To assess changes in the morphology and relationship between the maxillary central incisor and incisive canal (U1-IC) and estimate root resorption following non-extraction orthodontic treatment with fixed appliances, based on sex, age, and skeletal sagittal and facial classifications.

All patients treated with maxillary incisors retraction using fixed appliances without extraction between 2015 and 2023 and met the inclusion criteria were included in the study. Measurements, including IC width, root-IC distance, cortical bone width (CBW), IC height, and root resorption, were taken before (T1) and after (T2) treatment at specified heights (H1, H2, and H3; 2mm, 4mm, and 6mm above the labial cementoenamel junction of U1).

The sample included 44 patients (88 maxillary central incisors), with 18 males, 26 females, 27 teenagers, and 17 adults. Of these, 10 were Class I, 34 Class II, 7 low facial, 27 average facial, and 10 high facial groups. Significant reductions were observed in IC width, CBW, root-IC distance, and U1 length/width after treatment, across sexes, age groups, sagittal classes, and facial groups, particularly at most heights. Class II subjects showed greater reductions in U1 root-IC distance, length, and width compared to Class I. The average root resorption was 1.01±0.68mm post-treatment, with higher resorption in males (P=0.004), Class II (P<0.001), and low facial groups (P=0.007). The post-treatment U1-IC relationship showed the highest rates of approximation (68.2%) at the H2 and H3 levels, while contact and invasion rates were highest at the H1 level (13.6% and 9.1%, respectively). U1 tooth movement, U1 length, IC height, and ABO index were significant predictors of a negative U1-IC relationship.

Maxillary incisors retraction during fixed orthodontic treatment impacts the U1-IC relationship and root resorption, with variations across demographic and skeletal groups, underscoring the importance of considering individual anatomical and skeletal factors in treatment planning.