Standardisation of the Polishing and Post-Processing Sequence for Occlusal Splints: Formlabs SLA (Stereolithography) vs. PMMA 5-Axis Milled Fabrication

1. Introduction

2. Materials Overview

2.1 Formlabs SLA Dental Resins for Splints

• Dental LT Clear Resin (V2): A rigid, fracture-resistant methacrylate-based photopolymer for long-term hard occlusal splints. It prints optically clear and polishes to high transparency.
• Dental LT Comfort Resin: A flexible, durable formulation for soft/semi-rigid splints, bleaching trays, and night guards that require compliance.

2.2 CAD/CAM PMMA for 5-Axis Milled Splints

3. Clinical Rationale for Standardised Post-Processing

1. Surface roughness reduction - Ra < 0.2 µm target to minimise biofilm formation and Streptococcus mutans adhesion. Studies confirm that polishing protocol is the single most statistically significant determinant of Ra in 3D-printed splints (p < 0.001), overriding layer thickness and build orientation (MDPI Polymers, 2025).
2. Surface hardness - Vickers hardness (VHN) must be adequate to resist bruxing forces. Turker Kader et al. (J Esthet Restor Dent, 2026, PMID: 42087479) demonstrated that surface finishing method significantly affects VHN (p < 0.001), with glaze-coated SLA specimens achieving the highest hardness while polishing yielded the smoothest, most clinically consistent surfaces.
3. Staining and optical properties - Milled PMMA splints exhibit significantly less coffee staining than 3D-printed materials under equivalent conditions. Among printed splints, polishing and glycerin-curing are the most effective strategies for reducing staining susceptibility (Rueda et al., J Esthet Restor Dent, 2025, PMID: 40181636).
4. Biocompatibility and residual monomer elimination - Uncured surface resin in SLA parts contains unreacted photoinitiator and monomers. Complete post-cure is mandatory prior to any intraoral use.

4. Standardised Post-Processing and Polishing Protocol: Formlabs SLA Occlusal Splints

Phase 1: Part Removal

• Allow the Form 4B printer to complete the print cycle fully before opening.
• For the Form 4 Flex Build Platform: push the quick-release handles to flex the platform and release the part without stress fracture.
• For the standard Form 4 Build Platform: use a dedicated metal spatula tool at the attachment points, applying lateral pressure rather than direct prying force.
• Do not apply force across the occlusal table - this can introduce micro-fractures in thin regions.
• Wear nitrile gloves throughout; uncured resin is a skin sensitiser and potential allergen.

Phase 2: IPA Washing (Solvent Cleaning)

• Do not mix the biocompatible wash with tanks used for non-biocompatible resins (e.g., engineering resins).
• After washing, do not proceed directly to post-cure - residual IPA must evaporate.

Phase 3: Drying

• Air dry at room temperature for minimum 30 minutes.
• Do not use compressed air directly on the splint surface - this may leave particulate or moisture.
• Inspect under bright light: the surface should appear fully matte and consistent, not glossy or tacky. Tacky areas indicate residual uncured resin or IPA - return to wash cycle.
• Dental LT Comfort Resin may feel slightly tacky at this stage due to oxygen inhibition - this is normal and will resolve post-cure.

Phase 4: Post-Curing (Photo-Polymerisation Completion)

Phase 5: Support Removal

• After post-cure, use a support removal tool (dedicated pliers or flush cutters) to remove all support structures.
• Cut supports at the attachment point (touchpoint), not at the splint surface.
• Do not use a scalpel blade for bulk removal - this creates unpredictable cleavage planes.
• Some supports may leave a small nub or dimple; these are addressed in Phase 6.

