Radiographic Prevalence and Anatomical Variations of the Medial Clinoid Process: Implications for Endoscopic Endonasal Skull Base Surgery

Introduction: The middle clinoid process (MCP) is a bony prominence on the body of the sphenoid bone, located at the anterolateral margin of the sella, between the anterior and posterior clinoids. It has been identified in previous anatomical studies in ~40–50% of the patients. The MCP is relevant in endonasal approach due to the prevalence of a caroticoclinoidal ring, which is defined as an osseous continuation between the anterior and MCP. The surgical implications are especially important when neurosurgeons perform an endonasal endoscopic middle clinoidectomy and it is recommended to perform a preoperative identification to prevent inadvertent damage to the internal carotid artery (ICA) Objectives: Report the prevalence and describe the anatomical characteristics of MCP of patients in a Mexican Referral Center. Methods: The MCP was evaluated using head CT scans (slice of 1 mm) of 300 patients. The MCP was classified as unilateral or bilateral and its anatomical variation as absent, small (if it extended by 50% of the diameter of the parasellar ICA) ([ Fig. 2 ]), or a caroticoclinoidal ring (if it surrounded the ICA for 360 degrees between the anterior and middle clinoids). Patients who had undergone previous sphenoidal surgery or had sellar, parasellar or suprasellar pathology were excluded. Results: The most common anatomical variation was a small unilateral clinoid, identified in 98 cases. Bilateral caroticoclinoidal rings were present in 58, while unilateral caroticoclinoidal rings were present in 30 patients. ( Table 1 ) Discussion: This study highlights the importance of the MCP in endonasal approaches to the sellar, parasellar and suprasellar regions. It serves as a key landmark for identifying the anteromedial roof of the cavernous sinus and the transition between intracavernous and paraclinoidal ICA segments. Middle clinoidectomy improves access, especially in cases of tumor invasion or lateral extension. This study found a higher frequency of caroticoclinoidal rings than previous studies. Recognizing its presence not only provides a reliable landmark for orienting the cavernous and paraclinoidal ICA segments and helps anticipate potential challenges during the surgery. Conclusion: At least one MCP was present in 72.3% of the patients and 29.3% had caroticoclinoidal ring. Given this prevalence, incorporating such evaluation into routine preoperative planning becomes crucial, as it strengthens surgical precision, maximizes exposure, and enhances the safety of endonasal procedures involving the sellar, parasellar, and suprasellar regions. Table 1 Prevalence of MCP MCP Characteristic Patients ( n = 300), n (%) Clinoids ( n = 600), n (%) Absent Unilateral 78 (26) 244 (40.6) Bilateral 166 (55.3) Small Unilateral 98 (32.6) 176 (29.3) Bilateral 78 (26) Prominent Unilateral 55 (18.3) 92 (15.3) Bilateral 37 (12.3) Ring Unilateral 30 (10) 88 (14.6) Bilateral 58 (19.3) Publication History Article published online: 27 February 2026 © 2026. Thieme. All rights reserved. Georg Thieme Verlag KG Oswald-Hesse-Straße 50, 70469 Stuttgart, Germany