Summary Introduction. Although severe root resorption is rare, it is a side effect of orthodontic treatment which affects tooth prognosis. Patients with severe dentofacial deformity, for whom orthodontic treatment and orthognathic surgery was done at the age of 18 and later, had long duration orthodontic treatment and orthognathic surgery, and are at a high risk of root resorption. The impact of orthognathic surgery on root resorption has not been sufficiently studied, and therefore is an interesting topic to research. Aim of the Study. To identify the risk factors for apical root resorption of maxillary incisors and canines as a result of orthodontic and surgical treatment of Class III malocclusion involving LeFort I osteotomy. Material and methods. The root lengths of upper incisors and canines were measured on cone beam computer tomography (CBCT) scans obtained from a database of orthognathic surgery patients. As a criteria for root resorption was chosen the difference in root lengths between different time points. The measurements were performed using the scans taken before orthodontic treatment (T1), before surgery (T2), and after post surgery orthodontic treatment (T3), of 28 subjects, aged 20.5 ± 3.81 years, with the mean presurgery treatment time of 19.9 ± 8.8 months, and post-surgery time of 7.1 ± 3.1 months. Changes in root lengths during different time spans were correlated with treatment duration, the initial crown/root ratio, and the severity of dentofacial deformity (Wits appraisal, ANB angle, and overjet). Results. During T1 - T2 the roots of the lateral incisors shortened by a maximum of 0.78 ± 0.83 mm (p < 0.001), at a rate of 0.04 mm per month. During T2 - T3 the lengths of the central incisor roots decreased most by 0.49 ± 0.52 (p < 0.001) at a rate of 0.07 mm per month. The resorption speed for canines increased from 0.03 mm to 0.1 mm per month before and after surgery. There were statistically significant correlations between the crown-root ratio and the incisor root length (r = 0.319 for lateral and r = 303 for central, both p<0,05) and for canines (r = 482, p<0.01). The associations between the shortened root length, in different time spans for different teeth, and the severity of malocclusion were inconsistent. Conclusions. Overall, the shortened root length during combined orthodontic and surgical treatment might not be clinically significant. After surgery, the rate of root resorption (mm per month) increased, especially for canines. The teeth with initially shorter roots showed more resorption during treatment.

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