Biomechanical analysis of variable thickness direct-printed aligners for mandibular intrusion

Purpose This in vitro study evaluated the effect of global and localized thickness modifications in direct-printed aligners (DPAs) on force and moment systems during mandibular intrusion for curve of Spee leveling. A secondary objective was to compare DPA forces and moments against a thermoformed aligner (TFA) control under selected activation conditions. Methods A multi-axis sensor was used to measure three-dimensional forces and moments during programmed anterior intrusion. DPAs fabricated with TA-28 resin were tested in six configurations, all with 0.5 mm programmed intrusion and consisting of uniform thicknesses of 0.5 mm and 1.0 mm as well as selectively thickened designs. These groups were compared to a standardized TFA control group with 0.25 mm programmed intrusion. Results No statistically significant differences were observed among the DPAs configurations, indicating that localized and global thickness modifications did not significantly influence the generated forces and moments. Across all DPA groups, force magnitudes remained low and consistent. In contrast, TFA produced significantly higher and more variable forces and moments, despite a lower programmed activation. Additionally, lateral incisors consistently exhibited higher intrusive forces than central incisors, suggesting an asymmetric force distribution within the anterior segment. Conclusion The biomechanical performance of DPAs is primarily dictated by intrinsic material properties rather than localized thickness, as geometric modifications did not significantly alter force-moment expression. Compared to the TFA control, DPAs provided a more predictable and physiological force system, whereas the thermoforming process resulted in non-physiological loads despite lower activation levels. Clinically, the asymmetrical load distribution observed both in TFA and DPA, suggests that staged sequencing of the anterior teeth is necessary to optimize anchorage and improve predictability of curve of Spee leveling.