Effect of power arm length and attachment configuration on maxillary molar mesialization with clear aligners: a finite element analysis.

BACKGROUND

Maxillary molar mesialization with clear aligners is challenging because achieving controlled root movement without excessive mesial tipping remains difficult. Although power arms can modify the force system, the combined effects of power-arm length and attachment configuration on molar displacement and tipping remain insufficiently characterized. This study evaluated the effects of 6-, 8-, and 10-mm power arms combined with two attachment configurations during maxillary first molar mesialization using three-dimensional finite element analysis.

METHODS

The attachment configurations were selected to represent two clinically distinct approaches to integrating a vertical attachment with a power arm: a palatal vertical attachment combined with a buccal cut-out and a buccal vertical attachment with direct power-arm integration. Six three-dimensional finite element models were constructed by combining three power arm lengths (6, 8, and 10 mm) with two attachment configurations: a palatal vertical attachment with a buccal cut-out, and a buccal vertical attachment without a cut-out. A mesial force of 200 g was applied from a temporary anchorage device (TAD) to the power arm. Displacements were measured along the X (bucco-palatal), Y (mesio-distal), and Z (occluso-gingival) axes. Peak von Mises stress values in the periodontal ligament and aligner deformation were evaluated.

RESULTS

In all models, maxillary first molar displacement occurred predominantly in the mesial and gingival directions. Mesial crown displacement ranged from 0.068 to 0.074 mm, while distal displacement at the root apex ranged from 0.011 to 0.012 mm. Mesial tipping angles ranged from 0.155° to 0.170°, with higher tipping observed in models with shorter power arms. Increasing power arm length was consistently associated with reduced mesial tipping and lower periodontal ligament stress, but with reduced mesial crown displacement. Shorter power arms generated higher periodontal ligament stress, whereas longer power arms were associated with greater aligner deformation.

CONCLUSIONS

Power arm length and attachment configuration were associated with differences in initial tooth displacement patterns, periodontal ligament stress distribution, and aligner deformation during maxillary first molar mesialization with clear aligners. Longer power arms were associated with reduced mesial tipping and lower periodontal ligament stress, but also with reduced mesial crown displacement. The buccal vertical attachment configuration consistently produced lower mesial rotation than the palatal vertical attachment with buccal cut-out configuration at each corresponding power-arm length; this difference should be interpreted as a configuration-dependent finding within the initial biomechanical response.