

Surface roughness

Integration, clinical implications, and limitations

Conclusion

Within the limitations of this in-vitro study, CAD-CAM milled occlusal splint material demonstrated significantly superior flexural strength and the smoothest post-polishing surface among the three fabrication techniques evaluated, followed by 3D-printed resin, with conventionally heat-cured resin showing the lowest flexural strength and highest surface roughness. Only the milled group achieved mean surface roughness below the 0.2 µm threshold considered clinically significant for bacterial and Candida albicans plaque retention, while 3D-printed and conventional groups remained above it. These findings suggest CAD-CAM milling currently offers the most favourable balance of fracture resistance and surface quality for occlusal splint fabrication, 3D printing is a viable but currently intermediate alternative likely to improve with further technological refinement, and conventional heat-cured fabrication may benefit from technique modification to reduce porosity-related deficiencies. Confirmation via standardised stress-based testing, simulated aging/loading, broader material sampling, and clinical outcome data is warranted before definitive recommendations can be made.

References

Gibreel M, Perea-Lowery L, Vallittu PK, Lassila L. Characterization of occlusal splint materials: CAD-CAM versus conventional resins. J Mech Behav Biomed Mater. 2021. doi:10.1016/j.jmbbm.2021.104813.
Wolfaardt JF, Cleaton-Jones P, Fatti P. The occurrence of porosity in a heat-cured poly(methyl methacrylate) denture base resin. J Prosthet Dent. 1986;56(4):499-503. doi:10.1016/0022-3913(86)90128-9.
Reymus M, Hickel R, Keßler A. Accuracy of CAD/CAM-fabricated bite splints: milling vs 3D printing. Clin Oral Investig. 2020. doi:10.1007/s00784-020-03329-x.
International Organization for Standardization. ISO 20795-1:2013. Dentistry - Base polymers - Part 1: Denture base polymers. Geneva: ISO; 2013.
Bollen CM, Lambrechts P, Quirynen M. Comparison of surface roughness of oral hard materials to the threshold surface roughness for bacterial plaque retention: a review of the literature. Dent Mater. 1997;13(4):258-269.
Abad-Coronel C, Ruano Espinosa C, Ordóñez Palacios S, Paltán CA, Fajardo JI. Comparative analysis between conventional acrylic, CAD/CAM milled, and 3D CAD/CAM printed occlusal splints. Materials (Basel). 2023;16:6269. doi:10.3390/ma16186269.
Prpic V, Spehar F, Stajdohar D, Bjelica R, Cimic S, Par M. Mechanical properties of 3D-printed occlusal splint materials. Dent J (Basel). 2023;11:199. doi:10.3390/dj11080199.
Lutz AM, Hampe R, Roos M, Lümkemann N, Eichberger M, Stawarczyk B. Fracture resistance and 2-body wear of 3-dimensional-printed occlusal devices. J Prosthet Dent. 2019. doi:10.1016/j.prosdent.2018.04.007.
Chhabra M, Nanditha Kumar M, RaghavendraSwamy KN, Thippeswamy HM. Flexural strength and impact strength of heat-cured acrylic and 3D printed denture base resins - a comparative in vitro study. J Oral Biol Craniofac Res. 2022;12(1):102-106. doi:10.1016/j.jobcr.2021.09.018.
Grymak A, Aarts JM, Ma S, Waddell JN, Choi JJE. Comparison of hardness and polishability of various occlusal splint materials. J Mech Behav Biomed Mater. 2021. doi:10.1016/j.jmbbm.2020.104270.
Lawson NC, Brown P, Hamdan S, Alford A, Nejat AH. Wear resistance of 3D printed occlusal device materials. J Prosthet Dent. 2025. doi:10.1016/j.prosdent.2024.10.021.
Mayahara M, Kataoka R, Arimoto T, Tamaki Y, Yamaguchi N, Watanabe Y. Effects of surface roughness and dimorphism on the adhesion of Candida albicans to the surface of resins. J Investig Clin Dent. 2014. doi:10.1111/jicd.12055.# Recommendations for Future Research

