Force Expressed by 3D-Printed Aligners with Different Thickness and Design Compared to Thermoformed Aligners: An in Vitro Study

(1) Background: Clear aligners are favored for their aesthetics in orthodontics, with newer 3D-printed technologies allowing the design of aligners with differential thicknesses and materials, offering advantages in terms of force distribution on the teeth, thereby optimizing treatment biomechanics. This study aimed to compare the initial and final forces of three types of 3D-printed aligners (with different thickness gradients and gingival margins) and traditional thermoformed aligners (with different gingival margins), evaluating stress relaxation and force consistency to determine which material and configuration may be optimal for better force distribution; (2) Methods: Twenty-seven 3D-printed aligners with three design variations and 18 thermoformed aligners were analyzed. Customized models were used to assess force at specific points on the upper incisor (1.1) and molar (2.6). A 3 h stress-relaxation test was conducted at 37 °C, and force data were recorded every second using a motorized compression stand. Statistical analysis was performed using ANOVA, post hoc tests, and Kruskal–Wallis tests for comparisons; (3) Results and Conclusions: Aligners with vertical and horizontal thickness gradients and a gingival margin trimmed 2 mm above the gingival contour exerted the highest forces, particularly at incisal/occlusal points. No significant differences in stress relaxation were observed. The force applied to the molars was consistently higher than the force applied to the incisors. These 3D-printed aligners with both horizontal and vertical gradients may offer a viable alternative to thermoformed aligners.