The anisotropicity of the flexural properties of an occlusal device material processed by stereolithography

Although additive manufacturing enables the production of occlusal devices with stereolithography (SLA), not much is known about the mechanical properties of the device produced.

The purpose of this in vitro study was to evaluate the effect of SLA printing direction on flexural strength and modulus of elasticity. In addition, water sorption and surface topography were studied.

The material used in this study was an epoxy-based resin (Somos WaterShed XC 11122; DSM). Three groups of test specimens (n=10/group) were fabricated using different printing nozzle directions: horizontally, vertically, and at a 45-degree angle to the direction of the long axis of the test specimen. Control specimens were fabricated from an autopolymerizing acrylic resin (Palapress; Heraeus Kultzer GmbH). Water sorption of the 3 test groups was compared. For the 3-point bend test, dry and water stored (14 days) specimens were tested. Flexural strength and flexural modulus were measured. Surface topography was measured with a noncontacting optical profilometer. Two-way analysis of variance (ANOVA) followed by the Tukey HSD post hoc test was used to analyze the effect of different printing directions and the effect of water storage on the flexural strength and flexural modulus of the material. Two-way ANOVA was also performed to differentiate the effect of the material on flexural strength and flexural modulus (α=.05).

The flexural strength results for the dry test specimen groups were vertical 77.1 MPa, horizontal 73.4 MPa, 45 degrees 78.3 MPa, and control 83.8 MPa. After water sorption, the values were 45.1 MPa, 36.6 MPa, 41.7 MPa, and 80.6 MPa. The flexural moduli for the dry test specimen group were vertical 2.2 GPa, horizontal 2.0 GPa, 45 degrees 2.1 GPa, and control 2.3 GPa. After water sorption, the values were 1.3 GPa, 1.0 GPa, 1.1 GPa, and 2.3 GPa. Weight gain after 14 days of water sorption by group was 1.84% for vertical, 1.80% for horizontal, 1.75% for 45 degrees, and 1.54% for controls. The difference was not statistically significant when the 45- degree group and the vertical group were compared (P=.742) but was significant between the vertical and horizontal group (P<.001) and the horizontal and 45-degree group (P<.001). A statistical difference was found (P<.001) for both the storage (dry versus water storage) and the material (Somos WaterShed versus Palapress).

This study revealed that water storage had a stronger effect on the flexural properties of SLA-manufactured occlusal device material than different printing directions. However, anisotropicity was found with regard to the printing direction. Concerning the material's ability to resist fracturing, the optimal printing direction for an occlusal device is vertical.