Wear resistance and flexural properties of low force SLA- and DLP-printed splint materials in different printing orientations: An in vitro study

Objectives: To investigate the influence of material and printing orientation on wear resistance and flexural properties of one low force SLA- and two DLP-printed splint materials and to compare these 3D -printed splints to a subtractively manufactured splint material. Methods: Two DLP-printed (V -Print splint, LuxaPrint Ortho Plus) and one low force SLA-printed (Dental LT Clear) material, where specimens were printed in three printing orientations (0, 45, 90), were investigated. In addition, one milled splint material (Zirlux Splint Transparent) was examined. A total of 160 specimens were produced for both test series. The two -body wear test was performed in a chewing simulator (80 ' 000 cycles at 50 N with 5-55 C thermocycling). Steatite balls were used as antagonists. The wear pattern was analyzed with a 3D digital microscope in terms of maximum vertical intrusion depth (mm) and total volume loss (mm3). The flexural properties were investigated by three-point bending in accordance with ISO 20795-1: 2013 (denture base polymers). The flexural strength (MPa) and the flexural modulus (MPa) were measured. Two-way ANOVA was performed to investigate the effects of the two independent variables material and printing orientation for the three 3D -printed materials. The comparison of the printing orientations within one material was carried out with one-way ANOVA with post -hoc Tukey tests. Results: Two-way ANOVA revealed that wear and flexural properties are highly dependent on the 3D -printed material (p < 0.001). Across groups, a significant effect was observed for wear depth (p = 0.031) and wear volume (p = 0.044) with regard to printing orientation but this was not found for flexural strength (p = 0.080) and flexural modulus (p = 0.136). One-way ANOVA showed that both DLP-printed groups showed no significant differences within the printing orientations in terms of wear and flexural properties. Dental LT Clear showed that 90 oriented specimens had higher flexural strength than 0 oriented ones (p < 0.001) and 45 oriented specimens also showed higher values than 0 ones (p = 0.038). No significant differences were observed within the printing orientations for flexural modulus and wear behaviour within this group. T -tests showed that the milled splints exhibited statistically higher wear resistance and flexural properties compared to all three 3D -printed splint materials (p < 0.001) and that highly significant differences were found between the 3D -printed splint materials for both test series. Conclusion: Within the limitations of this in vitro study, it can be stated that wear behaviour and flexural properties are highly dependent on the 3D -printed material itself. Currently, milled splints exhibit higher wear resistance and flexural properties compared to 3D -printed splint materials. The printing orientation has a minor influence on the properties investigated. Nevertheless, two-way ANOVA also showed a significant influence of printing orientation in the wear test across groups and one-way ANOVA detected significant effects for SLA material in terms of flexural strength, with printing in 90 showing the highest flexural strength. Therefore, anisotropy was found in SLA material, but it can be limited with the employed printing parameters. Both DLPprinted materials showed no significant difference within the printing orientation.