Three-dimensional arch morphology differences in the invisible orthodontic treatment of extracted and non-extracted patients

To investigate the accuracy of the mandibular curve of Spee and mandibular curve of Wilson curve, mandibular posterior crown inclination, maxillary and mandibular arch width, and mandibular incisor labial inclination changes in the invisible orthodontic treatment of extracted and non-extracted patients to provide a basis for the study of the therapeutic efficacy of the invisible aligner.

This study was approved by the Unit Medical Ethics Committee. Adult bony Class Ⅰ patients treated with invisible aligners between 2016 and 2023 were selected and categorized into 32 cases in a group with four first premolar extractions and 33 cases in a non-extraction group. The initial scanning model (T1), the orthodontic plan design model (T2), and the scanning model after alignment and leveling (T3) were collected, and the scanning data at different time points were exported to STL format files. They were then analyzed and measured using GOM Inspect 2019 software; the clinical effect (T1-T3) was defined as AC, and the expected result (T1-T2) was defined as CC. To explore the mandibular curve of Spee, the leveling accuracy of the mandibular Wilson curve (AC/CC×100%), the change in mandibular posterior crown inclination, the change in maxillary and mandibular arch width and the change in mandibular incisor labial inclination. The results were tallied using R4.3.2 software.

The leveling accuracy of the mandibular curve of Spee was 3.2% and 10.1% in the extraction and non-extraction groups, respectively; the leveling accuracy of the mandibular first molar in the extraction and non-extraction groups was 9.5% and 4.2%, respectively, and the leveling accuracy of the mandibular second premolar was 32.8% and 25%, respectively. The leveling accuracy of the mandibular curve of Wilson was 126% in the extraction group compared to 704% in the non-extraction group. The maximum values of AC for crown inclination of the mandibular posterior teeth were all found in the first molar and the minimum values in the second premolar (P < 0.05); CC was the greatest in the second premolar and the least in the first molar in the extraction group (P < 0.05), and in the second molar and the least in the second premolar in the non-extraction group (P < 0.05). There was no significant difference in the change of maxillary and mandibular arch widths in the extraction group (P > 0.05); the AC of the arch widths at the maxillary first molar and at the mandibular second premolar and first molar in the non-extraction group was significantly greater than the CC (P < 0.05). The AC of the mandibular second premolar crown inclination was significantly greater in the non-extraction group than in the extraction group (P < 0.05), showing a more pronounced buccal inclination of the crowns; in contrast, there was no significant difference between the mandibular molar crown inclination AC and CC in the two groups (P > 0.05). The CC of mandibular curve of Spee in the extraction group was significantly greater than that in the non-extraction group (P < 0.05), whereas there was no significant difference in AC between the two groups (P > 0.05). In addition, the AC of the labial inclination of the mandibular incisors in the extraction group and the non-extraction group was smaller than the CC, but both were positive, indicating an increase in the degree of labial inclination of the mandibular incisors.

The invisible aligner mandibular curve of Spee leveling was poorly expressed, and there was a tendency for overexpression of the mandibular curve of Wilson leveling. Increased labial inclination of the mandibular incisors was observed in patients. The extraction group was accurately predicted in arch width control, there was overexpansion in the non-extraction group.