Dental 3D printing: surface treatment of resin objects

The surface treatment of the printed object also plays an important role in dental 3D printing. This article presents the study of a research team at LMU Munich that is working on the surface treatment of objects printed from resin. Polishing, varnishing or coating - which method has what influence on the properties of the printed object? Read more in this article.

Dental 3D printing: The advances in 3D printing technology and especially in the 3D Print of resin have made it possible to produce permanent fixed dental prostheses. The prerequisites for these restorations are essential material properties such as long-lasting color stability, high biocompatibility, strong mechanical properties, abrasion resistance and low surface roughness. In particular, 3D-printed restorations must have a high conversion rate for biocompatibility. In addition, a smooth surface with a roughness of no more than 0,2 μm is necessary to prevent plaque deposits and increased material wear.

Dental 3D printing with resins: surface treatment

However, resin restorations are still susceptible to color instability and discoloration, which can be influenced by factors such as material composition, post-processing and surface treatment. Surface treatments such as 

• Polishing,
• Paint, 
• Coating 

can improve the quality of polymer-based restorations. Polishing optimizes the aesthetic appearance, reduces microbial adhesion and improves color stability and mechanical properties. Applying a polymerizable, low-viscosity varnish can also improve the surface quality and offer another opportunity for customization.

Studies have shown that varnishing can produce smoother, more color-stable surfaces than conventional polishing. However, studies on varnishing or coating 3D printed restorations are currently limited. Therefore, the study presented here aimed to evaluate the effects of different surface treatments on the chemical and mechanical properties of a printed resin object and to compare these results with two veneering composites.

The study hypothesizes that neither the material type nor the surface treatment (painting, coating, polishing or no treatment) could affect the conversion rate, surface roughness, Martens parameters, flexural strength or material loss after 3-body wear.

material and methods

A total of 288 test specimens were produced in two different geometries (Fig. 1).

In vitro investigation

While one material has been manufactured using 3D technology (VarseoSmile Crown^more; BEGO GmbH & Co KG), two different veneering composites were polymerized in silicone molds using the incremental layering technique (GRADIA PLUS; GC EUROPE NV [abbreviated to: GC-VK] and VITA VM LC flow; VITA Zahnfabrik H. Rauter GmbH & Co KG [VITA-VK]). The surface of these materials was either

• varnished (OPTIGLAZE color; GC EUROPE NV [GC varnish], VITA AKZENT LC; VITA Zahnfabrik H. Rauter GmbH & Co KG [VITA varnish],
• polished (goat hair brush + polishing paste (Abraso Starglanz; bredent GmbH & Co KG), silicone polisher)

or

• coated (VarseoSmile Crown^more; 1999. 

Materials that did not receive any surface treatment were designated as control groups. After seven days of storage in water at 37 °C, the conversion rate, surface roughness, Martens parameters, biaxial bending strength and 3-body wear after 200.000 cycles were examined. The measured data were statistically evaluated using IBM SPSS Statistics v29.0 version, using Kolmogorov-Smirnov, Mann-Whitney U, Kruskal-Wallis and Spearman correlation tests (α=.05).

Results

After polishing, the printed object showed a higher degree of conversion (Fig. 2), increased surface roughness, higher 3-body wear, but lower Martens parameters compared to the two veneering composites. After polishing with a goat hair brush, the resin printed object had a lower flexural strength than VITA-VK. For the printed object, both polishing with the goat hair brush and the application of the GC varnish reduced the surface roughness, while the application of the VITA varnish resulted in the lowest 3-body wear (Fig. 3).

For both veneering composites, polishing with goat hair brushes resulted in low surface roughness and lower 3-body wear as well as high elastic penetration modulus and high flexural strength. Polishing with silicone polishers resulted in low elastic penetration modulus for both the resin-printed object and GC-VK as well as reduced flexural strength for the veneering composite. The coating process also produced a lower elastic penetration modulus compared to the control group and higher 3-body wear compared to the application of the GC varnish. Therefore, polishing with silicone polishers and the coating process cannot be recommended.

Conclusion

Dental 3D printing: Currently, resin printed objects have weaker chemical and mechanical properties compared to traditional veneer composites for permanent dental restorations. This requires subsequent surface treatment such as polishing to improve the material properties. Polishing with a goat hair brush can be recommended for all materials tested in this study, with painting being a promising alternative for the resin printed object especially in terms of surface roughness and 3-body wear.