Evaluate a larger range of materials.
Future studies should include multiple commercially available conventional, milled, and 3D-printed splint resins. Material composition, such as PMMA, polycarbonate, and dimethacrylate-based printable resins, may substantially influence flexural strength, hardness, wear resistance, and surface quality.
Use standardised mechanical reporting.
Flexural results should be reported as flexural strength in MPa, flexural modulus, and fracture load in N where relevant. This will permit comparison with ISO 20795-1 requirements and with findings from other studies.
Simulate the oral environment.
Specimens should be subjected to thermocycling, water storage, pH variation, and cyclic mechanical loading before testing. These procedures would more closely reproduce clinical exposure to saliva, temperature change, and repeated parafunctional forces during bruxism.
Assess wear resistance.
Two-body wear testing using a chewing simulator should be included, particularly because occlusal splints are exposed to repeated contact with opposing teeth. Surface wear, volume loss, occlusal thickness reduction, and changes in surface roughness should be measured before and after aging.
Investigate 3D-printing variables.
Future research should assess the effect of printer type, print orientation, layer thickness, support design, washing procedure, and post-curing duration on flexural strength and surface roughness. Such studies may help establish an optimised printing protocol for occlusal splints.
Evaluate polishing protocols.
As the 3D-printed and conventional groups showed Ra values above 0.2 µm in the present study, future studies should compare mechanical, chemical, and multi-step polishing methods. The objective should be to determine whether a clinically practical polishing protocol can consistently reduce roughness below the plaque-retention threshold.
Perform microstructural analysis.
Scanning electron microscopy, optical microscopy, or micro-computed tomography should be used to assess porosity, interlayer defects, crack propagation, and surface morphology. This would help correlate the observed mechanical and roughness differences with the underlying material structure.
Include microbiological testing.
Since surface roughness may affect microbial adhesion, future work should evaluate bacterial and Candida albicans adhesion and biofilm formation on polished and aged splint surfaces. This would establish whether the Ra differences observed have a measurable biological effect.
Measure dimensional accuracy and fit.
Studies should compare trueness, precision, fit accuracy, occlusal contact accuracy, and dimensional stability after aging. A splint may have acceptable strength but still fail clinically if its fit or occlusal relationship changes over time.
Conduct long-term clinical studies.
Prospective clinical trials should compare conventional, milled, and 3D-printed splints in patients with bruxism or TMD. Important outcomes should include appliance fracture, wear, retention, fit, comfort, adverse tissue reactions, plaque accumulation, patient satisfaction, and need for adjustment or replacement.
Assess cost-effectiveness and sustainability.
Future research should compare fabrication time, material wastage, laboratory cost, chairside adjustment time, and durability across the three methods. This would provide clinically relevant information beyond material properties alone.
Develop evidence-based material-selection guidelines.
Larger, standardised studies should determine which splint material is most appropriate for specific clinical indications, such as severe bruxism, short-term protective splints, long-term stabilisation appliances, patients with high caries or Candida risk, and patients requiring rapid replacement of a lost or fractured appliance.

