3D printing of prostheses: Mechanical properties

3D printing of dentures is increasingly becoming a focus of dental research and application. Additive manufacturing opens up new possibilities, particularly in the production of denture bases—but how does their mechanical strength compare to conventional materials?

Denture bases play an important role in removable prosthetics and are manufactured either using traditional methods such as injection molding or digitally – subtractive or additive (AM, 3D printing). 3D printing of dentures and denture bases, in particular, is a focus of interest. The most important additive manufacturing processes for resin-based dental materials are stereolithography (SLA) and digital light processing (DLP). According to the ISO/ASTM 52900 standard,^[1]  DLP and SLA work according to the principle of Vat polymerization, in which resin layers are cured by ultraviolet light or laser exposure.

Why 3D printing of prostheses is interesting

The advantages of additive manufacturing – including 3D printing of prostheses – lie in shortened production times and reduced material consumption. While milled denture bases are subject to material loss and tool wear, 3D printing of prostheses only results in material loss for the support structures. The denture bases are built up layer by layer from light-curing resins, then post-processed and polymerized.

Although the required physical properties of conventionally manufactured denture bases (e.g. injection-moldable plastics) are standardized by ISO 20795-1[2], some flexible base materials investigated in previous studies could not meet these requirements.[3] In addition, specific standards for testing plastics for 3D printing of definitive denture bases are currently lacking. 


material and methods

Three 3D printing resins for the additive manufacturing of the prosthetic bases 

• Lucitone Digital Print, LDP, Dentsply Sirona
• Flexcera Base, FCB, Desktop Health and 
• an experimental material, EXP

and a plastic for the injection process 

• IvoBase Hybrid, IBH, Ivoclar AG

were analyzed. Standardized test specimens (Fig. 1) were manufactured using additive manufacturing and injection molding, polished, tempered (37 °C in deionized water), and thermally stressed (5 °C to 55 °C in deionized water). Subsequently, tests were performed for:

• Flexural strength (σf)
• Elastic modulus (E)
• Fracture toughness (KIC)
• Fracture work (ωe)
• Hardness (HM)

Results

• The conventional material IBH showed the lowest strain and plastic deformation as well as the highest values for σ_f, E and HM.
• The K_IC– and ω_e-Values of IBH and EXP were lower than those of LDP and FCB.
• Aging affected the mechanical properties of the 3D-printed resins but had no measurable impact on the IBH.
• LDP and FCB can be considered as high-toughness materials since they meet the ISO 20795-1:2013 limits of 1,9 MPa√m for K_IC and 900 J/m² for ω_e exceed.
• The 3D-printable polymers showed σ_f-Test a high bending without fractures, with plastic deformation (Fig. 1, right) and σ_f-values below the 60 MPa standard for denture bases. 
• For flexible materials in 3D printing of prostheses or prosthetic bases, a tensile test design could therefore be more suitable than the classic bending test in the future.

Conclusion

3D printing of dentures and denture bases opens up new possibilities for the production of removable dental prostheses – particularly due to economic advantages and the high toughness of some resins. However, compared to an injection-molded reference polymer (IBH), the tested resins showed lower flexural strengths in some cases – some below the ISO limit of 60 MPa – and were sensitive to artificial aging. At the same time, LDP and FCB achieved high values for fracture toughness and fracture energy. 

The authors therefore advocate for adapted testing methods, such as tensile tests, to adequately evaluate ductile material behavior. 3D-printed prostheses are not a blanket replacement for proven methods, but they can be a sustainable addition – provided the testing standards and indication selection are appropriate for the material.