A Survey of the Variable Anatomy of the Parasellar Region with Specific Emphasis on the Middle Clinoid Process

The middle clinoid process (MCP) is a small bony projection from the sphenoid bone near the anterolateral margin of the sella turcica. The MCP is inconsistently present and varies in morphology. Ossification of the caroticoclinoid ligament, by which the MCP is attached to the anterior clinoid process (ACP), can result in the formation of a bony caroticoclinoid ring surrounding the adjacent internal carotid artery (ICA). This variability makes endonasal approaches to the skull base risky; we sought, therefore, to further illustrate the surgical anatomy of the parasellar region and quantify the incidence of MCP and caroticoclinoid rings in dried preserved human anatomical specimens. 2,726 cranial specimens including 2,249 males and 476 females were included, for a total of 5,352 parasellar regions with complete undamaged sella turcica for measurement in this study. The presence, dimensions, morphological classification (incomplete, contact, and caroticoclinoid ring) and intracranial relations of the middle clinoid were measured. A clinically relevant MCP was defined as height or base < 1.5mm. The stated age, gender, race, and relevant clinical information for each evaluated specimen were recorded. An MCP was found in 42% of specimens with 60% presenting bilaterally. In unilateral specimens, the MCP was found more commonly on the right side (p < 0.0001). 27% of MCPs were classified as fully ossified caroticoclinoid rings and 4% as contact. The average height of an incomplete MCP was 2.59 mm with a basal diameter of 4.16 mm. Processes exhibiting a contact morphology (MCP and ACP make contact but are not fully ossified into a ring) were found to be 3.65 mm in height with a 4.48 mm base. The diameter of the “pseudo-ring” of the contact MCP was 5.63 mm. Caroticoclinoid foramen were found to have a base diameter of 5.03 mm with a ring diameter of 5.52 mm. Ring forming MCPs exhibited a significantly greater basal diameter (p < 0.0001). Within the parasellar area, MCPs were found in the anterior third of the lateral bone window of the sella turcica. The superior aspect of the base of the MCP was found 6.25 mm posteromedial from the posterior border of the optic strut, 3.70 mm inferior to the opticocarotid elevation. There was no difference (p = 0.6841) in the presence of a clinically relevant MCP between men and women, however bilateral presentation was significantly more common in men (p < 0.0001). A clinically relevant MCP was identified in 52% of parasellar regions obtained from skulls recorded racially as “white,” as compared with 30% in parasellar regions from skulls recorded as “black” (p < 0.0001). An interclinoid bridge (ICB) was found in 4% of all parasellar regions examined; the incidence of an ICB was significantly associated with coincidence of an MCP (p < 0.0001), and specifically with ring forming morphologies (p < 0.0001). Variations in the formation of the MCP and especially formation of caroticoclinoid rings pose a risk for injury to the ICA during clinoidectomy in regional expanded endonasal approach and microsurgical procedures. An understanding of parasellar anatomy associated with racial and sexual variation is integral to surgical planning and patient counseling.