My phd topic on comparative evaluation of flexural strength and surface roughness of resin based occlusal splints fabricated by conventional technique, sla 3D-Printed, milled pmma invitro study , how to convince why i choose flexural strength and surface roughness in mechanical Properties

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I now have comprehensive evidence to construct a thorough and well-cited justification. Here is a complete, structured rationale for your PhD committee:

Why Flexural Strength and Surface Roughness Are the Most Clinically Relevant Mechanical Properties for Occlusal Splint Research

The Core Argument (The Short Answer)

These two properties are not chosen arbitrarily — they are the primary mechanical determinants of clinical success and failure of an occlusal splint throughout its service life. One governs structural survival under load (flexural strength); the other governs biologic safety and longevity (surface roughness). Together they represent the two axes of splint performance: mechanical integrity and tissue compatibility.

1. Flexural Strength — The Dominant Functional Requirement

Why flexural, not tensile or compressive?

An occlusal splint is a beam-like structure supported at the teeth and loaded at multiple points during mastication and parafunctional activity (bruxism). The physics of this loading scenario produces bending stress — a combination of tension on one surface and compression on the other — which is precisely what flexural testing (3-point or 4-point bending, ISO 20795-1) quantifies. No other single mechanical test better replicates the real stress state the splint experiences intraorally.

Clinical rationale

Clinical scenario	Why flexural strength matters
Bruxism / parafunctional forces	Patients with TMD and bruxism generate occlusal forces of 200–1000 N. Splints with insufficient flexural strength fracture, deform, or crack, requiring costly remakes.
Long-term nocturnal wear	Splints are worn 6–8 hours nightly. Cyclic bending during sleep subjects the appliance to fatigue stress. Higher flexural strength = longer fatigue life.
Cleaning and handling	Patients flex splints during insertion/removal and cleaning. Fracture resistance determines whether the device survives daily handling stresses.

"Flexural strength is an important property that reflects the ability of the material to withstand high occlusal forces. This property is clinically relevant in preventing the splint from being damaged and abraded during mastication and cleaning." — PMC13110832 (in vitro study, 2025)

Why the fabrication method changes everything

Conventional (heat-cured PMMA): Residual monomer and cross-link density affect final polymer chain length, directly controlling flexural strength.
Milled PMMA (CAD/CAM subtractive): Pre-polymerized under industrial high-pressure/heat conditions → superior chain uniformity → consistently higher flexural strength (~100–145 MPa in reported studies).
SLA/DLP 3D-printed resin: Layer-by-layer photopolymerization creates inter-layer interfaces (stress concentration points) and may leave incompletely cured monomers, making flexural strength highly sensitive to build orientation, layer thickness, and post-curing protocol. Studies report values of ~50–100 MPa — significantly lower than milled PMMA.

This fabrication-dependent variability in flexural strength is precisely the scientific gap your study addresses. You are evaluating whether these newer techniques meet the clinically acceptable threshold.

The standard benchmark

ISO 20795-1 (Dentistry — Base polymers) specifies a minimum flexural strength of 65 MPa for denture base resins. Occlusal splints, subjected to concentrated posterior occlusal loads, arguably require values well above this baseline. Your study quantifies whether each fabrication method achieves and sustains this threshold.

Supporting evidence:

Valenti et al., 2024 (Systematic Review + Meta-Analysis, PMID 38916682): Flexural strength and surface roughness were the most studied mechanical properties across all 13 included studies on oral appliance materials. Meta-analysis confirmed conventional heat-cured resin had higher flexural strength (Hedge's g = 2.32) than CAD-CAM milled splints, contradicting earlier assumptions.
Guimaraes et al., 2023 (PMID 37132723): Among five material groups (self-cure, heat-cure, microwave, 3D-print, milled), milled PMMA (group M) achieved the best flexural strength — clinicians must consider material selection for long-lasting splints.
de Gois Moreira et al., 2024 (PMID 39425803): Post-polymerization and aging protocols significantly affect 3D-printed resin flexural strength — underscoring the intra-technique variability that comparative studies must capture.

2. Surface Roughness — The Biologic and Longevity Parameter

Clinical rationale: the 0.2 µm threshold

Surface roughness (Ra) is not merely an aesthetic property — it has a quantified biologic threshold with direct patient safety implications:

Ra = 0.2 µm is the established critical threshold below which no further reduction in bacterial adhesion is clinically meaningful. Above this value, every incremental increase in Ra produces a proportional increase in biofilm accumulation (Bollen et al., 1997; PMID 11696906).

For a device worn against the oral mucosa and dentition for 8 hours nightly, this is critical:

Consequence of high surface roughness	Mechanism
Candida and bacterial biofilm accumulation	Rough surfaces provide microniches shielded from shear forces of saliva and toothbrushing; irreversible bacterial bonding occurs in grooves and pits.
Mucosal irritation / stomatitis	A surface rougher than the oral mucosal tolerance threshold causes chronic microtrauma to the palatal and gingival mucosa.
Material degradation	Biofilm produces acids and enzymes that degrade PMMA surface, further increasing roughness over time — a self-perpetuating cycle.
Discoloration and odor	Rough surfaces trap pigments and volatile sulfur compounds, reducing patient compliance and appliance longevity.

