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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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PMID: 39127617

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PMID: 39414438

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PMID: 37295454

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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

  1. 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.
  2. 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.
  3. 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.
  4. 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.
  5. 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.
  6. 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.
  7. 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.
  8. 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.
  9. 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.
  10. 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.
  11. 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.
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