Defining the Nasopalatine Line: The Limit for Endonasal Surgery of the Spine

Objectives: The expanded endoscopic endonasal approach (EEA) to the odontoid process is performed for decompression of the brainstem and to access tumors at the foramen magnum. Caudal exposure is limited by the nasal bones anteriorly and the hard palate posteriorly. We define the line connecting these two points as the nasopalatine line (NPL) and the nasopalatine angle (NPA) as the angle between the nasopalatine line and the plane of the hard palate. Study Design: This study was a retrospective cohort study. Methods: Pre and post-operative computed tomographic (CT) scans of 17 patients who underwent transodontoid EEA were reviewed. The position of the odontoid and the inferior extent of the tumor and surgical dissection were compared to the NPL. Factors affecting the posterior projection of the NPL, including basilar invagination and head position, were examined. Results: The mean NPA was 27.1 degrees (range 2131). The NPL intersects the spinal column at 8.9 mm (range -9.0-8.7 mm) above the base of the C2 body. The base of the odontoid process and the inferior extent of surgical dissection were always above this line. Both basilar invagination and head position affect the relative position of the NPL. Patients with basilar invagination demonstrated a significantly lower posterior projection of the NPL than those without (P < .01). Maximal cervical flexion afforded more caudal exposure than cervical extension. Conclusions: The NPL accurately predicts the most inferior extent of surgical dissection. Further caudal dissection may require the use of angled instruments or a transoral approach.