Torque expression in directly printed and thermoformed clear aligners: a controlled in vitro biomechanical comparison of power ridge effects

BACKGROUND: Torque control of anterior teeth remains a major challenge in clear aligner therapy. Power ridges have been introduced to improve buccolingual root control; however, their biomechanical performance in directly printed aligners remains insufficiently investigated. The aim of this study was to compare the torque efficiency of power ridges in thermoformed and directly printed aligners. METHODOLOGY: A controlled in-vitro study was conducted using a standardised maxillary model requiring incisor torque correction. Four aligner groups were evaluated: Zendura Flex thermoformed aligners (Bay Materials LLC, USA) with (ZF-PR) and without (ZF-C) power ridges, and two directly printed aligners with power ridges fabricated from TA-28 (Graphy Inc., Seoul, South Korea) and DCA (LuxCreo Inc., Bellevue, WA, USA) resin materials. Power ridges were digitally positioned on the labial gingival third of the maxillary central incisors. Biomechanical testing was performed using the Orthodontic Measurement and Simulation System (OMSS) to assess torque expression, root displacement, and force-moment systems under controlled temperatures (25 °C and 37 °C). RESULTS: Torque expression demonstrated a clear material-dependent pattern. ZF-C, used as the baseline, showed moderate and consistent torque expression (38-39% of the programmed torque) across temperatures, with no significant temperature-dependent differences. DCA exhibited the highest torque expression, reaching 62% at 25 °C and 47% at 37 °C. ZF-PR showed intermediate performance (32-42%) with moderate temperature-related variation. TA-28 demonstrated the lowest torque expression (10-17%) with no significant temperature effect. Secondary outcomes supported these findings, with moment generation and displacement patterns correlating with torque expression, whereas force magnitude did not. CONCLUSIONS: Torque expression in clear aligner systems is primarily governed by material properties and thermomechanical behaviour. Design features alone cannot fully compensate for limitations in material performance. Effective torque delivery depends on the generation and transmission of an appropriate force-moment system rather than on force magnitude alone.