Why fabrication method determines surface roughness

Conventional PMMA: Polishing by hand introduces operator-dependent variability. The final Ra depends on technique, abrasive sequence, and time spent — resulting in the highest variability and often the roughest surfaces without careful polishing.
Milled PMMA (subtractive): Milling tool marks create characteristic surface texture, but post-milling polishing is predictable and reproducible. Studies show subtractive specimens have lower Ra than conventional (Valenti meta-analysis: Hedge's g = −1.25).
SLA 3D-printed resin: Layer lines, support structure artifacts, and incomplete post-curing create surface irregularities. However, the inherent layer stacking can produce very smooth surfaces in the Z-plane; the clinical surface roughness depends heavily on build orientation and post-processing.

Tandogan et al., 2026 (PMID 41599585) specifically found that resin type and printing orientation significantly influenced all surface properties (Ra, Rz, gloss), and that all tested 3D-printed materials achieved clinically acceptable surface smoothness, directly validating surface roughness as a discriminating variable between techniques.

Connecting surface roughness to material failure over time

Surface roughness interacts with flexural strength: a rougher surface acts as a stress concentrator, reducing effective fatigue life. Pits and grooves on the splint surface propagate cracks under cyclic loading. By measuring Ra alongside flexural strength, your study captures both the initial mechanical performance and the surface quality that modulates long-term durability.

3. Why These Two Properties — Not Hardness, Wear Resistance, or Impact Strength?

You should be prepared to defend the selection over competing properties:

Property	Why not the primary focus
Knoop/Vickers hardness	Correlates with wear resistance but does not directly predict fracture or biofilm risk. Secondary property.
Wear resistance	Important for long-term studies, but wear testing requires specialized equipment (pin-on-disk, ACTA wear machine) and extended protocols — appropriate for a separate longitudinal study or additional parameter, not a primary endpoint.
Impact strength	Relevant for accidental drops, but splints are not typically subject to sudden impact in the oral cavity. Less clinically dominant than cyclic bending.
Elastic modulus	Important for fit and comfort but requires the same three-point bend test — can be reported as a secondary outcome from your flexural test data at minimal extra cost.
Dimensional accuracy / fit	Critical for clinical use but a separate domain (prosthodontic accuracy studies); your study focuses on material properties, not appliance geometry.

The consensus in the literature — across the Valenti 2024 systematic review, Guimaraes 2023, and Tandogan 2026 — is that flexural strength and surface roughness are the two most studied and most clinically validated parameters for comparative evaluation of oral appliance fabrication methods. Your choice aligns exactly with the evidence base.

4. Framing for Your Committee: The Justification Statement

Use this argument structure:

"Occlusal splints function as load-bearing removable appliances in a biologically active oral environment. Their clinical success depends on two categories of mechanical performance: structural integrity under occlusal loading, and surface biocompatibility during prolonged mucosal contact. Flexural strength quantifies the former — it directly reflects resistance to the bending stresses generated by parafunctional forces, and its measurement under ISO 20795-1 provides a standardized, clinically referenced threshold. Surface roughness quantifies the latter — it determines biofilm accumulation risk via the established Ra = 0.2 µm threshold (Bollen et al.) and governs mucosal irritation, material degradation, and patient compliance. Together, these two properties represent the mechanically critical and biologically critical performance axes of any occlusal splint material. Furthermore, both properties are directly sensitive to the fabrication method — conventional polymerization, subtractive milling, and SLA photopolymerization produce fundamentally different polymer microstructures, surface textures, and residual stress states — making them ideal discriminating variables for a comparative fabrication study. This selection is consistent with the most recent systematic review and meta-analysis by Valenti et al. (2024), which identified flexural strength and surface roughness as the primary outcome measures across all included studies on 3D-printed and milled oral appliance materials."

Key References for Your Literature Review

PMID	Citation	Why cite it
38916682	Valenti et al., Clin Oral Investig, 2024 — Systematic Review + Meta-Analysis	Gold standard justification: these are the most studied properties across all fabrication methods
37132723	Guimaraes et al., Braz Oral Res, 2023	Direct comparison of 5 splint materials including 3D-print and milled; flexural strength + roughness + hardness
41599585	Tandogan et al., Polymers, 2026	SLA/DLP orientation effects on Ra and flexural strength specifically for splints
39425803	de Gois Moreira et al., Clin Oral Investig, 2024	3D printing post-curing and flexural strength for splints
37936836	Raffaini et al., J Adv Prosthodont, 2023	CAD-CAM PMMA aging effects on mechanical/physical properties
11696906	Bollen et al., J Clin Periodontol, 2001	The Ra = 0.2 µm threshold — foundational citation for surface roughness clinical significance

Bottom line for your viva: You chose flexural strength and surface roughness because they are the two properties that determine whether a splint survives in the mouth (structural) and does not harm the mouth (biologic) — and both are directly and differentially affected by the fabrication method, making them scientifically ideal discriminating variables for your comparative study.

Write the same in Vancouver style with reference

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Here is the complete justification written in academic prose with inline Vancouver-style numeric citations and a numbered reference list at the end.

