The Naso-Axial Line: a new method of accurately predicting the inferior limit of the endoscopic endonasal approach to the craniocervical junction

Background: Endoscopic approaches to anterior pathology of the craniocervical junction (CVJ) have arisen as alternatives to open approaches. Understanding and predicting the limits to the endoscopic approach to CVJ is important in surgical planning. The endoscopic endonasal approach (EEA) is commonly used, thus we sought to develop a method that accurately predicts the inferior limit of this to the CVJ. The method developed was compared with methods currently used to predict the same. Methods: Nine fresh-frozen adult cadaver heads were used for anatomical dissection. Preoperative and postoperative volumetric computerized tomographic scans were performed to compare the naso-axial line (NAL) vs. naso-palatine (NPL, or Kassam, line) used to predict the inferior limit of the EEA to the actual extent of surgical dissection. Results: The mean differences between the NAL and the actual inferior surgical exposure of EEA at the C2 cortex (anterior and posterior) were both 0.3 mm. Anatomically, the actual inferior surgical limit ranged from the dens to the upper half of the C2 body, which matched that predicted by the NAL. The NPL predicted an inferior EEA limit at a much lower range than the actual—from the lower half of the C2 body to the superior endplate of C3. Using the least squares means (LSM) method to estimate the deviation from postsurgical measurements, we found no difference between the NAL and the actual inferior surgical limit (LSM, 0; P = 1.0). In contrast, the NPL predicted a significantly lower EEA limit than the actual surgical limit (LSM, 13.3; P < 0.001). Conclusion: The naso-axial line accurately predicts the inferior limit of the EEA, which ranges from the dens to the upper half of C2. This new method can be used in presurgical planning to assess the suitability of the EEA to craniovertebral junction pathology.