A thorough knowledge of the anatomy of the craniocervical junction and its variations is vital to operative success in this area, and the reader is urged to study some of the many detailed anatomical reviews available in the literature [1 ^– 4].

The craniocervical junction is bounded by the ventral rim of the foramen magnum, the occipital condyles, the arch of C1, the odontoid peg, and the base of the second cervical vertebra (Fig. 4.1). At the arch of C1, the vertebral artery is 24 mm from the midline, whereas at the C2–C3 disc and the ventral rim of the foramen magnum it lies 11 mm from the midline. The distance between the foramen magnum and the base of C2 is about 40 mm and that between the lateral masses of C1 is 27 mm. The wedge-shaped clivus is 38–42 mm long, 22 mm wide, and 4 mm thick at the foramen magnum and 20 mm thick dorsal to the pituitary fossa (Fig. 4.2). Rostrally, the unprotected “knuckle” of the carotid as it leaves the petrous canal to enter the cavernous sinus is 11 mm from the midline arch of C1 (Fig. 4.3). To this are attached the longus coli muscles and the anterior longitudinal ligament. A midline raphe can be seen extending to the tubercle. Above this are attached the longus capitis and rectus capitis anterior muscles; dorsal to them lies the pharyngobasilar fascia. The vertebral arteries run about 14 mm on each side lateral to the tubercle. Deep to the bones are the tectorial membrane, the posterior longitudinal ligament, and the dura, which is often attached to the ventral rim of the foramen magnum. Within this exposure, provided that there is no rotation of C1, there are no major neural or vascular structures that cross the operative field (it should be noted, however, that venous bleeding can be encountered from the dural marginal sinus and bony diploic channels in the clivus).

Details of each surgical procedure are discussed later, but broad definitions follow. In the absence of basilar invagination, a transoral approach with elevation of the soft palate will allow access to the arch of C1 and the odontoid peg. A midline split of the soft palate will gain the access to the anterior aspect of the foramen magnum. A division of the hard and soft palate will gain access to the lower half of the clivus but has such limited lateral extension because of the upper teeth that it is now used very little. The extended “open door” maxillotomy (a combination of the Le Fort 1 osteotomy and a midline palatal split) will provide access from the sphenoid down to C3; this surgical procedure was developed in response to the challenge posed by basilar invagination. A transmandibular, transglottic approach is combined with a posterior approach for 360-degree eradicational surgery for tumors of the upper cervical spine.

Plain lateral films of the craniocervical junction in flexion and extension (Fig. 4.4), together with an anteroposterior view (including a good view of the peg), allow for assessment of the bony limitations of the surgical field and provide a baseline for subsequent rapid office assessment. Magnetic resonance imaging (MRI) provides exquisite soft tissue images (Fig. 4.5), but computed tomography (CT) and particularly computed myelography [5] still provide the most accurate means of identifying bony abnormalities and also provide absolute measurements of the structures involved. These methods also afford easy assessment of the craniocervical angles and the angle of the neuraxis. New software packages now allow the generation of sophisticated three-dimensional surfaces, rendering images that can be evaluated from any angle. Vertebral angiography is required to outline tumor circulation or as part of a trial vertebral artery occlusion by balloon.