Microsurgical anatomy of the dural collar (carotid collar) and rings around the clinoid segment of the internal carotid artery

OBJECTIVE: To examine the relationship of the clinoid segment of the internal carotid artery to the structures in the roof of the cavernous sinus and to determine whether this segment is neither intradural nor intracavernous, as recently proposed. METHODS: The region of the roof of the cavernous sinus was dissected and examined using 3 to 40 x magnification and micro-operative techniques. RESULTS: The clinoid segment was located within a collar formed by the dura lining the medial surface of the anterior clinoid process, the posterior surface of the optic strut, and the upper part of the carotid sulcus. The clinoid segment and the collar were defined above by the upper ring formed by the dura extending medially from the upper surface of the anterior clinoid process to surround the artery and below by the lower ring formed by the dura extending medially from the lower surface of the anterior clinoid process. The upper ring was adherent to the wall of the artery, but the lower dural ring was separated from the lower margin of the clinoid segment by a narrow space that admitted venous tributaries of the cavernous sinus, called the clinoid venous plexus. This venous plexus narrowed as the upper ring was approached and became wider at the lower ring, where the plexus communicated with the venous channels of the cavernous sinus. The upper and lower dural rings were best defined along the lateral and anterior margins of the artery, were less distinct medially, and disappeared posteriorly, where the dura forming the upper and lower rings came together. CONCLUSION: The clinoid segment is intracavernous, being located within a collar of dura in which venous tributaries of the cavernous sinus course. The implications of these findings for surgery are reviewed.