Mechanical properties of materials for 3D printed orthodontic retainers

Aim: The purpose of this study was to compare the mechanical properties of materials used for orthodontic retainers made by direct 3D printing and thermoforming. Materials and methods: Twenty-one specimens (n=7) from 3 different materials (Formlabs Dental LT Clear V2 - Formlabs Inc., Somerville, Massachusetts, USA; NextDent Ortho Flex - Vertex-Dental B.V., Soesterberg, The Netherlands, and Erkodent Erkodur - ERKODENT, Germany) were manufactured and their mechanical properties were evaluated. Two of the specimen groups were 3D printed and the other one was fabricated using a material for thermoforming. The statistical methods we applied were descriptive statistics, the Kruskal-Wallis and Dunn’s post-hoc tests. Results: With respect to Young’s modulus (E), the Kruskal-Wallis test (df=2, χ2=17.121, p=0.0002) showed a significant difference between the materials for direct 3D printing of orthodontic retainers (E=2762.4 MPa±115.16 MPa for group 1 and 2393.05 MPa±158.13 MPa for group 2) and thermoforming foils (group 3, E=1939.4 MPa±74.18 MPa). Statistically significant differences were also found between the flexural strength (FS) (Kruskal-Wallis test, df=2, χ2=17.818, p=0.0001) and F(max) (Kruskal-Wallis test, df=2, χ2=17.818, p=0.0001). Conclusions: The materials tested in the current study showed statistically significant differences in their Young’s modulus, flexural strength, and F(max).