Three dimensional analysis of maxillary retromolar alveolar bone before and after en‐masse distalization

Purpose: The objectives of this study were: 1.To study the changes in the maxillary retromolar alveolar bone dimensions in response to Modified Palatal Anchorage Plate (MPAP) assisted maxillary en‐masse dental distalization, and 2. To evaluate the bone level at the distal surface of the distalized second molar. ❧ Methods: In this retrospective study, the pre‐ and post‐treatment CBCTs of 41 Class II subjects treated with the MPAP appliance were analyzed using Invivo 3D cephalometric analysis software (version 5.3; Anatomage, San Jose, Calif). Four landmarks for reference, five landmarks bordering the three dimensional limits of the maxillary retromolar bone, and the CEJ at the distal surface of the second molar were digitized for each subject pre‐ and post‐treatment. The following linear measurements were calculated: length, height and width of the retromolar alveolar bone, and the bone level at the distal surface of the second molar. The pre‐treatment retromolar alveolar bone dimensions were compared with that of 54 normal occlusion subjects to rule out abnormal maxillary growth in class II subjects. A paired t‐test was used to analyze the changes in retromolar alveolar bone dimensions before and after treatment with significance at α=0.05. ❧ Results: The mean (±SD) pretreatment maxillary retromolar alveolar bone measurements (width, height, and length) for the class II malocclusion group were as follows: 13.36 ± 2.06 mm, 7.86 ± 1.99 mm, 7.29 ± 3.14 mm. The values were not significantly different from that of the normal occlusion group (12.98 ± 2.02 mm, 7.92 ± 1.96 mm, 7.88 ± 2.01 mm). After undergoing treatment, the retromolar alveolar bone width, height and length significantly decreased (p<0.001) to 12.08 ± 2.58 mm, 6.36 ± 2.21 mm and 5.14 ± 2.71 mm, respectively. Statistical comparison between the actual post‐treatment length 5.14 mm and the expected post‐treatment value of 4.57 mm (pre‐treatment length 7.29 mm minus amount of molar distalization 2.72 mm) showed significant increase in the total bone length during treatment by 0.57mm. Bone coverage at the distal surface of the maxillary second molar (2.04 ± 1.33 mm for pre and 2.03 ±1.28 mm post treatment) did not show significant change. ❧ Conclusion: As maxillary teeth are distalized, they move into bone that is smaller in the dimensions of length, width and height. In all 41 treated cases, there was sufficient bone length remaining post‐distalization. Together with the observed bone deposition at the leading end, it can be concluded that there is sufficient retromolar bone length to accommodate maxillary dental distalization. This study also showed that maxillary en‐masse distalization with the MPAP appliance does not lead to periodontal defects at the distal surface of the second molar. However, further studies must be done to evaluate if there is active bone remodeling in all three dimensions to maintain the bony housing around the distalized second molar. It would be beneficial for clinicians to use the method described in this study to assess the available retromolar alveolar bone when planning maxillary en‐masse distalization.