Do clear aligners distalize maxillary molars bodily or by distal tipping? A systematic review introducing a prespecified biomechanical descriptor.

Abstract

Objectives: To characterize the sagittal translation-tipping balance associated with reported maxillary molar distalization outcomes during clear aligner therapy (CAT) using a prespecified model-based descriptor. Materials and Methods: A systematic review was conducted according to PRISMA guidelines and registered in PROSPERO (CRD420261306794). PubMed, Scopus, and WOS were searched from inception to February 2026. Studies reporting maxillary molar distalization by CAT and providing crown displacement (Dcrown) and angulation change (θ) were included. A prespecified sagittal rigid-body approximation was applied. Tipping-related displacement was estimated as Dtip = Leff·tan(|θ|), where Leff was set as crown-to-center of resistance lever arm. Bodily displacement was defined as Dbod = Dcrown-Dtip. The primary outcome was the BF (Dbod/Dcrown). Descriptive analyses were performed separately for maxillary first (U6) and second (U7) molars. Results: Twelve studies were included. Median Dcrown was 1.78 mm for U6 and 1.72 mm for U7. Median BF was 0.70 for U6 and 0.78 for U7. Using prespecified BF thresholds, translation-dominant movement was observed in 56.3% of U6 arms and 60.0% of U7 arms. Mixed behavior was observed in 37.5% of U6 arms and 26.7% of U7 arms, whereas tipping-dominant behavior occurred in 6.3% of U6 arms and 13.3% of U7 arms. Internal validation against root displacement showed that 66.7% of predictions were within 0.50 mm and 88.9% within 0.75 mm of observed apical movement. Conclusions: Maxillary molar distalization by CAT is more often translation-dominant than tipping-dominant; however, mixed movement patterns are common and tipping-dominant behavior can occur, particularly in U7.