Aim: The aim of this study is to retrospectively determine the incidence of severe external root resorption (grade 4 according to Malmgren et al.), in maxillary incisors, during fixed orthodontic treatment and to evaluate the possible predisposing factors for root resorption. Subject and Method: The treatment records of 7000 patients who have been treated between years 1990 and 2019 were examined to determine potential predisposing factors of external root resorption, the following data was retrieved from the patients\' records : age at the beginning of the treatment, gender, root morphology, overjet, overbite, treatment modality (extraction, non-extraction), treatment duration, buccal and palatal alveolar bone thickness in the maxillary incisors region, and amount of movement of the incisal root apices and incisal edges. Patients with missing records were excluded. Surgical, removable and unfinished cases were excluded, too. All subjects with severe root resorption (Grade 4) were identified and a matching group of control subjects with minimal root resorption (Grade 1) were selected for statistical comparisons. Root morphology was assessed on panoramic radiographs according to the classification proposed by Consolaro as follows: triangular, rhomboid, pipette and dilacerated. Overjet, overbite, buccal and palatal maxillary alveolar bone thickness, and amount of movement of the incisal root apices were assessed on cephalometric radiographs. The relationship between root resorption and the predisposing factors was assessed using chi square test. Result: Severe apical root resorption was detected in 120 patients. The results have also demonstrated significant difference between the groups for the variables: increased overjet, treatment modality (extractions), increased treatment duration, cortical thickness of the alveolar bone, and amount of incisor movement at the end of the treatment. Conclusion: Extractions, intrusion, increased treatment duration, thin alveolar bone, and excessive incisor movement represent risk factors for severe root resorption in maxillary incisors following orthodontic treatment.
