Methods: Pre-treatment CBCT images (Newtom VG, 0.25mm voxel-size) of 150 Chinese orthodontic patients (12-30 years, either gender) with unilateral impacted canines were randomly coded and analyzed in Dolphin-3D and Mimics programs. Maxillary lateral (U2), central (U1) incisors and first premolar (U4) with or without RR (surface defects>1mm in length and depth) on the impaction and contralateral normal sides were counted. RR prevalence was defined as Numberteeth with RR/Numberteeth examined. Based on the position of the canine cusp-tip relative to U2 root long-axis and mesial surface, impacted canines were categorized into 6 groups: mid-buccal, disto-buccal, mesio-buccal, mid-palatal, disto-palatal and mesio-palatal positions. RR prevalence difference between impacted and control sides was compared and RR distribution with impacted-canine positions were analyzed by Chi-square and Fisher’s exact tests.
Results: Overall RR was detected on the impaction-side U2/U1/U4 roots with a 25%/14%/10% prevalence, respectively, significantly greater than their control-side counterparts (1%/1%/0%, respectively). Impaction-side RR prevalence varied significantly (P<0.05) with impacted-canine positions. RR on U2 was most prevalent when canines were at mid-buccal (50%), mesio-palatal (37.4%) and mid-palatal (33.3%) positions; RR on U1 was most prevalent when canines were at mesio-palatal (34.6%) and mesio-buccal (23.8%) positions; RR on U4 was most prevalent when canines were at disto-palatal (23.5%) and disto-buccal (14.9%) positions. When canine positions were combined, RR at U2/U1/U4 were most prevalent when canines were at middle/mesial/distal (42.1%/29.8%/17.2%, Chi-square tests, P<0.05) positions, respectively.
Conclusions: Maxillary canine impaction increased the prevalence of root resorption at adjacent lateral, central incisors and first premolars. The mesio-distal proximity of these roots to impacted maxillary canine cusp-tip influences their relative risk of root resorption.