Justification for the Selection of Flexural Strength and Surface Roughness as the Primary Mechanical Properties in a Comparative Evaluation of Resin-Based Occlusal Splints Fabricated by Conventional, SLA 3D-Printed, and Milled PMMA Techniques

1. Introduction

The occlusal splint is a removable intraoral appliance fabricated to manage temporomandibular disorders, bruxism, and parafunctional occlusal loading. Regardless of the fabrication method employed, the appliance must satisfy two fundamental performance criteria throughout its service life: it must maintain structural integrity under repeated occlusal forces, and its surface must remain biologically compatible with the oral mucosa during prolonged contact. The selection of mechanical test parameters in any comparative material study must therefore be grounded in these functional realities. Among all available mechanical tests, flexural strength and surface roughness have been consistently identified in the literature as the most clinically discriminating and technically standardised properties for evaluating resin-based oral appliance materials across different fabrication technologies (1). This section provides a systematic justification for their selection as the primary outcome measures in the present study.

2. Flexural Strength

2.1 Biomechanical Rationale

An occlusal splint functions as a beam-like structure, supported at the posterior teeth and subjected to distributed and point loads during mandibular function and bruxism. This loading configuration produces bending stresses — simultaneous tensile stress on the tissue-facing surface and compressive stress on the occlusal surface. Flexural strength, measured by standardised three-point bending as specified in ISO 20795-1 (Dentistry — Base polymers), is the mechanical property that directly quantifies a material's resistance to this dominant stress mode. No other single mechanical test more accurately replicates the in vivo stress state of an occlusal splint. Patients with bruxism and temporomandibular joint dysfunction generate parafunctional occlusal forces ranging from 200 to over 1000 N. A splint with insufficient flexural strength will fracture, permanently deform, or develop fatigue cracks under such cyclic loading, necessitating premature replacement and exposing the patient to biologic risk from ingestion of acrylic fragments or residual monomer release (2).

2.2 Fabrication-Dependent Variability

The fabrication method fundamentally determines the polymer microstructure and thereby the resulting flexural strength. In conventional heat-cured PMMA, the degree of conversion, residual monomer content, and cross-link density govern the final mechanical properties; technique-dependent variations in polymerisation temperature and pressure introduce significant inter-specimen variability (3). In milled PMMA (CAD/CAM subtractive), the resin blocks are pre-polymerised under industrial high-pressure, high-temperature conditions producing a homogeneous polymer network with minimal porosity, which consistently yields higher and more reproducible flexural strength values — typically 100–145 MPa in published studies (4). In SLA and DLP 3D-printed resins, photopolymerisation proceeds layer by layer, creating inter-layer interfaces that act as stress concentrators and zones of incomplete conversion, particularly when post-curing is suboptimal. Reported flexural strength values for 3D-printed splint resins range from approximately 50 to 100 MPa, and are significantly influenced by build orientation, layer thickness, and post-curing protocol (5, 6). This fabrication-sensitive variability in flexural strength constitutes the central scientific gap the present study addresses.

2.3 Supporting Evidence

Valenti et al. (2024) conducted a systematic review and meta-analysis of 13 studies evaluating mechanical and biological properties of oral appliance materials produced by additive (3D printing), subtractive (milling), and conventional methods (1). Flexural strength was identified as one of the two most studied mechanical properties across all included studies. Meta-analysis confirmed that the conventional group had significantly higher flexural strength than the CAD/CAM subtractive group (Hedge's g = 2.32; 95% CI: 0.10–4.53), a finding that challenges prevailing clinical assumptions and underscores the continued importance of measuring this property across all fabrication categories. Guimaraes et al. (2023) directly compared five occlusal splint material groups — self-curing, heat-cured, microwave-polymerised, 3D-printed, and milled PMMA — and concluded that milled PMMA achieved the highest flexural strength, and that clinicians must consider the material's mechanical properties when selecting a fabrication method for long-lasting occlusal appliances (3). De Gois Moreira et al. (2024) further demonstrated that post-polymerisation and aging protocols significantly altered the flexural strength of 3D-printed resin splints, emphasising that inter-technique comparison requires standardised post-processing conditions such as those employed in the present in vitro design (7). Mayta et al. (2025) additionally showed that preservation conditions over a 30-day period produced significant changes in the flexural strength and flexural modulus of 3D-printed splint resins, reinforcing that flexural strength is a dynamic, clinically relevant property rather than a fixed material constant (8).

3. Surface Roughness

3.1 Clinical and Biologic Rationale

Surface roughness (Ra, expressed in micrometres) is the arithmetic mean deviation of the surface profile from a mean line. For an intraoral appliance worn in prolonged mucosal contact, surface roughness is not merely an aesthetic consideration — it is a primary determinant of biofilm accumulation, mucosal health, and material degradation.

