Endoscopic endonasal inferior transclival approach, step-by-step dissection. The inferior clivus has been drilled away while leaving the anterior margin of the foramen magnum. The apical ligament is identified running from the odontoid tip to the dorsal surface of the anterior margin of the foramen magnum. The odontoid tip is covered anteriorly by the superior tubercle of the atlas. Note that the inferior clivectomy has extended laterally up to the level of the petroclival fissure. The occipital condyles have not been drilled, but drilling has extended to the jugular tubercles supero-laterally. (Image courtesy of JC Fernandez-Miranda)