Endoscopic Endonasal Anterior Clinoidectomy: Volumetric Assessment and Feasibility

Abstract

Transcranial removal of the anterior clinoid process (ACP) provides access to the clinoidal segment of the internal carotid artery (ICA) as well as superolateral decompression of 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 which could be safely resected via an EEA. A cadaveric anatomical study was performed with removal of the ACP in 3 major steps in order of difficulty and risk. At each step, the removal was stopped when no more bone of the ACP could be seen without traction on neural structures. After each step, a CT scan was performed to allow volumetric measurement of the remaining ACP. Twenty ACP in 10 cadaveric heads were removed to various degrees using the described stepwise technique. The mean portion of the ACP resected by each step was 21%, 46% and 27%, respectively. Cumulated ACP removal at the end of step 3 was 94%, with complete removal achieved in 35% of the specimens. Using the safe route above the optic canal, removal of 21% of the ACP can be achieved via EEA. Although substantially more of the ACP 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, the third cranial nerve and the optic nerve.