Endoscopic endonasal inferior transclival approach, step-by-step dissection. The anterior atlanto-occipital membrane, which continues laterally with the capsule of the atlanto-occipital joints, has been removed. This allows identification of the anterior arch of C1 and the anterior aspect of the foramen magnum. The anterior arch of C1 occasionally presents a bony prominence on its upper surface, the superior tubercle of C1, which covers the tip of the odontoid process. The supracondylar groove, at the base of the condyle, is now identified. This groove correlates with the location of the hypoglossal canal posteriorly. Laterally, we can identify the fibrous tissue that fills the exocranial aspect of the petroclival fissure. This fissure is a key landmark that represents the ideal lateral margin of an inferior clivectomy. (Image courtesy of JC Fernandez-Miranda)