The Petrous Process of Sphenoid Bone: A Surgical Landmark for the Identification of Abducens Nerve in Endoscopic Endonasal Surgery

Objective: The abducens nerve is the most medial of the midline cranial nerves and has several segments; from its origin in the pontomedullary fissure to the lateral rectus muscle. Its identification during expanded endonasal approaches in the coronal plane can be challenging, with the interdural segment being at most risk. The objective of this study was to provide a detailed investigation of the surgical anatomy and anatomical relationships of the interdural segment of the abducens nerve, based on anatomical dissections, anatomical models and imaging studies. In addition, we describe and study the petrous process of sphenoid bone (PPsb), as we believe it could be a key and reproducible landmark to safely identify the abducens nerve during endoscopic endonasal surgery.

Method: Eight fresh human silicon-injected heads underwent an endonasal transclival approach. 16 cranial nerves were dissected to expose the interdural segment of the 6th nerve and its relationship with the petrous process of the sphenoid bone. The distance between the sphenoid sinus floor and petrous process and from petrous process to sellar floor was measured, and anatomical relationships and variations were recorded. The length of the interdural segment was registered. Additionally, we compared the findings with an open transcranial approach ([Fig. 1]).

Results: The petrous process of the sphenoid bone (PPsb) is a bony prominence in the lateral aspect of the sphenoid body on either side of the dorsum sellae that could be identified in all 8 specimens. It articulates with the apex of the petrous portion of the temporal bone laterally. The sixth nerve was located above the PPsb in all the specimens, just above the petroclival dural fold. Its identification probed to be reliable, as none of the nerves were damaged during the procedure or PPsb drilling. The average height from the sphenoid sinus floor to the PPsb apex was 7 mm and the average distance to the sellar floor was 6 mm. The interdural segment had an oblique trajectory and the average length from its dural entry point to PPsb apex was 8 mm. PPsb was located posteromedial to the paraclival ICA and medial to the foramen lacerum. Gruber's ligament was located posterior to the abducens nerve in all 16 sides, running from the petrous apex to posterior clinoid process ([Figs. 2] and [3]).

Conclusion: This study identifies the PPsb as an important and reproductible landmark in the early identification of the abducens nerve in endoscopic endonasal procedures, which should potentially minimize the risk of injury.

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