Abstract
BACKGROUND: Clear aligner therapy has become increasingly common for extraction cases, yet achieving controlled bodily mesialization of posterior teeth remains challenging, particularly in moderate anchorage situations. Clinical experience suggests that aligners often cause molar tipping and loss of anchorage, but few studies have quantified this problem under realistic clinical conditions. This study aimed to evaluate the predictability of posterior tooth movement with Invisalign in moderate anchorage cases and to compare the performance of different attachment designs. METHODS: This retrospective study analyzed 32 patients (24 females, 8 males; mean age 23.9 ± 6.4 years) treated with bilateral first premolar extractions and Invisalign aligners. Digital dental models before and after treatment were superimposed using a surface-based registration protocol to assess tooth movement in three dimensions. The designed and achieved displacements, inclinations, rotations, and tipping angles of posterior teeth were compared using paired t-tests and one-way ANOVA. RESULTS: Significant differences were observed between predicted and achieved values for mesial displacement and tipping in most posterior teeth. Maxillary first molars showed greater mesial movement and tipping than planned, while mandibular molars exhibited less movement but still notable tipping. Among attachment designs, the double attachment provided superior control of molar tipping compared to power arm and single attachment. Vertical skeletal pattern significantly influenced the expression of tipping and buccolingual inclination. CONCLUSION: In moderate anchorage extraction cases, posterior mesialization with clear aligners showed limited predictability, particularly due to molar mesial tipping. The use of double rectangular attachments appeared to provide better control of crown angulation than single attachment or power arm. These findings may help inform digital treatment planning and attachment selection for extraction cases requiring controlled posterior movement.