Are aligners capable of inducing palatal bodily translation or palatal root torque of upper central incisors? A biomechanical in vitro study.

Objectives: Previous studies have shown that aligners have limited ability to control root movements. The purpose of this study was to investigate which modification geometry and foil thickness are optimal for generating the force-moment (F/M) systems required for palatal root torque of maxillary central incisors.

Materials And Methods: Tooth 11 was separated from a maxillary acrylic model and connected to a movement unit via a 3D F/M sensor. Different modification geometries (crescent, capsular, double-spherical) with different depths were digitally implemented in the labio-cervical region of tooth 11 to induce an increased contact force. We evaluated the F/M systems exerted by aligners with thicknesses of 0.4-1.0 mm. F/M measurements were taken with tooth 11 in the neutral position and during palatal displacement of tooth 11 (simulating its initial clinical movement).

Results: The mechanical requirements of palatal root torque are a palatally directed force (- Fy) and a palatal root torquing moment (- Mx). These requirements were reliably achieved with modification depths > 0.5 mm. The modification depth and foil thickness had a significant influence on - Fy magnitudes (linear mixed-effect models, p < 0.01). With the 0.75-mm aligners combined with 1.5-mm deep modifications, the palatal root torque range (palTR) started after an initial palatal crown displacement of 0.09, 0.12, and 0.12 mm for the capsular, crescent, and double-spherical modification geometries, respectively.

Conclusions: A relatively early start of the palatal torque range (after a 0.1-mm palatal crown displacement) and appropriate - Fy magnitudes were achieved with 0.75-mm-thick aligners containing 1.5-mm deep capsular or crescent pressure regions. Subsequent clinical trials are required to confirm the clinical effects of these modifications.

Clinical Relevance: In vitro testing indicated that modified aligners are capable of generating the F/M components required for palatal root torque of upper central incisors.