BACKGROUND: The middle clinoid is an osseous prominence that arises from the body of the sphenoid bone at the anterolateral margin of the sella.
OBJECTIVE: To illustrate the radiological and surgical anatomy of the middle clinoid and describe the technical nuances for endonasal endoscopic middle clinoid removal.
METHODS: The fine-cut head CT-angiogram scans of 100 patients and 50 anatomic specimens were examined. The middle clinoid was categorized as: absent, small, prominent, or caroticoclinoidal ring. Ten colored silicon-injected anatomic specimens were used to study the surgical anatomy for the endonasal middle clinoidectomy. Extensive surgical experience allowed for intraoperative observations regarding the surgical anatomy of the middle clinoid and the technical nuances for its removal.
RESULTS: The middle clinoid was identifiable in 60% of scans (bilateral in 35%), and 20% had at least one caroticoclinoidal ring (bilateral in 6%). When present, the middle clinoid is located at the transition between the intracavernous internal carotid artery (ICA) and paraclinoidal ICA, and covers the anteromedial roof of the cavernous sinus. Endonasal removal of the middle clinoid improves access to the parasellar region. The middle clinoidectomy is completed exposing the following structures sequentially: sellar dura, anterior wall of the cavernous sinus, dura of the lateral tuberculum sella, and paraclinoidal ICA. When a caroticoclinoidal ring is identified, progressive reduction of the middle clinoid can be achieved without fracturing the ring.
CONCLUSION: Recognition of the middle clinoid and caroticoclinoidal ring on preoperative imaging is critical for surgical planning and middle clinoid removal in endonasal skull base surgery.
ABBREVIATION: ICA, internal carotid artery