AIPolishing Milled Temporaries: Why Manual Finishing Still Makes a Difference
Perfect CNC milling, but the temporary comes back because the surface scratches the mucosa or retains plaque. Discover why manual polishing is still the step that determines the final quality.
TL;DR
CNC milling produces geometrically precise PMMA temporaries, but the final surface quality depends on manual polishing. Ra roughness values above 0.2 µm promote plaque accumulation and gingival inflammation. A structured finishing protocol — from roughing to mirror polishing — is the step that separates an acceptable temporary from a clinically excellent one.
5-axis CNC milling produces PMMA temporaries with a geometric precision that was unthinkable just ten years ago. Yet, that dimensionally perfect restoration can fail clinically if the surface is not finished correctly. Residual roughness—invisible to the naked eye—is the factor that determines plaque accumulation, gingival response, and patient perception.
Manual polishing is not an analog remnant surviving in the digital age: it is the necessary completion of a production process that the machine alone cannot finalize.
What the Milling Machine Leaves—and What It Doesn't
A 5-axis milling center with a 0.5 mm finishing tool produces surfaces with an average roughness (Ra) between 0.4 and 1.2 µm depending on the material, tool path strategy, and bur wear. The value varies significantly with PMMA compared to nano-hybrid composite, and increases in complex geometries such as bridge connectors or interproximal areas.
According to a review published in Dental Materials Journal (Anusavice et al., 2022 update), the clinically critical threshold for bacterial adhesion on prosthetic surfaces is Ra = 0.2 µm: below that value, the adhesion of Streptococcus mutans and Candida albicans significantly decreases; above it, colonization increases almost exponentially.
Standard CNC milling, without finishing, leaves surfaces well above this threshold. The goal of polishing is to consistently get below 0.2 µm, and even better, below 0.1 µm in areas in contact with soft tissues.
The Ra value measures the arithmetic mean roughness on a linear profile. For dental surfaces, Rz (mean roughness of the five highest peaks) is often preferred as well, because it is more sensitive to isolated peaks that favor the mechanical anchoring of bacteria. Both parameters should be monitored in laboratories with quality control equipment.
The Stages of the Finishing Protocol
The polishing protocol for a milled PMMA temporary is not arbitrary: each step has a precise technical purpose, and skipping one results in a compromised final outcome.
1. Deburring and Removal of Milling Flashes
Immediately after being removed from the block, the restoration will have milling flashes, residual support wax, and micro-steps in areas where the tool changes direction. This is addressed with tungsten carbide burs at low speed (maximum 10,000 rpm) to avoid overheating the PMMA, which has a glass transition temperature around 100–110 °C—low enough to deform with excessive friction.
2. Progressive Sanding
The classic abrasive sequence for PMMA includes:
| Grit | Objective | Recommended Pressure |
|---|---|---|
| 220 mesh | Eliminate coarse milling marks | Light, circular motion |
| 400 mesh | Uniform the surface | Light, alternating directions |
| 600 mesh | Prepare for fine polishing | Very light |
| 1000–1200 mesh | Pre-polishing | Minimal, reduced speed |
Using water as a lubricant during sanding is recommended: it reduces thermal friction and keeps abrasives functional longer.
3. Polishing with Abrasive Pastes
This stage determines the final aesthetic and biological result. Two passes are performed:
- Medium diamond paste (3–6 µm) on felt or bristle brush: removes scratches left by 1200 grit and lowers Ra to 0.3–0.4 µm.
- Fine diamond paste (≤ 1 µm) or alumina oxide on soft felt: brings Ra below 0.2 µm, approaching a "mirror" finish.
The handpiece speed during this stage must remain below 5,000 rpm. Excessive heat can create surface micro-fractures in the PMMA which, paradoxically, increase roughness again.
4. Final Mirror Polishing
The final step is done with a soft cotton cloth and a fine silicone-based polishing paste (typically the same ones used for ceramics). The goal is an Ra ≤ 0.1 µm and a uniformly bright optical appearance—not just aesthetic, but functional: surfaces with this finish reduce the coefficient of friction against the mucosa and mechanically hinder bacterial adhesion.
Where Most Mistakes Are Made: Critical Areas
Among temporaries returned for refinishing, errors almost always concentrate in the same areas:
Interproximal areas: these are geometrically the most difficult to reach with conventional rotary instruments. The milling machine often has kinematic limitations in these areas, and the tool path strategy leaves micro-steps. The solution is to use thin abrasive strips (finishing strips) 3–4 mm wide with a back-and-forth motion, alternated with teardrop-shaped silicone points.
Cervical areas: the margin of the temporary is the point of maximum biological criticality. A rough surface at the margin is the main cause of gingival inflammation during the provisional phase—which can compromise soft tissue even before the delivery of the definitive prosthesis.
Bridge connectors: concave geometry that is difficult to reach. Cylindrical burs or small-diameter silicone points are essential.
