Roadmap to Endoscopic Endonasal Occipitocervical Fusion following Odontoidectomy: Anatomical Evaluation of Buccopharyngeal Flap Exposure in a Cadaveric Study

Objective: It is common for patients to develop iatrogenic instability at C1–C2 and/or O-C1 following endonasal/transoral odontoidectomy. While this instability has historically been treated in a delayed manner utilizing staged posterior instrumentation, a small number of recent reports have described odontoidectomy and C1–C2 arthrodesis through a single-stage endonasal or transoral approach. There is a paucity of information in the literature regarding the optimal methods used to expose the ventral craniocervical junction in a safe manner to facilitate subsequent arthrodesis. Currently, no instrumentation exists to perform O-C1 fusion following endonasal odontoidectomy. The aims of this study are to (1) validate the use of regional anatomic landmarks to provide adequate operative exposure of the ventral craniocervical junction; (2) obtain anatomic measurements assessing the distance from the required pharyngeal incisions to adjacent, eloquent neurovascular structures; and (3) document the degree of exposure of the O-C1 and C1–C2 joint spaces provided by the aforementioned approach to promote additional research of endoscopic endonasal-compatible methods for occipitocervical fusion.

Methods: Four fresh, frozen cadaveric specimens were utilized for dissection. An inverted U-shaped buccopharyngeal flap was fashioned using a superior incision at the level of the pharyngeal tubercle of the clivus and lateral incisions in Rosenmüller fossae that extended inferiorly until limited by palatal structures. The following flap characteristics were assessed using caliper instrumentation: width of exposure at the superior and inferior extent of the flap, extent of O-C1 joint and C1–C2 joint exposure. Additional anatomic measurements included: the distance between the medial aspect of the carotid arteries at the level of the pharyngeal tubercle and C1, and the distance between the hypoglossal canals. The risk of injury to the hypoglossal nerves and parapharyngeal carotid arteries and the ability to perform a transarticular endonasal C1–C2 screw based on the method proposed by Mendes et al. were assessed based on the proposed flap dimensions.

Results: On average, the superior and inferior extents of flap exposure averaged 25.5 and 30.5 mm, respectively. The flap provided, on average visualization of the medial 8.5 mm of the O-C1 joint and 9.9 mm of the C1–C2 joint. The mean distance between the parapharyngeal carotid arteries was 36.8 mm at C1 and 37.5 mm at the pharyngeal tubercle of clivus. In all instances, the parapharyngeal carotid arteries were lateral to the dimensions of the flap. In one specimen, the flap incision posed risk to the underlying hypoglossal nerve.

Conclusion: The inverted U-shaped buccopharyngeal flap described in this paper poses minimal risk to the adjacent parapharyngeal carotid artery and provides important information on how to safely manage the close proximity of the hypoglossal canal. This study paves the way for potential future research that includes investigation of a low-profile construct able to facilitate endoscopic endonasal O-C1 arthrodesis.

Fig. 1 (A) Endonasal view of the posterior nasopharynx. (B) An inverted U-shaped buccopharyngeal flap as described in methods above. (C, D) The longus colli and capitis musculature are elevated and the anterior arch of C1, the O-C1 and C1–C2 joints are identified. (E) Additional lateral dissection is limited by the parapharyngeal carotid arteries.

No conflict of interest has been declared by the author(s).