Three-dimensional retrospective analysis of root resorption and incisive canal changes following maxillary incisor retraction in class I and class II extraction cases: a comparison of clear aligners and fixed appliances

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Abstract
This study aimed to compare the effects of clear aligners and fixed appliances on root resorption, incisive canal (IC) morphology, and the spatial relationship between the maxillary central incisors (U1) and the IC following incisor retraction in dental/skeletal Class I and II extraction cases. Sixty-six adult patients (132 maxillary central incisors) with comparable baseline characteristics (based on the ABO Discrepancy Index) were retrospectively analyzed. Patients received either clear aligners (n = 33) or fixed appliances (n = 33) following first premolar extraction. Pre- and post-treatment CBCT measurements were recorded at three vertical reference planes (H1, H2, H3: 2 mm, 4 mm, and 6 mm above the labial cementoenamel junction), including IC width, cortical bone width (CBW), U1–IC distance, IC height, and U1 root length/width. Both groups showed reductions in IC width, CBW, and U1–IC distance, with greater changes in the fixed appliance group. U1–IC distance was significantly reduced in the fixed group at H1 (1.79 ± 0.76 mm vs. 1.24 ± 1.04 mm), H2 (1.78 ± 1.08 mm vs. 1.33 ± 0.86 mm), and H3 (1.61 ± 1.35 mm vs. 1.16 ± 0.90 mm) (P < 0.05). Root resorption was also significantly higher in the fixed group (1.81 ± 1.55 mm vs. 0.87 ± 0.77 mm, P < 0.001). Root–IC proximity patterns included 7.6% separation, 63.1% approximation, 15.9% contact, and 13.4% invasion, with contact and invasion more prevalent in the fixed group. Root resorption increased with IC proximity, peaking at 3.65 ± 1.97 mm in invasion cases. Regression analysis identified tooth movement, root length, and inter-root distance as predictors of U1–IC proximity, while root resorption was associated with treatment duration, U1 movement, IC height, and incisor inclination (U1–SN angle). These findings underscore the importance of individualized planning and appliance selection in cases requiring maximum anterior retraction to reduce the risk of root resorption and IC-related complications.

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