Surgical microanatomy of the anterior clinoid process for paraclinoid aneurysm surgery and efficient modification of extradural anterior clinoidectomy.

Background Anatomic variations of the anterior clinoid process (ACP) should be recognized before clinoidectomy to ensure a safe approach. This study describes the incidence of caroticoclinoid foramen (CCF), interclinoid osseous bridge, and pneumatization of the ACP during extradural anterior clinoidectomy. The problems and technical issues encountered in such cases are described. Methods Using multidetector-row computed tomography, 144 sides in 72 cases of paraclinoid aneurysm treated by extradural anterior clinoidectomy were analyzed preoperatively. Results CCF, interclinoid osseous bridge, and pneumatization of the ACP were observed in 16.6%, 2.77%, and 27.7% of cases. Pneumatized patterns were divided into 3 groups according to route: pneumatization via the optic strut (in 74.1%), pneumatization via the anterior root (in 14.8%), and pneumatization via optic strut and anterior root (in 11.1%). CCF and interclinoid osseous bridge represent obstacles to complete extradural removal of the ACP. The ACP should not be moved even after drilling the lateral wall of the ACP, orbital roof, and optic strut, so an intradural approach is sometimes needed. A CCF warrants careful removal to open the distal dural ring. Awareness of the routes of pneumatization for the ACP should reduce the risk of tears in the paranasal mucosa. If tears arise in the mucosa, suturing and closure are needed to prevent liquorrhea. Conclusions Preoperative computed tomography is useful to detect variations in the anatomy around the ACP. When performing extradural anterior clinoidectomy in such anomalous cases, appropriate modifications are needed to ensure a safe approach.