A neurosurgical view of anatomical evaluation of anterior C1–C2 for safer transoral odontoidectomy

An anatomical study for evaluation of anterior C1–C2. To provide essential anatomic data for safer transoral odontoidectomy. The surface dimensions of the atlas vertebra and the transoral approach for odontoidectomy have been described in detail. Anterior arcus of C1 must be drilled out to reach odontoid process for transoral odontoidectomy. The thickness of anterior ring of C1 has not been studied before. Sixty, dried adult atlas and 60 axis vertebrae and ten cadaveric craniocervical specimens were measured for the following: (1) bony drilling depth (BDD), the distance from the anterior wall of anterior ring of C1 to anterior wall of odontoid; (2) minimum drilling diameter (MDD), distance of minimum C1 anterior ring removal for odontoid resection on horizontal plane; (3) maximum bony drilling diameter (MBDD), distance of maximum C1 anterior ring removal for odontoid resection on horizontal plane. Lateral border of this diameter is limited by medial borders of the lateral mass; (4) the widest odontoid diameters (WOD) on coronal sections were measured. On 60 atlas and axis vertebrae, the BDD was 7.0 ± 1.2 mm on dry bones, the distance between the medial borders of the lateral mass (MBDD) was 16.1 ± 1.5 mm, and the WOD on coronal sections (WOD) was 9.8 ± 0.8 mm. On cadavers, the distance between the two edges of C1 anterior ring removal for odontoid resection (MDD) was 10.8 ± 1.1 mm and the WOD on coronal sections (WOD) was 10.1 ± 1.4 mm. An odontoid surgery through transoral approach is safe and feasible. A quantitative understanding of the anterior anatomy of C-1 and C-2 is necessary when considering transoral odontoid resection. In this study the authors define safe zones for anterior atlas and axis.