Changes in occlusal force and contact area after extraction of premolars in orthodontic treatment : 2-year follow-up

Abstract

This study aims to evaluate the changes in occlusal contact area and force after orthodontic treatment accompanied with premolar extraction. Female patients who completed orthodontic camouflage treatment were categorized into non-extraction group (n = 36), two maxillary premolar extraction group (n = 18), and four premolar extraction group (n = 31) and measured for occlusal function before, immediately after, and 2 years after orthodontic treatment. Patients with mandibular prognathism treated with intraoral vertical ramus osteotomy, as well as Le Fort I osteotomy as necessary, were categorized into male non-extraction (n = 45), male two maxillary premolar extraction (n = 23), female non-extraction (n = 31), and female two maxillary premolar extraction groups (n = 29) and measured for the occlusal function before treatment, 1 month after orthognathic surgery, immediately after orthodontic treatment, and 2 years after orthodontic treatment. Occlusal function was measured by asking the subjects to bite the pressure-sensitive sheet at maximal strength. The measured values were analyzed using linear mixed model and Bonferroni post hoc test. 1. In female patients who underwent orthodontic camouflage treatment, there was no between-group difference in the occlusal contact area and force in each treatment period, and pattern of change (P > 0.05). 2. In female patients who underwent orthodontic camouflage treatment, the occlusal contact area and force at 2 years after the completion of orthodontic treatment recovered to the level prior to orthodontic treatment in the non-extraction group and the two maxillary premolar extraction group (P > 0.05), but the occlusal contact area in the four premolar extraction group were significantly smaller at 2 years after orthodontic treatment compared to before orthodontic treatment (P < 0.05). 3. In patients with mandibular prognathism treated with surgical-orthodontic treatment, the pattern of changes in occlusal force and occlusal contact area did not show a significant difference between groups in both male and female patients (P > 0.05). 4. In patients who received surgical-orthodontic treatment for mandibular prognathism, there was no significant difference between groups in occlusal contact area and force by treatment period in both male and female patients (P > 0.05), except after orthodontic treatment in male patients. In male patients, the occlusal contact area in the extraction group was significantly greater than in the non-extraction group after the completion of orthodontic treatment (P < 0.05). Among female patients who underwent orthodontic camouflage treatment, non-extraction group and two maxillary premolar extraction group recovered to pre-treatment level of occlusal force at 2 years after orthodontic treatment. In patients with mandibular prognathism treated with intraoral vertical ramus osteotomy, maxillary premolar extraction during presurgical orthodontic treatment did not cause significant difference in the changes in occlusal function 2 years after orthognathic treatment.