Biomechanical Assessment of Root Torque in Clear Aligners: Impact of Trimline Design, Engagers, and Pressure Points...

Root torque control is essential for guiding the proper movement of anterior teeth, primarily to prevent periodontal complications. This movement, in clear aligner therapy (CAT), requires precise force application to minimize unintended displacements. This study examines the influence of trimline design, engager placement, and pressure points on torque efficiency in an upper central incisor, introducing buccopalatal compensation movement (pCM), a deliberate buccopalatal over-correction of the aligner to enhance torque expression.

Finite Element Analysis, revealing internal stress patterns not easily captured in bench testing, evaluated two design factors (trimline design flat/scalloped, engager presence with/without), and two force modifiers (pressure point configuration none/buccal/palatal/both, pCM 0.03-0.36 mm). We recorded incisor biomechanics at the incisal edge: distal force (Fd), palatal force (Fp), extrusive force (Fe), palatal root moment (Mp), distal root moment (Md), and distolingual moment (Mdl), as well as strain across periodontal ligaments and peak stress in the aligner.

Flat trimline (FT) aligners outperformed scalloped (ST) designs, generating higher Mp (4.74 Ncm vs. 1.51 Ncm), particularly when combined with two pressure points (2PP). Engagers did not consistently enhance root torque expression and often increased unwanted forces. Introducing pCM significantly reduced side effects while maintaining overall Mp to similar levels. Aligner stress exceeded the plastic deformation threshold in several configurations, predominantly those with engagers or buccal pressure points.

Trimline configuration, pressure point placement, and pCM significantly influence root torque control in CAT. Optimal force systems were achieved with specific combinations: FT designs with 0.18 mm pCM and dual pressure points without engagers, and ST designs with 0.15 mm pCM and buccal pressure points with engagers. These findings provide biomechanical evidence for optimizing aligner design, thereby enhancing the predictability of root movements and paving the way for greater efficiency of CAT as well as improved overall treatment quality.