Volumetric Assessment of Endoscopic Endonasal Anterior Clinoidectomy

Background: Transcranial removal of the anterior clinoid (AC) allows access to the clinoidal segment of the internal carotid artery (ICA) as well as laterosuperior decompression of the optic nerve in the optic canal. Endoscopic endonasal approaches (EEAs) can access the entire medial and inferior portions of the optic canal, but no data exists to support what proportion of the anterior clinoid could be safely resected via an EEA.

Methods: A prospective cadaveric anatomical study was performed. Removal of the AC followed three major steps in order of difficulty and risk. Following medial canal decompression, the first step consisted of drilling the optic canal roof maximally into the medial anterior clinoid; the second step consisted of drilling the optic strut (via the lateral opticocarotid recess) and sphenoid wing attachment; the third step consisted of opening of the proximal dural ring and medial transposition of the ICA. At each step, the removal was stopped when no more bone of the AC could be seen without traction on neural structures. After each step, a CT scan was performed to allow volumetric measurement of the remaining AC.

Results: Twenty ACs in 10 cadaveric heads were removed to various degree using the described stepwise technique. The mean percentage of resection of the AC provided by each step was 21, 46, and 29%, respectively (p < 0.001). The mean percentage of removal at the end of the three steps was 96%, with complete removal achieved in 40% of the specimens only at the end of the third step. The most common locations for bony remnant were the lateral optic canal and the tip of the AC.

Conclusion: Using the safe route above the optic canal, removal of only 21% the AC can be achieved via an endoscopic endonasal approach, allowing medial and superior decompression of the optic nerve. Although substantially more of the AC can be drilled by accessing the optic strut, the benefits of pursuing additional removal must be weighed against the significant risks of drilling in this narrow corridor bordered by the ICA, third cranial nerve, and optic nerve.