Does the Phase-One Functional Therapy Increase the Risk of an External Apical Root Resorption Following the Phase-Two Fixed Orthodontic Treatment? A Pilot Study.

: This retrospective study aimed to analyze the frequency and extent of apical root resorptions (EARR) during orthodontic treatment in the upper and lower incisors, as well as lower molars, using orthopantomograms (OPG). Potential influencing factors such as age, gender, root shape, type of orthodontic appliance, and treatment duration were examined as well. : A total of 57 patients who completed their treatment at the orthodontic department of the Goethe University of Frankfurt between 2011 and 2018 were included in the study. These patients had a combined total of 570 teeth, which were divided into two groups. Group 1 consisted of 20 patients (average age at T0: 10.1 ± 1.2 years old) received a one-phase fixed orthodontic treatment using passive self-ligating Damon bracket system (average duration of 2.1 years ± 6 months), while group 2 consisted of 37 patients (average age at T0: 12.4 ± 2.8 years old) underwent a two-phase therapy, which involved a phase-one functional therapy (average duration of 1.7 years ± 6 months) prior to the phase-two fixed orthodontic treatment with the Damon system (average duration of 1.5 ± 4 months) with a total treatment time of 3.2 years ± 7 months. To determine the extent of post-treatment root resorption of the upper and lower incisors, as well as the first lower molars, crown-root ratio was calculated for each tooth using the pre- and post-treatment OPGs. Additionally, each tooth was assigned a degree of resorption according to the Levander and Malmgren classification. The inter-group comparisons were conducted using the Wilcoxon Mann-Whitney U test. Spearman's correlation analysis was used to assess the relationship between age, treatment duration, and EARR. The association between gender, root morphology, and EARR was evaluated using the Wilcoxon Mann-Whitney U test. For nominally scaled variables, the Chi-square test was used. The statistical significance was set at < 0.05. : No statistically significant differences were seen between groups 1 and 2 regarding the degree of root resorption ( = 0.89). The study found that the average root resorption for all examined teeth was -5.14%, indicating a slight reduction in the length of the tooth roots after orthodontic treatment. However, no significant differences were observed concerning gender, age, type of orthodontic appliance or treatment duration. Comparisons between upper and lower jaws also did not yield statistically significant differences. The majority of teeth in the study exhibited a normal root shape. The short root length and a pipette formed roots were significantly associated with a higher risk of root resorption ( = 0.001). : The study's findings suggest that the two-phase orthodontic treatment does not increase the risk of EARR compared to one-phase therapy significantly. Some degree of root resorption occurred as a result of orthodontic treatment in both groups. Notably, abnormal root forms were identified as influential factors that could help predict the likelihood of root resorption following orthodontic treatment.