Endonasal access to the upper cervical spine, part one: radiographic morphometric analysis.

Objectives To determine the anatomical relationships that may influence endonasal access to the upper cervical spine. Setting We retrospectively analyzed computed tomography of 100 patients at a single institution. Participants Participants included adults with imaging of the hard palate, clivus, and cervical spine without evidence of fracture, severe spondylosis, or previous instrumentation. Main Outcome Measures Morphometric analyses of hard palate length and both distance and angle between the hard palate and odontoid process were based on radiographic measurements. Descriptive zones were assigned to cervical spine levels, and endoscopic visualization was simulated with projected lines at 0, 30, and 45 degrees from the hard palate to the cervical spine. Results We found an inverse relationship between hard palate length and the lowest zone of the cervical spine potentially visualized by nasal endoscopy. The distance between the posterior tip of the hard palate and the odontoid tip, and the angle formed between the two, directly influenced the lowest possible cervical exposure. Conclusions Radiographic relationships between hard palate length, distance to the odontoid, and the angle formed between the two predict the limits of endonasal access to the cervical spine. These results are supported by cadaveric data in Part Two of this study.