Measurements of the anatomical structures of the mandibular buccal shelf for the safe insertion of mini-implants: a cone-beam computed tomography retrospective study.

Background: The purpose of this study was to obtain measurements of the anatomical structures of the mandibular buccal shelf (MBS) using cone-beam computed tomography (CBCT) for the safe insertion of orthodontic mini-implants.

Methods: The sample consisted of the CBCT records of 100 subjects. The scans were imported into a reconstruction program. Measurements were taken in the coronal plane for three different roots: the mandibular first molar distal root (6D), second molar mesial root (7 M), and second molar distal root (7D). The inferior alveolar nerve canal (IANC) was used as a reference to determine the safe insertion depth. The roots of the mandibular molars were used as a reference to determine the maximum safe insertion angle. Analysis of variance with post hoc analysis and Kruskal-Wallis H test with post hoc analysis were used for data analysis.

Results: For the 6Ds, the average bone depth was 20.33 ± 3.6 mm, the average depth above the IANC plane was 9.55 ± 2.97 mm, and the average maximum safe insertion angle was 47.47 ± 18.94°. For the 7Ms, the average bone depth was 22.06 ± 2.85 mm, the average depth above the IANC plane was 10.71 ± 2.67 mm, and the average maximum safe insertion angle was 43.14 ± 15.26°. For the 7Ds, the average bone depth was 23.75 ± 2.86 mm, the average depth above the IANC plane was 12.12 ± 2.61 mm, and the average maximum safe insertion angle was 28.39 ± 10.96°. From anterior to posterior, the buccal bone depth and depth above the IANC gradually increased (P < 0.01), there was no significant difference in the maximum safe insertion angle between the 6Ds and 7Ms (P = 1.000), but the significant difference was observed between 6Ds and 7Ds, 7Ms and 7Ds (P < 0.000).

Conclusions: The optimal implantation site for mini-implant in the MBS is at the buccal site corresponding to the second molar. A maximum insertion length of ≤ 12 mm and a maximum insertion angle which respects the long axial of the tooth root of ≤ 28° are recommended for the safe insertion of mini-implants in the MBS.