Purpose The purpose of this study was to assess differences in the fundamental mechanical properties of resin-made three-dimensional (3D) printed orthodontic aligners according to the printing orientation. Methods Twenty resin 3D-printed dumbbell-shaped specimens and 20 orthodontic aligners were fabricated and postcured in nitrogen. Half of the specimens and aligners were built in horizontal (H), the other half in vertical (V) directions. The dumbbell-shaped specimens were loaded in a tensile testing machine, while parts of the aligners were embedded in acrylic resin, ground, polished, and then underwent instrumented indentation testing (IIT). Mechanical properties that were assessed included the yield strength (YS), breaking strength (BS), plastic strain (epsilon), Martens hardness (HM), indentation modulus (E-IT), elastic index (eta(IT)), and indentation relaxation (R-IT). Data were analyzed statistically with independent t-tests or Mann-Whitney tests at alpha = 5%. Results No significant differences were found between specimens or aligners printed either in a horizontal or a vertical direction (P > 0.05 in all instances). Overall, the 3D-printed aligners showed acceptable mechanical propertied in terms of YS (mean 19.2 MPa; standard deviation [SD] 1.7 MPa), BS (mean 19.6 MPa; SD 1.2 MPa), epsilon (mean 77%; SD 11%), HM (median 89.0 N/mm(2); interquartile range [IQR] 84.5-90.0 NN/m2), E-IT (median 2670.5 MPa; IQR 2645.0-2726.0 MPa), eta(IT) (median 27.5%; IQR 25.9-28.1%), and R-IT (mean 65.1%; SD 3.5%). Conclusion Printing direction seemed to have no effect on the mechanical properties of 3D-printed resin aligners, which are promising for orthodontic use.
