Association between gonial angle and bone thickness in the infrazygomatic crest and mandibular buccal shelf regions for mini-screw insertion in adults: A retrospective study

Abstract

Introduction: Temporary anchorage devices (TADs) improve orthodontic biomechanics, with the infrazygomatic crest (IZC) and mandibular buccal shelf (MBS) as common extra-alveolar sites. Skeletal morphology, especially gonial angle as a vertical facial dimension indicator, may influence bone availability. This study investigated the correlation between IZC and MBS bone thickness and gonial angle using cone-beam computed tomography (CBCT).

Methods: CBCT scans from September 2019 to September 2021 were analysed. Bone thickness in IZC and MBS regions was measured at standardized reference points on predefined coronal and axial planes. Participants were consecutively selected from a radiology archive according to inclusion/exclusion criteria. Normality was checked with Kolmogorov-Smirnov test, and correlations were assessed using Spearman's test (P<0.05) in SPSS v22.

Results: A total of 120 adult CBCT scans were evaluated (≥18years). The mean gonial angle was 120.5° (95% CI: 119.0-122.0). IZC thickness decreased cranially, showing no significant correlation with gonial angle (P>0.05); the maximum thickness was 5.01±1.65mm at the mesiobuccal root of the maxillary first molar. In the MBS region, bone thickness, depth, and angulation increased posteriorly and all showed significant inverse correlations with gonial angle (P<0.05). Maximum MBS thickness was 7.12±2.31mm at the distal root of the mandibular second molar; minimum thickness was 1.53±0.94mm at the mesial root of the first molar.

Discussion and conclusion: The distal mandibular second molar offers the most favourable site for extra-alveolar TAD placement due to greater thickness, depth, and angulation. Inverse correlation with gonial angle suggests improved primary stability in patients with reduced vertical facial height. IZC thickness is less affected by gonial angle, but careful angulation is required to avoid sinus perforation. Individualized CBCT assessment is crucial for optimizing TAD placement, enhancing biomechanical outcomes, and minimizing procedural risks.

Keywords: Bone screws; Cone-beam computed tomography; Infrazygomatic; Mandible; Mandibular buccal shelf; Miniscrews; Orthodontic anchorage procedures; Orthodontics; Zygoma.