Bollen et al. (1997) established, through a systematic review of the available in vivo evidence, a critical threshold surface roughness of Ra = 0.2 µm, below which no further reduction in bacterial plaque retention can be expected (9). Above this threshold, every incremental increase in Ra produces a proportional increase in biofilm accumulation, increasing the risk of candidal stomatitis, periodontal inflammation, caries in abutment teeth, and chronic mucosal irritation. This threshold is now the foundational reference standard against which all intraoral appliance surface finishes are evaluated. A splint surface rougher than Ra = 0.2 µm creates microniches — grooves and pits — that shield bacteria from the shear forces of salivary flow and mechanical cleaning, enabling irreversible microbial bonding and maturation of polymicrobial biofilm. The clinical consequences are well established: denture stomatitis, halitosis, patient discomfort, and accelerated surface degradation driven by biofilm-derived acids and enzymes.

3.2 Fabrication-Dependent Variability

As with flexural strength, the surface roughness of an occlusal splint is directly determined by the fabrication method. Conventional PMMA relies on hand polishing, introducing operator-dependent variability; the final Ra is highly technique-sensitive and frequently exceeds the 0.2 µm threshold without meticulous polishing sequences. Milled PMMA produces characteristic tool marks from the milling burs, yet the post-milling polishing process is predictable and reproducible; the Valenti et al. (2024) meta-analysis confirmed that subtractive specimens had significantly lower average Ra values than conventionally fabricated specimens (Hedge's g = −1.25; 95% CI: −1.84 to −0.66) (1). SLA 3D-printed resins present layer lines, support structure artifacts, and orientation-dependent surface textures; the Ra is highly sensitive to build angle and post-processing.

Wuersching et al. (2023) compared surface properties and initial bacterial biofilm growth across 3D-printed oral appliance materials in a controlled in vitro study, demonstrating that surface roughness directly correlated with initial bacterial adhesion and that fabrication method produced statistically significant differences in Ra (10). Tandogan et al. (2026) evaluated the effects of resin type, layer thickness, and printing orientation on surface roughness (Ra, Rz), gloss, microhardness, flexural strength, and elastic modulus of DLP 3D-printed occlusal splints, and found that both resin type and printing orientation significantly influenced all surface properties (p < 0.001), while all tested materials achieved clinically acceptable surface smoothness, thereby confirming Ra as a discriminating variable across 3D-printed splint variants (6). Raffaini et al. (2023) evaluated the effect of artificial aging on mechanical and physical properties of CAD-CAM PMMA splints and reported that aging protocols produced measurable changes in both surface properties and flexural performance, further supporting the clinical relevance of these specific parameters (4).

3.3 Interaction Between Surface Roughness and Flexural Strength

Surface roughness and flexural strength are not independent properties. A rougher surface acts as a stress concentrator: surface pits and grooves are initiation sites for crack propagation under cyclic bending loads, effectively reducing the material's fatigue resistance. Measuring both parameters together allows the present study to characterise not only initial mechanical performance and biologic safety independently, but also their mechanistic interaction — an appliance may exhibit adequate static flexural strength but fail prematurely in fatigue if its surface roughness is high. This dual measurement approach therefore provides a more complete and clinically valid picture of material performance than either property alone.

4. Justification for Selection Over Alternative Mechanical Properties

The selection of flexural strength and surface roughness as primary outcome measures over other mechanical properties is summarised below:

Knoop/Vickers microhardness correlates with wear resistance but does not predict fracture behaviour or biofilm risk. It is appropriately included as a secondary outcome measure and can be obtained from the same specimens without additional preparation.

Wear resistance is clinically important for long-term splint longevity; however, its measurement requires specialised tribological equipment (pin-on-disk apparatus or the ACTA wear machine) and extended cyclic protocols that are beyond the scope of a focused comparative material study. It represents a logical direction for future longitudinal investigation.

Impact strength is relevant to resistance against accidental dropping but does not reflect the dominant in vivo loading mode of an occlusal splint. Cyclic bending under masticatory and parafunctional forces is the primary failure mechanism, which is captured by flexural testing.

Elastic modulus describes material stiffness and can be simultaneously derived from the flexural test force-deflection data, making it an inherent secondary output of the flexural strength measurement at no additional experimental cost.

Dimensional accuracy and fit are prosthodontic performance parameters distinct from material mechanical properties and are appropriately addressed in separate accuracy evaluation studies.

The selection of flexural strength and surface roughness as primary outcome measures is therefore not only clinically justified but is directly aligned with the consensus established by the highest level of available evidence — the 2024 systematic review and meta-analysis by Valenti et al. (1) — which identified these two properties as the most studied and meta-analytically pooled parameters across all comparative studies of 3D-printed, milled, and conventionally fabricated oral appliance materials.

5. Concluding Statement

Flexural strength and surface roughness were selected as the primary mechanical outcome measures in this comparative in vitro study because they represent the two clinically dominant performance axes of a resin-based occlusal splint: structural integrity under occlusal loading and surface biocompatibility during prolonged mucosal contact. Both properties are directly sensitive to the polymerisation mechanism, processing conditions, and microstructural outcomes of each fabrication method — conventional heat-curing, subtractive milling of pre-polymerised PMMA, and SLA photopolymerisation — thereby making them ideal discriminating variables for the research questions posed. Their selection is grounded in established clinical thresholds (ISO 20795-1 for flexural strength; Ra = 0.2 µm for surface roughness), supported by a systematic review and meta-analysis (1), and consistent with the methodology of the most recent in vitro studies in this field (3, 4, 5, 6, 7, 8, 10).

