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

Abstract

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.

Keywords: Caudal limits; Craniovertebral junction; Endoscopic endonasal surgery; Hard palate length; Multiport surgery; Radiographic anthropometric lines.

Fig. 1

A. Sagittal midline view. The rhinion (pink point) marks the nasal bone tip. The hard palate is represented by a yellow line connecting the anterior (red point) and posterior (blue point) nasal spines. A green scale indicates positions relative to the hard palate, with negative values below and positive values above. B. Area of exposure measurement. Upper fixed points are centered on the jugular tubercles bilaterally. Lower variable points represent the maximum caudal reach of the navigation probe in the same sagittal plane as the jugular tubercles on each side. C. Endoscopic endonasal view of the craniovertebral junction post-dissection. Key structures labeled; PC.ICA: paraclival internal carotid artery, JT: jugular tubercle, OC: occipital condyle, AOJ: atlantooccipital joint, PFD: posterior fossa dura, Od: odontoid, Alar Lig.: alar ligament, Apical L.: Apical ligament, 9 CN: glossopharyngeal, 10 CN: vagus nerves, 11 CN: spinal accessory nerve, 12 CN: hypoglossal nerve, RCA: rectus capitis anterior, LM of C1: lateral mass of C1, AT of C1: anterior tubercle of C1

Fig. 2

Radiographic caudal predictive lines and associated angles. A. Illustration of all radiographic caudal predictive lines described in the literature, showing their start points and relationships. Lines: NasoPalatine (red), NasoAxial (blue), RhinoPalatine (green), and Nostril (yellow). B, C, D, E. Calculated angles at the posterior nasal spine level relative to the hard palate line (dashed black line)