Surgical anatomy and efficient modification of procedures for selective extradural anterior clinoidectomy

Objective: The surgical anatomy of structures surrounding the anterior clinoid process (ACP) was examined in 39 cadavers to enable safe performance of selective extradural anterior clinoidectomy. Methods: The lateral portion of the optic canal (OC) at the end of the orbit was defined as point A, and the lateral margin of the dural insertion into the superior orbital fissure (SOF) as B. A vertical line from A to the lateral margin of the ACP was assumed to define a crossing, defined as C. Distances between A and B, C and B, and C and A were measured. The length and width of OC and the optic strut were also measured. Results: The mean distances between A and B, C and B, and C and A were 11.8, 8.9 and 6.3 mm, respectively. The mean length and width of OC were 8.9 and 5.7 mm, and those of the optic strut 5.4 and 2.4 mm, respectively. Conclusion: We propose a modified, efficient procedure for selective extradural anterior clinoidectomy, as follows. Drilling is started from the point approximately 9 mm posterior to the lateral margin of the dural insertion into the SOP, and pointed medially in the direction with a right angle to the lateral margin of the ACP. After drilling about 6 mm to reach the lateral border of the OC, unroofing of the OC is carried out to remove the ACP en bloc by fracturing of the optic strut. Using the present procedure, the distance of drilling of the lesser wing of the sphenoid bone is minimized.