Phase 6: Touchpoint Smoothing (Coarse Finishing)

1. Medium Scotch-Brite-type rotary wheel - for bulk smoothing of touchpoint residues
2. Green stone silicone carbide bur - effective for spot reduction of raised support nubs
3. Rubber silicone polishing bur (coarse/grey) - smooths the transition zone around removed supports

Phase 7: Progressive Polishing Sequence

Route A: Handpiece Polishing (Chair-Side / Lab Handpiece)

Route B: Lathe Polishing (Dental Laboratory Standard)

Optional: Glaze or Resin Coating

• Glaze coating (e.g., dental glaze, light-cured surface sealant): Produces the highest VHN but does not achieve the lowest Ra. The glaze fills surface micropores and micro-scratches. Indicated when maximum hardness is the priority (severe bruxism patients).
• Resin coating: Application of a thin layer of unfilled or lightly filled resin, followed by light-curing. Produces smooth surfaces and improves stain resistance. The coating must be cured in glycerin (or under nitrogen) to prevent an oxygen-inhibited surface layer (Turker Kader et al., 2026; Rueda et al., 2025).
• Polishing without coating remains the most practical and clinically reproducible approach, yielding smooth surfaces with consistent and acceptable Ra values (Turker Kader et al., 2026).

Phase 8: Final Inspection and Quality Control

• Measure Ra with a surface profilometer (contact or optical) - target Ra ≤ 0.2 µm at occlusal and buccal surfaces.
• Verify occlusal contacts with 8-µm articulating foil.
• Check for any remaining support marks, bubbles, or uneven translucency.
• Disinfect with appropriate agent per Formlabs biocompatible material guidance (quaternary ammonium or peracetic acid solutions; avoid prolonged alcohol immersion post-completion as it may cause surface cloudiness in LT Clear).

5. Standardised Post-Processing and Polishing Protocol: PMMA 5-Axis Milled Occlusal Splints

Phase 1: Milling Completion and Part Separation

• The 5-axis CNC milling machine (e.g., Roland DWX-53D, Yenadent, Ivoclar PrograMill) completes the milling cycle automatically, including finishing toolpath passes.
• The splint is typically held in the puck by a retention peg or connector bridge (varies by machine and nesting software).
• Separate the splint from the puck using a dedicated separating disc (thin diamond disc, 0.3 mm width) or a pre-cut groove left by the CAM software. Never break the connector by hand - this risks fracture into the splint body.
• Remove any visible machining chips with a dry bristle brush.

Phase 2: Removal of Machining Marks - Coarse Grinding

Phase 3: Progressive Wet Grinding / Sanding

Phase 4: Pumice Polishing Phase

• The wheel must be loaded with pumice slurry before contact with the splint - a dry wheel on acrylic generates heat and causes surface burn/crazing.
• Maintain constant motion; dwelling on one area causes localised softening.
• If the 5-axis milling used a finishing toolpath with 0.05-0.1 mm step-over (fine setting), the as-milled surface may already be near-smooth enough to skip the coarser wet sanding grades and begin directly at 600-grit equivalent.

Phase 5: Final High-Shine Polishing

Phase 6: Occlusal Adjustment (if required)

• Mount splint on articulator or check directly in patient's mouth.
• Mark occlusal contacts with 8-µm articulating foil (Shimstock) and 40-µm paper.
• Adjust with fine-grain diamond bur or acrylic trimmer; any adjusted area must be re-polished through at minimum Phase 5 (pumice + shine compound) to restore surface smoothness.
• This is a frequently omitted step in clinical practice, leading to uneven Ra and focal rough zones that act as plaque traps.

Phase 7: Final Quality Control

• Surface profilometry (Ra target ≤ 0.2 µm).
• Visual inspection under magnification (x3-5 loupes) for scratches, peg remnants, and dull zones.
• Dimensional check: verify thickness in the posterior occlusal region (minimum 1.5-2 mm for bruxism splints).
• Disinfect: autoclave (if manufacturer permits; most PMMA blanks are not autoclavable due to thermal deformation at 121°C) or cold disinfection with 2% glutaraldehyde, peracetic acid, or sodium hypochlorite solution per manufacturer instructions.

