A Morphometric Analysis of Axis Vertebra for Microsurgical Posterior Midline Transaxis Odontoidectomy Surgery

Odontoidectomy is a challenging technique due to anatomical complexity and complications associated with preferred approaches like transoral–transpharyngeal or posterolateral techniques. The posterior midline transaxis approach offers bilateral access without further complications of anterior approaches and offers craniovertebral instrumentation option at the same session. A CT-based morphometric analysis of 40 cadaveric specimens evaluated with various morphometric parameters, including distances and angles between the base and tip of the odontoid process and the lateral masses, to analyze the surgical corridor utilized in the transaxis odontoidectomy procedure. Toshiba Aquilion CT (Computed Tomography) scanner with 80 detectors at 0.5 mm slice thickness was utilized. Electronic images were generated using 3D (3-Dimension) Slicer software. The specified measurements were conducted on the generated images based on 25 different morphometric landmarks. Key measurements included a PMEP-to-FT distance of 5.21 mm (right) and 6.37 mm (left), PMEP-to-odontoid tip distance of 26.74 mm, and a mean odontoid height of 15.47 mm. Instruments directed from the PMEP should be angled medially at 55° in the axial plane and 16° upward in the parasagittal plane to reach the odontoid base. Odontoidectomy requires detailed anatomical knowledge, extensive training, and precise use of intraoperative navigation systems. Existing studies highlight the posterior midline transaxis approach as morphologically compatible and practical. However, limited research on PMEP angles and distances prompted this morphometric analysis to enhance understanding of the surgical access route and anatomical boundaries.