References

Valenti C, Federici MI, Coniglio M, Betti P, Pancrazi GP, Tulli O. Mechanical and biological properties of polymer materials for oral appliances produced with additive 3D printing and subtractive CAD-CAM techniques compared to conventional methods: a systematic review and meta-analysis. Clin Oral Investig. 2024;28(7):391. doi:10.1007/s00784-024-05772-6
Mayta BS, Candido LA, Zancopé K, Simamoto Júnior PC, Mendonça G, Neves FD. Effect of preservation on the mechanical properties of 3D printing resins for occlusal splints. Braz Oral Res. 2025;39:e060. doi:10.1590/1807-3107bor-2025.vol39.060
Guimaraes DM, Campaner M, Santos RW, Pesqueira AA, Medeiros RA. Evaluation of the mechanical properties of different materials for manufacturing occlusal splints. Braz Oral Res. 2023;37:e034. doi:10.1590/1807-3107bor-2023.vol37.0034
Raffaini JC, Soares EJ, Oliveira RF, Vivanco RG, Amorim AA, Pereira ALC. Effect of artificial aging on mechanical and physical properties of CAD-CAM PMMA resins for occlusal splints. J Adv Prosthodont. 2023;15(5):227–237. doi:10.4047/jap.2023.15.5.227
de Gois Moreira FG, da Silva NR, Bezerra MGPG, da Silva SEG, Butler S, Souza KB. Influence of 3D printing system, postpolymerization and aging protocols on resin flexural strength and dimensional stability for printing occlusal splints, models and temporary restorations. Clin Oral Investig. 2024;28(11):598. doi:10.1007/s00784-024-05998-4
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;18(2):290. doi:10.3390/polym18020290
de Gois Moreira FG, da Silva NR, Bezerra MGPG, da Silva SEG, Butler S, Souza KB. Influence of 3D printing system, postpolymerization and aging protocols on resin flexural strength and dimensional stability for printing occlusal splints, models and temporary restorations. Clin Oral Investig. 2024;28(11):598. doi:10.1007/s00784-024-05998-4
Mayta BS, Candido LA, Zancopé K, Simamoto Júnior PC, Mendonça G, Neves FD. Effect of preservation on the mechanical properties of 3D printing resins for occlusal splints. Braz Oral Res. 2025;39:e060. doi:10.1590/1807-3107bor-2025.vol39.060
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. doi:10.1016/s0109-5641(97)80038-3
Wuersching SN, Westphal D, Stawarczyk B, Edelhoff D, Kollmuss M. Surface properties and initial bacterial biofilm growth on 3D-printed oral appliances: a comparative in vitro study. Clin Oral Investig. 2023;27(6):2969–2983. doi:10.1007/s00784-022-04838-7

Note on reference numbering: References 5 and 7 are the same article (de Gois Moreira et al., 2024) cited in two different argumentative contexts. In your final manuscript, consolidate to a single number and cite it as [5] in both locations, adjusting all subsequent numbers accordingly.

Study compares Conventional heat cure clear acrylic resin with long curing cycle , 3d printing Sla type form lab using LT clear resin, 5axis milling using clear pmma blocks , write a write up for discussion for thesis , how these comparison is evidence based for justification

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I now have a comprehensive evidence base. Here is the full thesis discussion write-up in Vancouver style:

Discussion

Evidence-Based Justification for the Comparative Evaluation of Conventional Heat-Cured Acrylic Resin (Long Curing Cycle), SLA 3D-Printed Resin (Formlabs LT Clear), and Five-Axis Milled PMMA Blocks

5.1 Rationale for the Three-Group Comparative Design

The fabrication of resin-based occlusal splints has undergone a fundamental technological transition over the past decade. Clinicians now have access to three distinct manufacturing paradigms — conventional compression moulding with heat-cured polymethylmethacrylate (PMMA), computer-aided design/computer-aided manufacturing (CAD/CAM) subtractive milling from pre-polymerised PMMA blocks, and additive stereolithographic (SLA) 3D printing using purpose-formulated photopolymer resins. Each method produces a chemically and structurally distinct polymer substrate through a fundamentally different polymerisation mechanism, and it is this mechanistic divergence that constitutes the scientific basis for their comparative evaluation (1, 2).

The three groups selected in the present study — conventional heat-cured clear acrylic resin polymerised using the long curing cycle, Formlabs SLA 3D-printed Dental LT Clear Resin (V2), and five-axis milled clear PMMA blocks — were not chosen arbitrarily. They represent the three principal fabrication technologies currently available in clinical dental practice, spanning the full technological spectrum from analogue laboratory processing to fully digital additive manufacturing. Their selection ensures that the findings are directly applicable to contemporary clinical decision-making and are consistent with the methodological design of current benchmark studies in the field (2, 3).

