3D analysis of incisive canal morphological changes and proximity to maxillary incisor roots following anterior tooth movement: a comparison of aligner and fixed appliance therapy

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Abstract
This study aims to assess morphological changes, the spatial relationship between the maxillary central incisor (U1) and the incisive canal (IC), and root resorption in non-extraction cases treated with either clear aligners or fixed appliances using 3D-CBCT. Two hundred eight maxillary central incisors from 104 patients underwent maxillary incisors retraction without extraction using clear aligners (n = 60) or fixed appliances (n = 44). Measurements, including IC width, root-IC distance, cortical bone width, IC height, and root resorption, were taken before (T1) and after (T2) treatment at specified heights (H1, H2, and H3; 2, 4, and 6 mm above the labial cementoenamel junction of U1). IC width increased with aligners but decreased with fixed appliances, particularly at H1. IC cortical bone width decreased at various levels, with more pronounced reductions at H1 and H3 in the fixed group. The U1 root-IC distance also decreased significantly in both groups, especially in the fixed group at H1 (0.73 ± 0.86 mm vs 0.40 ± 58 mm, P = 0.042), H2 (0.88 ± 0.94 mm vs 0.47 ± 0.78 mm, P = 0.024), and H3 (0.73 ± 0.92 mm vs 0.30 ± 0.98 mm, P = 0.008). Root-IC proximity patterns varied notably, with higher separation rates in the aligner group and greater approximation, contact, and invasion rates in the fixed group. Root resorption was more significant in the fixed group than in the aligner group (1.01 ± 0.74 mm vs 0.32 ± 0.56 mm, P < 0.001). Factors such as treatment duration, U1 movement, and U1-IC distance were identified as primary predictors of root-IC proximity changes. These results highlight the need for careful treatment planning to preserve root health and prevent IC-related risks. Clear aligners may be preferable in non-extraction cases, particularly when the upper incisor is near the IC or requires more retraction, as they could reduce root resorption while maintaining IC dimensions better than fixed appliances.

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