6. Comparative Summary: SLA vs. PMMA Milled Polishing Protocols

7. Evidence Summary and Key Literature

1. Turker Kader I et al. (2026) (J Esthet Restor Dent, PMID: 42087479) - 96 SLA-printed disc specimens (n=12/group) evaluated across as-printed, polished, resin-coated, and glaze-coated groups with air vs. glycerin post-cure. Conclusion: Surface finishing significantly affects Ra and VHN (p < 0.001); post-curing environment has no significant effect. Polishing provides the smoothest, most clinically consistent results. Glaze coating achieves highest VHN.
2. Rueda SR et al. (2025) (J Esthet Restor Dent, PMID: 40181636) - DLP-printed vs. milled splint materials; evaluated as-printed, optical polish, resin-coated, and polished specimens with/without glycerin curing. Conclusion: Polishing and resin-coating optimise roughness and translucency. Glycerin curing reduces staining except in polished specimens. Milled materials (ProArt CAD Splint) stained significantly less than any 3D-printed material.
3. Yan X et al. (2026) (J Prosthet Dent, PMID: 42002494) - Systematic review (82 studies; PRISMA); surface quality of 3D-printed resin restorations. Conclusion: Surface quality is determined synergistically by material composition, printing parameters, and post-processing. Post-processing (polishing, chemical coating, post-polymerisation) is most decisive for esthetic and biophysical properties.
4. MDPI Polymers study (2025) - Combined effect of build orientation, layer thickness, and polishing protocol on Ra and S. mutans adhesion for 3D-printed occlusal splints. Conclusion: Polishing protocol alone significantly affects Ra (p < 0.001). Combined centrifugation + mechanical polishing (C + M) produces the lowest Ra (0.646 ± 0.059 µm). Polishing is independent of build orientation; it can be re-applied when necessary.
5. Raffaini JC et al. (2023) (J Adv Prosthodont, 15(5):227-237) - Compared heat-cured, milled PMMA, and 3D-printed PMMA splints under artificial aging. Conclusion: Surface roughness inversely correlates with flexural strength. Milled PMMA demonstrates superior Ra stability under aging conditions. Rough surfaces increase biofilm accumulation and staining susceptibility.
6. Formlabs Dental Application Guide (2024) - Official manufacturer protocol for Dental LT Clear Resin and Dental LT Comfort Resin splints: dual IPA wash, ≥30 min drying, 60°C/60 min Form Cure post-cure, support removal, handpiece and lathe polishing sequences.
7. Quirynen M & Bollen CML (1995) (J Clin Periodontol 22:1-14) - Foundational paper establishing Ra 0.2 µm as the threshold below which further reduction does not decrease bacterial adhesion; values above 0.8 µm substantially increase plaque retention.

8. Practical Recommendations for Standardisation

1. Document every post-processing step with timestamps and operator initials as part of the digital manufacturing record (MDR) - relevant for CE/FDA Class II device documentation.
2. Maintain material-specific post-cure records for SLA splints (temperature, duration, batch number of resin) - post-cure inadequacy cannot be detected visually but substantially affects biocompatibility.
3. Use a profilometer for batch verification - at minimum, one specimen per batch should be measured before clinical delivery. A Ra > 0.2 µm on the tissue-fitting surface warrants return to polishing.
4. Never skip the dry step between pumice and high-shine compound - pumice contamination of the shine compound is the most common source of sub-optimal gloss in both SLA and milled PMMA splints.
5. Re-polish all occlusal adjustment areas - whether done intraorally or on the articulator, any grinding must be followed by at minimum pumice + high-shine polish. Providing patients with an unpolished adjusted splint is a recognised clinical error.
6. Store SLA splints away from UV light - ongoing photopolymerisation from ambient UV can cause post-delivery changes in surface hardness and dimensional stability.
7. For milled PMMA, use wet grinding throughout the sanding sequence - heat generation from dry grinding on pre-polymerised PMMA causes surface smearing (thermoplastic flow) that paradoxically increases Ra at high grit levels.
8. Train all laboratory personnel in the sequence using the same instruments, compounds, and speeds - the literature confirms that operator technique and protocol adherence are the dominant variables in achieving target Ra in both fabrication methods.

Comparative Summary of Polishing Protocols for Occlusal Splints: Conventional Heat-Cure, Formlabs SLA 3D-Printed, and PMMA 5-Axis Milled Fabrication

1. Introduction and Rationale for Comparison

1. Conventional heat-cure acrylic (long-cure protocol) - compression-moulded poly(methyl methacrylate) powder-liquid system, polymerised in a water bath under heat and pressure.
2. Formlabs SLA 3D printing - layer-by-layer stereolithographic photopolymerisation of a biocompatible dental resin (Dental LT Clear Resin V2 / Dental LT Comfort Resin).
3. PMMA 5-axis CNC milling - subtractive machining of a pre-polymerised, industrially manufactured PMMA puck using a multi-axis milling machine.