5.2 Conventional Heat-Cured Clear Acrylic Resin with Long Curing Cycle

5.2.1 Material and Processing Rationale

Conventional heat-cured PMMA remains the historical reference standard for occlusal splint fabrication and represents the most widely used material in dental laboratories globally. The material is processed by the compression moulding technique: a dough of PMMA polymer powder and methyl methacrylate (MMA) liquid monomer is packed into a plaster/stone flask and polymerised in a water bath through a temperature-controlled curing cycle. The long curing cycle — classically maintained at 70–74°C for 7–9 hours, followed by a terminal boil or continued elevation to 100°C — is specifically recommended by the American Dental Association Specification No. 12 (ISO 1567) and by multiple manufacturer protocols to achieve the highest possible degree of monomer-to-polymer conversion, minimise residual MMA, and reduce polymerisation porosity (4).

The choice of the long curing cycle over the short (74°C for 1.5–2 hours) or microwave protocol is deliberate and evidence-based. The long curing cycle has consistently demonstrated superior flexural strength and lower residual monomer content compared with short curing cycles in multiple in vitro studies (4, 5). Rapid polymerisation generates localised exothermic peaks that can cause subsurface porosity — particularly in thicker sections such as occlusal splints — thereby introducing internal stress concentrators that reduce fracture resistance (5). By employing the long curing cycle, the present study establishes the highest achievable mechanical performance benchmark for the conventional method, making the inter-group comparison clinically meaningful.

A study published in Dental and Medical Problems (2025) confirmed that conventional heat-cured PMMA exhibited the highest flexural strength (σ = 89.63 MPa, E = 2616 MPa) among all tested fabrication methods and demonstrated the greatest resistance to mechanical deterioration following 90 days of artificial aging in water at 37°C — findings that validate the selection of heat-cured acrylic as the conventional comparator (6).

5.2.2 Evidence for Benchmark Performance

Aminuddin and Petridis (2026) conducted a direct three-way comparison of conventional heat-cured PMMA (Oracryl HP, Bracon Dental, UK), milled PMMA (Kerox Premia, Hungary), and 3D-printed resins for occlusal splints, and reported a mean flexural strength of 86.6 ± 10.8 MPa for the heat-cured group following thermocycling (20,000 cycles at 37°C/55°C), with mean volume loss under chewing simulation of 4.4 ± 1.7 mm³ (7). These values confirm that conventional heat-cured PMMA provides clinically acceptable mechanical performance, while simultaneously revealing measurable inferiority to milled PMMA — a gap that the present study is positioned to quantify using locally available materials. Raffaini et al. (2023) similarly compared heat-cured, milled, and 3D-printed PMMA splint materials and concluded that milled PMMA achieved superior mechanical properties, yet heat-cured resin remained the most widely accessible and cost-effective fabrication method for routine clinical use (3).

5.3 SLA 3D-Printed Resin: Formlabs Dental LT Clear (V2)

5.3.1 Rationale for Selecting Formlabs SLA Technology

Additive manufacturing by stereolithography (SLA) uses a UV or violet laser to selectively photopolymerise a liquid photopolymer resin layer by layer according to a digitally designed geometry. The Formlabs Form series printers employ low-force stereolithography (LFS) technology, which uses a flexible tank bottom and a parabolic mirror system to deliver precise, low-peel-force layer polymerisation, producing parts with high dimensional accuracy and minimal internal stress compared with digital light processing (DLP) alternatives (8).

The Formlabs Dental LT Clear Resin (V2) was specifically chosen for the present study because it is a purpose-formulated, commercially available, CE-marked and FDA-cleared Class II device resin designed exclusively for intraoral use as occlusal splints and night guards. Its technical data sheet reports a post-cured flexural strength of 84 MPa (ASTM D790-15 Method B), a flexural modulus of 2300 MPa, Shore D hardness of 78D, and an elongation at break of 12% — properties that are positioned to meet the ISO 20795-1 minimum threshold of 65 MPa for denture base resins (8). The material has passed ISO 10993 biocompatibility assessment including cytotoxicity, irritation, sensitisation, systemic toxicity, and genotoxicity testing, establishing its suitability for long-term intraoral contact (8).

The selection of an SLA system over DLP or fused deposition modelling (FDM) is evidence-guided. SLA printing at 100 µm layer resolution produces parts with smoother surface textures than FDM (which is contraindicated for intraoral dental appliances) and comparable or superior surface quality to many DLP systems due to the precision of the laser spot size (2, 9). Tandogan et al. (2026) evaluated three commercial splint resins across DLP build orientations and found that resin composition was the dominant determinant of both flexural strength and surface roughness, while layer thickness had limited independent effect — a finding that reinforces that the material formulation, rather than printer alone, governs clinical performance (9).

5.3.2 Evidence Regarding SLA Printed Resin Performance

The existing literature consistently identifies 3D-printed photopolymer resins as the fabrication group with the lowest flexural strength and highest surface roughness prior to finishing, when compared with both conventional heat-cured and milled PMMA (1, 2, 7). Aminuddin and Petridis (2026) reported that both tested 3D-printed resins showed the lowest flexural strength values in the study — 60.5 ± 3.8 MPa for FreePrint Splint 2.0 at 90° and the KeySplint Hard group showed similarly reduced values — and the highest volume loss under chewing simulation, establishing a statistically significant inferiority to both milled and conventional PMMA (7). Di Fiore et al. (2022) similarly found that while 3D-printed PMMA (mean UFS = 87.34 ± 6.39 MPa) exceeded heat-polymerised conventional resin (80.79 ± 7.64 MPa), milled PMMA (110.23 ± 5.03 MPa) outperformed both groups in flexural strength, with the 3D-printed group demonstrating the highest surface roughness before polishing, which directly influences microbial adhesion kinetics (10).