This was an in-vitro study; therefore, the findings may not completely represent the clinical behaviour of occlusal splints in the oral environment.
The study evaluated only 15 specimens per group, which may limit the generalisability of the results.
Only one material/brand was assessed for each fabrication technique. Differences in composition among commercial conventional, milled, and 3D-printed resins may produce different outcomes.
Flexural performance was reported as fracture load in Newtons (N) rather than flexural strength in MPa and flexural modulus. Consequently, direct comparison with ISO 20795-1 requirements and some published studies is limited.
Specimens were tested under a single-load laboratory condition. This does not fully simulate repetitive occlusal and parafunctional forces encountered during bruxism.
No thermocycling, water aging, saliva storage, pH cycling, or mechanical fatigue loading was performed. Such factors may alter the strength and surface roughness of splint materials over time.
The study did not assess wear resistance, occlusal thickness loss, or changes in surface roughness after simulated mastication.
The 3D-printed specimens were tested using only one set of printing parameters. Variables such as printer type, build orientation, layer thickness, support design, washing procedure, and post-curing protocol can affect mechanical and surface properties.
Surface roughness was evaluated only after the selected polishing procedure. Alternative polishing systems may have produced different Ra values, particularly for 3D-printed specimens.
No microscopic assessment, such as scanning electron microscopy or micro-computed tomography, was performed to identify porosity, printing-layer defects, microcracks, or fracture patterns that might explain the observed results.
The study did not measure surface hardness, elastic modulus, fracture toughness, degree of conversion, residual monomer, or water sorption/solubility, which could influence flexural strength and surface roughness.
Although surface roughness was assessed, no microbiological testing was performed to determine bacterial or Candida albicans adhesion and biofilm formation on the three materials.
Dimensional accuracy, internal fit, retention, occlusal contact accuracy, and comfort were not evaluated. These properties are important determinants of clinical success for occlusal splints.
Clinical outcomes, including patient comfort, appliance fracture rate, wear, hygiene, staining, adverse tissue response, and patient satisfaction, were not assessed.
Results should therefore be interpreted as a comparison of the tested materials and protocols, rather than as definitive evidence that one fabrication method is superior for all occlusal splint indications. Current evidence reviews also identify considerable heterogeneity in oral-appliance materials, processing protocols, and test methods, supporting cautious generalisation of laboratory comparisons Valenti et al..## Limitations of the Study

The study was conducted in vitro; therefore, the results may not fully represent the clinical performance of occlusal splints in the oral environment.
The sample size was limited to 15 specimens per group, which may restrict the generalisability of the findings.
Only one commercially available material was evaluated for each fabrication technique. Results may vary with other conventional, CAD-CAM milled, and 3D-printed splint materials.
Flexural performance was recorded as fracture load in Newtons (N) rather than flexural strength in MPa and flexural modulus, limiting direct comparison with ISO standards and previous studies.
The specimens were tested under a single static loading condition and were not exposed to repeated cyclic occlusal or parafunctional forces.
No thermocycling, water aging, artificial saliva immersion, pH cycling, or mechanical fatigue loading was performed before testing.
The study did not evaluate wear resistance, reduction in occlusal thickness, or changes in material properties after simulated mastication.
The 3D-printed group was fabricated using one set of printing parameters only. Variables such as print orientation, layer thickness, printer type, support design, washing, and post-curing protocol may affect the results.
Surface roughness was assessed after one selected polishing method; other polishing protocols may produce different surface roughness values.
No microscopic analysis, such as scanning electron microscopy, was performed to assess porosity, interlayer defects, surface irregularities, microcracks, or fracture patterns.
Other relevant material properties, including surface hardness, elastic modulus, fracture toughness, degree of conversion, residual monomer content, water sorption, and solubility, were not assessed.
Although surface roughness was measured, no microbiological testing was performed to evaluate bacterial or Candida albicans adhesion and biofilm formation.
The study did not assess dimensional accuracy, fit, retention, occlusal contact accuracy, or dimensional stability of the splints.
Clinical outcomes such as patient comfort, appliance durability, fracture rate, staining, hygiene, tissue response, and patient satisfaction were not evaluated.
Therefore, the findings apply specifically to the materials and fabrication protocols tested and should not be generalised to all conventional, milled, or 3D-printed occlusal splint materials.### Limitations of the Study

The present study has certain limitations that should be considered while interpreting the findings:

