Vat photopolymerisation 3D-printing processes for dental application

This article was first published in the ADA's News Bulletin, March 2023.

The profession of dentistry has always been keen to progress the adoption of new materials and techniques. Materials science has continued to assess the suitability of such new materials and assist their introduction. Various computerised technologies for indirect creation of restorations or restorative tools have been utilised since the 1980s. An international standard for additive manufacturing ISO 52900:2021 exists for the current range of such techniques, which include: binder jetting, directed energy deposition, material extrusion, material jetting, powder bed fusion, sheet lamination, and vat photopolymerisation (VP).

A recent review article in Dental Materials considered potential areas for further research in VP 3D printing. VP uses a layer-by-layer curing of liquid photopolymer resin by ultraviolet (UV) light. Many other review articles have covered clinical application or assessment of current technology.

This review searched PubMed and Scopus databases for articles from 2005 to 2022. A significant increase in articles occurred after 2015.

Mechanical properties

Use of VP has been explored for construction of temporary restorations. Surface roughness and strength are equal, or sometimes superior to conventional techniques. It has been observed that the orientation of print layers may be a significant consideration in achieving suitable strength.

Colour stability is comparable to conventionally heat-cured resins.

Various resin formulations have been explored for clinical applications. No standard formulation appears to have emerged. Commonly recognised dental polymer bases such as PMMA, BisGMA, UDMA, and TEGDMA among others have been explored. A significant challenge to research is the prevalence of trade secrets in manufacture of current materials.

Fillers have been explored to improve clinical properties. Titanium dioxide, silicon dioxide, seipiolite (magnesium silicate clay), graphene oxide and graphite nanoplatelets have all been successfully evaluated as having no effect on printing techniques. Pure graphene affects printability. Zirconia fillers are affected by print
orientation. Bioactive glass is now receiving attention in research, particularly with the opportunity to stimulate hydroxyapatite crystal formation.

Post curing with UV of the completed structure and heat curing have shown to improve hardness, internal fit, completeness of polymerisation, tissue compatibility and colour stability. Material flexibility was unchanged.
 

Above left: Printing direction of fabricated dentures. Above right: The trueness indicates the average deviation between the experimental data and the master data as follows: positive deviation, yellow to red; negative deviation, light blue to blue; and acceptable deviation, light green to green.

Images reproduced under terms of the CC-BY licence from: Hada T, Kanazawa M, Iwaki M, Arakida T, Soeda Y, Katheng A, et al. Effect of printing direction on the accuracy of 3D- printed dentures using stereolithography technology. Materials 2020;13:1–12.
 

Biocompatibility and antimicrobial properties

Tissue culture testing of materials has shown they are currently safe for short term contact, but may exhibit mild cytotoxicity after several days of use. Individual materials may therefore not be suitable for construction of splints or orthodontic appliances.

Washing of uncured monomer with ethanol, isopropyl alcohol or tripropylene glycol monomer has also been found to improve biocompatibility.

The use of antimicrobial agents as components in VP resin is also being explored. No mention has been made of the length of antimicrobial activity.

It has been observed that despite the relative surface roughness of VP surfaces, there may be less microbial adhesion demonstrated than with conventional dental materials. No papers on in vivo testing for biocompatibility or antimicrobial activity were found by the authors.
 

Accuracy and surface quality

Two standards currently exist relating to accuracy of fit: ISO 5725-1:1994 and ISO 12836:2015 The review of papers showed much study had been carried out on different
additive manufacturing techniques, but relatively little on VP. Studies have proved diffcult to compare.

The chief factors in maximising accuracy of any 3D printing system seem to relate to minimising distortion in the Z (vertical) axis of print. Careful design requires addition of suffcient thick support structures to maintain stability of the structure, and oblique orientation of the main appliance axis. Possibly one might think of an angled path of construction similar to the considerations of path of insertion in prosthesis design.

The authors noted that current testing again has not been performed in vivo, which may introduce other considerations as to required print accuracy.
 

Patent activity

The number of patents lodged found in the Patentscope database in 2018-21 was 38 compared to 1 in the period 2000-2005. The majority of these patents relate to materials rather than manufacturing systems. Dentistry can expect significant materials progress in the near future.
 

Conclusions

The field of 3D printing is still exploring different additive manufacturing techniques. Many commercial 3D printers are being adapted from other applications to be evaluated in dentistry. It is not possible to compare different manufacturing systems with the same test methods. VP may well prove to be a significant contributor to additive manufacturing in dentistry. The level of increasing research suggests that careful attention should be paid to this sector in the next few years.