2. Material Microstructure and its Polishing Implications

2.1 Conventional Heat-Cure Acrylic

• Heterogeneous microstructure with PMMA bead boundaries visible under SEM
• Inherent microporosity from trapped air during packing - a significant source of elevated baseline Ra
• Residual monomer content of 0.5-5% (higher in poorly cured batches or fast-cure cycles)
• As-deflasked Ra typically 2.0-4.5 µm from gypsum surface texture transfer, flash lines, and porosity
• Surface hardness lower than pre-polymerised milled PMMA due to the free-radical powder-liquid polymerisation mechanism
• Dimensional inaccuracy from polymerisation shrinkage (0.3-0.5%) and thermal contraction on cooling - requiring remounting and occlusal correction before polishing

2.2 PMMA 5-Axis Milled

• Minimal residual monomer (< 0.2%)
• Flexural strength ≥ 100 MPa; modulus ≥ 2,800 MPa; hardness ≥ 140 MPa (manufacturer specifications, Ivoclar ProArt)
• As-milled Ra of 0.4-1.5 µm from tool scallop marks - significantly lower baseline than heat-cure
• No flash lines, no plaster inclusion marks, no packing porosity

2.3 Formlabs SLA Printed

• Layer-step artefacts create an anisotropic surface texture, Ra 1.5-3.0 µm as-printed
• Oxygen inhibition layer at all exposed surfaces creates a tacky, incompletely cured zone that must be removed before polishing begins
• Support touchpoints leave residual nubs requiring targeted mechanical removal
• Baseline surface hardness lower than milled PMMA until post-cure is complete

3. Full Polishing Protocol Comparison

3.1 PHASE 0: Pre-Polish Processing (Fabrication-Dependent)

3.2 PHASE 1: Gross Finishing / Removal of Fabrication Artefacts

3.3 PHASE 2: Progressive Abrasive Reduction (Wet Grinding / Sanding)

3.4 PHASE 3: Pumice Polishing Phase (Lathe or Handpiece)

3.5 PHASE 4: High-Shine Polish (Lathe)

3.6 PHASE 5: Optional Enhancement Steps

4. Occlusal Correction and Re-Polishing

5. Surface Property Outcomes: Evidence-Based Comparison

5.1 Surface Roughness (Ra)

5.2 Surface Hardness (Vickers Hardness, VHN)

5.3 Staining Resistance

5.4 Flexural Strength

6. Time, Labour, and Workflow Comparison

7. Special Considerations for the Conventional Long-Cure Protocol

• Higher degree of monomer conversion (lower residual MMA)
• Less internal porosity
• More dimensionally stable result compared to the fast (1-hour) cycle

8. Condensed Master Protocol Comparison Table

9. Summary of Key Evidence

10. Clinical Recommendations

1. Conventional heat-cure splints require the greatest pre-polish investment (curing cycle + remounting + occlusal correction). This step is non-negotiable and must precede the polishing sequence. Shortcutting the long-cure cycle to the fast cycle trades polishing ease and material quality for time savings and is not recommended for definitive occlusal splints.
2. SLA-printed splints require the strictest protocol adherence in the pre-polish phase (wash quality, drying completeness, post-cure parameters). Any deviation from the validated post-cure cycle directly compromises surface hardness, biocompatibility, and polishability. Once post-cure is complete, the polishing sequence mirrors that of heat-cure acrylic.
3. Milled PMMA splints enter the polishing sequence with the best baseline surface and the fewest mandatory pre-polish steps. They achieve the most consistent post-polish Ra and the highest intrinsic stain resistance. The main polishing challenge is the peg connection zone and any steep concave milled surfaces.
4. Regardless of fabrication method, any occlusal adjustment made after polishing must be followed by re-polishing from the pumice phase at minimum. Providing a patient with an unpolished adjusted zone is a recognised source of early biofilm accumulation, discomfort, and splint failure.
5. Surface profilometry verification at Ra ≤ 0.2 µm before delivery should be the laboratory standard for all three fabrication routes, particularly for high-bruxism patients where surface degradation is accelerated by occlusal loading and repeated cleansing.