Evaluation of Optimal Sites for the Insertion of Orthodontic Mini-implants at Mandibular Symphysis Region Through Cone-beam Computed Tomography

Background: This study evaluates the overall bone thickness (OBT) and cortical bone thickness (CBT) of mandibular symphysis and to determine the optimal sites for the insertion of orthodontic mini-implants.Methods: Cone-beam computed tomography (CBCT) images of 32 patients (18 males, 14 females), including 16 adults and 16 adolescents were included in this study. The sample was further categorized into three facial types: low angle, average angle, and high angle. OBT and CBT were measured at the mandibular symphysis region. All measurements were performed at six different heights (2 mm, 4 mm, 6 mm, 8 mm, 10 mm, and 12 mm from cementoenamel junction [CEJ]) and at seven different angles (0, 10, 20, 30, 40, 50, 60 degrees to the occlusal plane). Results: Neither OBT nor CBT was influenced by age or sex, except for that CBT was significantly thicker in adults than in adolescents. OBT and CBT were significantly thicker in low-angle cases than in average- and high-angle cases. Both OBT and CBT were significantly influenced by insertion locations, insertion heights, insertion angles, and their interactions. CBT and OBT gained the greatest value at the location between two lower central incisors, and became greater with the increase of insertion height and insertion angle. Both recommended and optimal insertion sites were mapped. Conclusions: Mandibular symphysis region was suitable for the placement of orthodontic mini-implants. The optimal insertion site was 6-10 mm apical to CEJ between two lower central incisors with an insertion angle being 0-60 degrees to the occlusal plane.