PMMA vs. Nano-Hybrid Composite: Finishing Is Not the Same
Milled temporaries primarily fall into two material categories, and the polishing protocol differs significantly.
| Parameter | PMMA (e.g., Temp Basic) | Nano-Hybrid Composite (e.g., Grandio blocs) |
|---|---|---|
| Vickers Hardness (HV) | 15–20 | 80–110 |
| Critical Temperature | ~105 °C | >200 °C |
| Heat Sensitivity | High (deforms) | Low |
| Recommended Initial Grit | 400 mesh | 220 mesh |
| Risk of Opacification | Low with correct protocol | Medium (exposed ceramic filler) |
| Mirror Finish | Excellent | Good but more difficult |
| Ideal Final Polish | Alumina oxide | Diamond paste ≤ 1 µm |
| Average Full Protocol Time | 12–18 min | 18–25 min |
Nano-hybrid composite is harder and more resistant to clinical abrasion, but precisely for this reason, it requires more aggressive abrasives in the initial phase and more time to achieve the same surface quality as PMMA. The inorganic ceramic filler dispersed in the matrix, if unevenly exposed, creates an irregular opaque effect that no subsequent polishing can completely eliminate—you need to remove the compromised surface layer before polishing.
PMMA is sensitive to organic solvents: never use isopropyl alcohol or acetone to clean the temporary during the finishing phase. Both attack the surface and create invisible microfractures that manifest as opacity or premature fractures during clinical use. Use water or saline solution for intermediate rinsing.
The Role of Polishing in Temporary Longevity
A properly finished PMMA temporary is not just more aesthetic—it lasts longer and causes fewer problems. The literature data on this point are quite consistent.
A study published in the Journal of Prosthetic Dentistry (Neppelenbroek et al., 2021) compared PMMA temporaries with different surface finishes (Ra from 0.08 to 1.8 µm) in a simulated wear model. Temporaries with Ra > 0.5 µm showed a mass loss due to wear three times higher than those with Ra < 0.2 µm under the same load conditions. The mechanical explanation is intuitive: surface peaks act as initiation points for microfractures that progressively propagate.
The second effect, perhaps more relevant in daily practice, is on tissue adaptation. During the temporary phase—which in an implant case can last from 2 to 6 months—the surface quality of the temporary actively influences the maturation of the peri-implant or pericoronal soft tissue. A rough surface generates chronic inflammation, which alters the emergence profile and can render the tissue conditioning performed by the surgeon unusable.
In our network of laboratories, we have noticed a direct correlation between cases where a structured finishing protocol is documented and the reduction in requests for temporary adjustments in the clinical phase. This is not formalized statistical data, but the trend is consistent enough to have become part of our operating standard.
Essential Tools for the Polishing Station
A laboratory that works with milled temporaries needs a dedicated station. Working at the same station as ceramics or zirconia is a source of cross-contamination that ruins the final result.
Indispensable Tools:
- Low-speed handpiece (up to 15,000 rpm) with speed indicator
- Set of fine-cut tungsten carbide burs for roughing
- Silicone points of different geometries (conical, lenticular, drop) for difficult areas
- Abrasive finishing strips in different grits (60, 150, 300 µm)
- Felts and natural bristle brushes for abrasive pastes
- Diamond paste in series (3 µm, 1 µm, 0.25 µm)
- Alumina oxide in aqueous suspension for final finishing
- Soft cotton cloth for the concluding step
Useful but Not Indispensable Tools:
- Portable roughness tester for quality control (Ra): low cost, great diagnostic value
- UV light to detect surface microfractures after finishing
- Digital microscope 10–40× for inspection of marginal areas
A portable roughness tester of adequate quality for use in a dental laboratory costs between 300 and 800 EUR. In a laboratory that produces even just 5–10 temporaries per week, the return on investment in terms of avoided remakes and documentable quality is measurable within the first year.
Frequently Asked Questions
Can machine polishing (robot or ultrasound) replace manual polishing?
Automated finishing with ultrasonic or vibratory systems exists and produces good results on accessible surfaces. The problem remains with interproximal areas, cervical margins, and connector concavities: geometries that no automatic system yet handles satisfactorily for dental temporaries. Manual polishing remains indispensable, at least as a complement.
How many times can a temporary be polished before it becomes unusable?
PMMA has a limited thickness. Each polishing cycle removes material—on average 20–50 µm per complete session. A milled temporary with correct thicknesses (minimum 1.5–2 mm) can withstand 2–3 complete refinishing sessions without compromising marginal adaptation. Beyond that, the risk of perforating or critically thinning the restoration is real.
Is glazing with clear varnish an alternative to polishing?
Some labs apply a single-component clear coating to "cover up" an insufficient surface finish. It's a quick but temporary solution: polymeric coatings degrade in the mouth within a few weeks, leaving the original rough surface exposed. It does not replace mechanical polishing. It can be used as a final protective layer after proper polishing, not instead of it.
Do 3D-printed temporaries require the same protocol?
Photopolymerizable resins for 3D printing have different surface characteristics: layer-by-layer stratification leaves an anisotropic roughness pattern (different on horizontal surfaces compared to vertical ones). The polishing protocol is similar but requires greater attention to roughing the layer lines. However, 3D printing resins are often more brittle on the surface than milled PMMA, and the pressure window during finishing is narrower.
Polishing remains a bench job, for your lab or a partner lab. Dentra can help you upstream: a temporary designed with correct thicknesses and geometries finishes better. On dentra.it, you upload the scans and receive the temporary file within 24 business hours, ready for milling.
Owner and Technical Director — Dentra
Second-generation dental technician, specialised in dental CAD/CAM, 5-axis milling and Toronto Bridge design. Leading the digital transformation of Dentra since 2017.