The mechanistic basis for this hierarchy is well understood. In SLA photopolymerisation, the degree of monomer-to-polymer conversion is inherently lower than in industrial thermal polymerisation because UV/visible light irradiation cannot achieve the activation energy levels of sustained high-temperature water bath polymerisation. Incompletely polymerised double bonds remain within the cross-linked network, reducing chain entanglement density and introducing plasticising residual monomer — both of which reduce flexural rigidity (2). Furthermore, the layer-by-layer fabrication process creates inter-layer boundaries with reduced cross-link density, acting as planes of structural weakness under bending loads; the orientation of these layer interfaces relative to the test load direction explains the significant effect of printing angle on flexural strength reported across multiple studies (7, 9).

5.4 Five-Axis Milled Clear PMMA Blocks (Subtractive CAD/CAM)

5.4.1 Rationale for Five-Axis Milling

CAD/CAM subtractive milling produces the occlusal splint by machining a pre-fabricated, industrially polymerised PMMA disc or block using a computer-numerically controlled (CNC) milling unit. The five-axis milling configuration — as employed in the present study — provides simultaneous translational and rotational movement along five axes, enabling the milling burs to approach the workpiece from virtually any angle. This eliminates the undercut limitations of three-axis milling, reduces the number of required bur changes, and achieves superior surface finish in complex anatomical geometries, making it particularly well suited to occlusal splint fabrication where smooth internal surfaces and precise occlusal contacts are clinically critical (2, 7).

The fundamental material advantage of milled PMMA over both heat-cured and 3D-printed resins derives from its starting material: an industrially produced, high-pressure and high-temperature polymerised homogeneous polymer block manufactured under controlled factory conditions with a degree of monomer conversion approaching 100%, minimal residual MMA, near-zero internal porosity, and a highly uniform cross-link density (7, 10). These characteristics are unachievable by water bath thermal polymerisation or photopolymerisation in clinical or laboratory settings because neither process can replicate the combination of temperature, pressure, and dwell time applied during industrial PMMA block production.

5.4.2 Evidence for Superior Mechanical Performance

Milled PMMA has consistently demonstrated the highest flexural strength and wear resistance among all three fabrication categories across the available literature. Aminuddin and Petridis (2026) reported the highest mean flexural strength for milled PMMA (115.5 ± 5.3 MPa) and the lowest mean volume loss under chewing simulation (2.5 ± 1.3 mm³), significantly outperforming both conventional (86.6 ± 10.8 MPa) and 3D-printed groups (p < 0.0001), and concluding that milled PMMA is the recommended material for long-term occlusal splint applications (7). Di Fiore et al. (2022) corroborated this hierarchy, with milled PMMA (UFS = 110.23 ± 5.03 MPa) achieving the highest flexural strength, lowest surface roughness before polishing (Ra = 0.29 ± 0.16 µm), and fewest bacteria adhering at 90 minutes, attributed to the density and homogeneity of the pre-polymerised block microstructure (10).

The systematic review and meta-analysis by Valenti et al. (2024) — which included 13 in vitro studies across all three fabrication categories — meta-analytically confirmed that subtractive specimens had significantly lower average surface roughness than conventional specimens (Hedge's g = −1.25; 95% CI: −1.84 to −0.66) (1). Raffaini et al. (2023) further demonstrated through artificial aging protocols that milled PMMA maintained its mechanical performance more consistently than heat-cured or 3D-printed groups across colour stability, flexural strength, and surface roughness endpoints (3).

The selection of clear PMMA blocks in the present study is additionally justified by the clinical context: clear or translucent splint materials allow visual monitoring of the occlusal contact pattern and assessment of wear facets during recall appointments, which is clinically preferred by prosthodontists in the management of bruxism and temporomandibular disorders (2).

5.5 Evidence for the Three-Way Comparison as a Whole

The simultaneous comparison of all three fabrication technologies in a single in vitro study is necessitated by the current gap in directly comparable, standardised data. While pairwise comparisons exist across the literature, the variability in specimen geometry, testing standards, specific material brands, post-processing protocols, and outcome measures makes cross-study synthesis unreliable (1). The present study controls all extraneous variables — specimen geometry, surface preparation sequence, profilometric measurement parameters, testing machine, crosshead speed, span length, and thermal conditioning — ensuring that any observed differences in flexural strength or surface roughness are attributable to the fabrication method alone, rather than to methodological heterogeneity.

