Defining the caudal limits of the endoscopic endonasal approach to the craniovertebral junction: anatomic study correlating radiographic measures

Objective

The endoscopic endonasal approach (EEA), has become the preferred alternative to traditional open and transoral approaches to the ventral craniovertebral junction (CVJ) region. However, preoperative prediction of the limitations of caudal reach remains challenging. This cadaveric study aimed to quantify the CVJ area of exposure and access afforded by the EEA, evaluate the accuracy of previously described radiographic anthropometric lines, and identify the lowest limit of the EEA corridor.

Methods

Endoscopic endonasal dissections of the CVJ were completed in 35 cadaveric specimens. The area of exposure (AoE) and caudal-most reach were measured using a navigation system. Radiographic measurements included the distance of the odontoid process from the hard palate, length of the hard palate, distance of the lowest point reached from the hard palate level, and angles such as the nasopalatine line (NPL) angle, nasoaxial line (NAxL) angle, nostril-hard palate line (NTL) angle, and rhinopalatine line (RPL) angle.

Results

The mean CVJ AoE was 931.22 ± 79.36 mm2. The NPL, NAxL, and RPL angles showed significant negative correlations with the distance of the odontoid process from the hard palate line (r = -0.521, p = 0.001; r = -0.538, p = 0.001; r = -0.500, p = 0.002, respectively), while the NTL angle did not (r = -0.241, p = 0.162). No significant correlation was found between achieved AoE via EEA and NPL, NAxL, NTL, or RPL (p > 0.05). Importantly, hard palate length was the sole predictor of CVJ AoE variability (r = -0.416, p = 0.013), with shorter lengths associated with increased exposure. The mean distance of the lowest point reached in the AoE from the hard palate level was 9.47 ± 1.24 mm.

Conclusions

This anatomic study highlights the variability in CVJ anatomy and the limitations of using previously defined radiographic anthropometric lines for predicting the caudal limits of the EEA. Hard palate length emerged as the only reliable predictor of the surgical area of exposure via the endonasal corridor. Clinical studies are warranted to validate these findings and define the potential need for adjunctive surgical routes in managing complex CVJ pathologies.