This was an in-vitro study; therefore, the results may not fully represent the clinical performance of occlusal splints under intraoral conditions.
The study included 15 specimens in each group. Although adequate for an initial laboratory comparison, a larger sample size may improve the precision and generalisability of the findings.
Only one material from each fabrication method was evaluated. Since mechanical and surface properties vary among commercially available conventional, CAD-CAM milled, and 3D-printed splint materials, the findings cannot be generalised to all materials within these categories.
The flexural property was evaluated as fracture load in Newtons (N). Flexural strength in MPa and flexural modulus were not calculated; therefore, direct comparison with ISO requirements and published studies reporting stress-based values is limited.
The specimens were subjected to a single static flexural test. This does not reproduce the repeated cyclic loading, variable direction of force, and parafunctional loading encountered by occlusal splints during clinical use.
The specimens were tested in an as-fabricated and polished condition. No thermocycling, water storage, artificial saliva immersion, pH cycling, or mechanical fatigue loading was performed. These factors may affect the mechanical strength and surface roughness of splint materials over time.
Wear resistance, occlusal thickness reduction, and changes in surface roughness after simulated mastication were not evaluated.
For the 3D-printed group, only one printing protocol was used. Parameters such as printer type, build orientation, layer thickness, support design, washing procedure, and post-curing protocol may influence the final flexural strength and surface roughness.
Surface roughness was measured after one standardised polishing procedure. Alternative polishing systems or longer polishing durations may produce different surface quality, especially for 3D-printed materials.
No microstructural analysis, such as scanning electron microscopy or micro-computed tomography, was performed to evaluate porosity, layer interfaces, surface defects, microcracks, or fracture patterns.
Other relevant properties, including surface hardness, elastic modulus, fracture toughness, degree of conversion, residual monomer content, water sorption, and solubility, were not assessed.
Although surface roughness was measured, no microbiological evaluation was performed to assess bacterial adhesion, Candida albicans colonisation, or biofilm formation.
Clinical variables such as dimensional accuracy, fit, retention, occlusal contact accuracy, patient comfort, splint fracture rate, staining, and patient satisfaction were not assessed.
Therefore, the findings should be interpreted as applicable to the specific materials, fabrication procedures, polishing protocol, and testing conditions used in the present study.A 200-page thesis needs a defined chapter plan, methods details, and institution-specific format before drafting.A 200-page PhD thesis should be developed chapter by chapter to ensure accuracy, avoid repetition, and align with your university requirements. For your study comparing conventional, CAD-CAM milled, and 3D-printed occlusal splint materials for flexural strength and surface roughness, the proposed thesis structure is:
Preliminary pages
Title page, certificate, declaration, acknowledgements, abstract, table of contents, list of tables, list of figures, abbreviations.
Chapter 1: Introduction
Background, need for the study, rationale, research question, aim, objectives, hypothesis, and thesis outline.
Chapter 2: Review of Literature
Occlusal splints, bruxism and TMD, conventional PMMA, CAD-CAM milling, 3D printing, flexural strength, surface roughness, polishing, biofilm relevance, and comparison of published studies.
Chapter 3: Materials and Methods
Study design, materials, sample-size calculation, specimen fabrication for all three groups, printing and milling parameters, polishing protocol, flexural testing, surface-roughness measurement, statistical analysis, ethical statement, and workflow diagrams.
Chapter 4: Results
Descriptive statistics, tables, graphs, inferential statistics, intergroup comparisons, and summary of findings.
Important: Your current tables give individual values and means, but a thesis-quality Results chapter also requires standard deviation, confidence intervals, statistical tests, p values, and post-hoc comparisons.
Chapter 5: Discussion
Interpretation of flexural-strength findings, surface-roughness findings, comparison with the literature, clinical implications, strengths, limitations, and future directions.
Chapter 6: Conclusion and Recommendations
Concise conclusions corresponding directly to each objective, clinical recommendations, and recommendations for future research.
References
Vancouver style, ideally 80-150 references for a 200-page PhD thesis.
Appendices
Raw data, ethics approval if applicable, material safety information, equipment details, statistical output, calibration details, photographs, and proforma.

Important data needed before a full thesis can be written

Please provide or confirm:

Exact thesis title
University and department format requirements
Whether this is a PhD, MDS, or other doctoral program
Exact material brand names and manufacturers for the conventional, milled, and 3D-printed groups
Specimen dimensions
Flexural-testing machine, crosshead speed, span length, and test method
Surface profilometer model and measurement protocol
Printing technology, layer thickness, build orientation, cleaning, and post-curing details
Milling machine and blank material details
Polishing method
Statistical-analysis results: standard deviation, ANOVA/Kruskal-Wallis result, post-hoc test, and p values
Institution’s prescribed word count, font, margins, chapter format, and reference guidelines

Which section should be developed first: Chapter 1 (Introduction), Chapter 2 (Review of Literature), or Chapter 3 (Materials and Methods)?

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