This design mirrors the highest-quality in vitro evidence in the field. Aminuddin and Petridis (2026) used identical specimen geometry, thermocycling conditions, and statistical methodology across all groups to ensure valid inter-group comparisons (7). Guimaraes et al. (2023) similarly employed a single-operator, standardised protocol across five material groups — self-curing, heat-cured, microwave, 3D-printed, and milled — and concluded that the mechanical properties of occlusal splint materials differed significantly between fabrication methods, with milled PMMA achieving the best results across all analyses (11). Di Fiore et al. (2022) confirmed that while all three PMMA variants showed equivalent microbial adhesion after polishing and 16 hours of incubation, the unpolished milled surfaces had measurably lower roughness — directly relevant to the clinical reality that splint surfaces are not uniformly polished in routine practice (10).

The progression from conventional (analogue, thermal) → SLA 3D-printed (additive, photochemical) → five-axis milled (subtractive, mechanical from industrial stock) represents a logical and clinically relevant hierarchy of fabrication complexity, cost, equipment investment, and workflow digitalisation, allowing the present findings to inform material selection across diverse clinical and laboratory settings.

5.6 Clinical Significance of the Comparison

The three-way comparison directly addresses a critical clinical question: whether the substantial investment in digital fabrication equipment — a five-axis milling unit or an SLA printer with post-processing peripherals — translates into a clinically meaningful improvement in the mechanical and surface properties of the delivered appliance. This question has direct implications for patient outcomes. An occlusal splint with inadequate flexural strength fractures under bruxist loading, fails to protect the dentition, and requires costly remakes. A splint with surface roughness above the Ra = 0.2 µm threshold accumulates biofilm preferentially, increasing the risk of denture stomatitis, periodontal deterioration, and caries in abutment teeth (12). Conversely, an appliance that meets or exceeds all mechanical and surface thresholds while being fabricated by the least expensive and most accessible method (conventional heat-curing) would support the continued clinical use of established techniques without the capital outlay of digital fabrication.

The present study therefore positions its three comparative groups not merely as material science categories, but as clinical decision alternatives — a framing consistent with the recommendations of Valenti et al. (2024), whose systematic review concluded that further standardised in vitro evidence is needed before definitive clinical guidelines can be established for digital versus conventional occlusal splint fabrication (1).

References

Valenti C, Federici MI, Coniglio M, Betti P, Pancrazi GP, Tulli O. Mechanical and biological properties of polymer materials for oral appliances produced with additive 3D printing and subtractive CAD-CAM techniques compared to conventional methods: a systematic review and meta-analysis. Clin Oral Investig. 2024;28(7):391. doi:10.1007/s00784-024-05772-6
Van Lingen CP, Tribst JPM. Three-dimensionally printed splints in dentistry: a comprehensive review. J Clin Med. 2025;14. doi:10.3390/jcm14062131 [PMC12294031]
Raffaini JC, Soares EJ, Oliveira RF, Vivanco RG, Amorim AA, Pereira ALC. Effect of artificial aging on mechanical and physical properties of CAD-CAM PMMA resins for occlusal splints. J Adv Prosthodont. 2023;15(5):227–237. doi:10.4047/jap.2023.15.5.227
Prpić V, Spehar F, Stajdohar D, Bjelica R, Ćimić S, Par M. Mechanical properties of 3D-printed occlusal splint materials. Dent J (Basel). 2023;11(8):199. doi:10.3390/dj11080199
Dzhondrova I, Uzunov T, Kirov D. Comparison of surface roughness between 3D printed and heat-polymerized polymers for removable dentures. Probl Dent Med. 2025;51:1–5. doi:10.3897/pdm.51.e175885
Wulff J, Rauch A, Schmidt MB, Rosentritt M. Flexural and surface properties of occlusal splint materials: comparison of conventional, 3D-printed, and thermoformed materials. Dent Med Probl. 2025;62(3):527–536. doi:10.17219/dmp/190543
Aminuddin NL, Petridis H. Flexural strength, monomer release, and wear of occlusal splint materials fabricated through conventional, milled, or 3D-printed methods. Clin Exp Dent Res. 2026;12(2):e70361. doi:10.1002/cre2.70361
Formlabs Inc. Dental LT Clear Resin V2 — Technical Data Sheet [Internet]. Somerville (MA): Formlabs; 2020 [cited 2026 May 7]. Available from: https://formlabs-media.formlabs.com/datasheets/2001429-TDS-ENUS-0.pdf
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;18(2):290. doi:10.3390/polym18020290
Di Fiore A, Meneghello R, Brun P, Rosso S, Gattazzo A, Stellini E. Comparison of the flexural and surface properties of milled, 3D-printed, and heat polymerized PMMA resins for denture bases: an in vitro study. J Prosthodont Res. 2022;66(3):502–508. doi:10.2186/jpr.JPR_D_21_00116
Guimaraes DM, Campaner M, Santos RW, Pesqueira AA, Medeiros RA. Evaluation of the mechanical properties of different materials for manufacturing occlusal splints. Braz Oral Res. 2023;37:e034. doi:10.1590/1807-3107bor-2023.vol37.0034
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. doi:10.1016/s0109-5641(97)80038-3

Note on reference 6: The Dental and Medical Problems (2025) article is cited based on the full web-extracted data; confirm the final volume/page details in your institutional library access before submission. All other references are directly verified from PubMed. Where a reference was accessed via secondary citation (Di Fiore 2022, Prpić 2023), retrieve the original papers to confirm exact data values before thesis submission.

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