Risk factors for external apical root resorption in orthodontics: the role of treatment modalities and tooth-related conditions

External apical root resorption (EARR) is a multifactorial and often irreversible complication observed during orthopaedic and fixed appliance treatment. This study aimed to identify possible risk factors for orthodontically induced EARR and to determine whether there is an increased risk for carious and restored teeth. In this retrospective cohort study, initial panoramic radiographs were used only to screen for missing or impacted teeth, whereas EARR and tooth-related conditions were assessed exclusively on paired initial and final periapical radiographs. EARR was scored with the modified Malmgren classification and dichotomized as absent (Grade 0) or present (Grades 1–3). Potential patient-, treatment-, and tooth-related factors were then evaluated for their associations with EARR. Associations were estimated with patient-clustered logistic generalized estimating equations (GEE) using robust standard errors. Among 3232 maxillary incisors from 808 patients, 1475 (45.6%) had Grade 1–3 EARR. In the adjusted GEE model, female sex was associated with lower odds (aOR, 0.70; 95% CI, 0.53–0.94), whereas older age at active-treatment start (per year: aOR, 1.14; 95% CI, 1.07–1.21), longer active-treatment duration (per year: aOR, 1.49; 95% CI, 1.20–1.84), premolar extraction (aOR, 2.17; 95% CI, 1.59–2.97), pointed roots (aOR, 1.51; 95% CI, 1.23–1.87), short roots (aOR, 1.89; 95% CI, 1.34–2.66), short-pointed roots (aOR, 2.06; 95% CI, 1.36–3.12), and Rapid Palatal Expansion (RPE) plus fixed treatment (aOR, 1.75; 95% CI, 1.26–2.43) were associated with higher odds. Caries/restoration status and restoration type were not statistically significant. Older age at active-treatment start, longer active treatment, premolar extraction, non-normal root morphology, and RPE plus fixed treatment were associated with higher EARR odds, whereas female sex was associated with lower odds. Caries/restoration status and restoration type were not associated with EARR after patient-clustered analysis. Pretreatment root morphology and anticipated treatment complexity and duration should inform EARR risk assessment and radiographic monitoring.