Figure 1.1 Muscular triangles of the neck.

The atlas

The atlas has no body (Figure 1.2). The anterior arch is faceted to receive the tip of the odontoid process, and the medial aspect of each articular mass is indented by the attachment of the transverse band of the cruciate ligament. The spinal canal at this level is spacious. Its sagittal diameter may be divided into three; the anterior third being occupied by the odontoid peg; the middle third by the cord; and the posterior third by the subarachnoid space. Cisternal puncture by the posterior or lateral route is therefore safe under normal conditions.

The oblique groove across the posterior arch of the atlas accommodates the vertebral artery after it has wound around the outside of the articular mass. The attachment of the posterior atlanto-occipital membrane is arched over the artery at this point, and this arch is sometimes outlined, completely or incompletely, by bone, to form the arcuate foramen. This bony arch is insignificant; but it has been said that its presence renders the atheromatous vertebral artery more vulnerable to compression during rotation of the head (Klausberger and Samec, 1975).

The side-to-side width of the atlas is greater than that of any other cervical vertebra, to increase the leverage of the muscles inserted into the transverse process. This transverse process is the only one in the cervical spine which is not grooved to allow egress of a nerve root. The articular masses are broader and deeper than any other because they shoulder the weight of the skull and because the odontoid process bears no weight.

The axis

The axis has stolen the body of the atlas (Figure 1.3) to form the odontoid peg which projects up from its centrum to lie behind the arch of the atlas. The tip of the odontoid is faceted in front to mate with its atlantic fellow, and behind to accommodate the synovial bursa which separates it from the transverse band of the cruciate ligament. On either side of the base of the odontoid, the centrum presents the inferior facets of the atlanto-axial joints. Below, the atlas begins to take on the characteristics of a typical cervical vertebra. Its laminae meet to project a bifid and massive spinous process whose depth and aquiline profile are very variable. The pedicles are thick and their upper margins continuous with that of the body. The inferior articular facet lies below and behind the superior, and subtends an angle of almost 90° with the transverse process. This articulotransverse angle is recessed at its apex to accommodate the tip of the pyramidal process of the third vertebra (Veleanu, 1975).

Vertebrae three to six are so similar that it is not easy, or necessary, to identify an individual bone (Figure 1.4). In the articulated column they increase in size from above downwards. The margins of the bodies are sharply defined, particularly around the superior rim where the posterolateral edge projects upwards to articulate with the body above. Gray does not give this projecting edge a discrete name (Gray’s Anatomy, 1969), but Frazer calls it the neurocentral lip (Frazer, 1958), and European anatomists refer to it variously as the unciform or uncinate process, or the semilunate process. It is a structure of sufficient identity to deserve a name, and it is a significant structure in the pathology of cervical spondylosis. In this book it will be referred to as the neurocentral lip. The antero-inferior margin of the body projects downwards. This normal epinasty increases with the development of spondylotic osteophytes, a point to be remembered during discography and anterior interbody fusion.

The spinal canal is large to accommodate the cervical enlargement of the cord. The laminae are slender, and in youth each slightly overlaps the one below. This overlap increases markedly with age.

The pedicles, apophyseal joints, transverse processes and neurocentral lips are peculiar and specific to the cervical spine (Figure 1.5). Together they constitute the boundaries of the intervertebral foramen and enclose the foramen transversarium. This foramen, which affords passage to the vertebral artery, separates the costotransverse bar from the pedicle. The groove which forms the floor and walls of the intervertebral foramen, becomes progressively more shallow as the vertebrae descend. Medial to the vertebral artery the groove is floored by the pedicle. Here lie the anterior root of the spinal nerve and the posterior root ganglion; the former usually, though not invariably, above the latter (Abdullah, 1958). Running above the nerve root are the radicular and spinal branches of the vertebral artery, and their accompanying veins. Tapering into the groove are the blending layers of the meninges and the nerve sheaths forming the dural root sleeve.