ANATOMY OF CRANIOCERVICAL JUNCTION
The craniocervical junction consists of the occiput and the first two cervical vertebrae (C1 and C2). It is the complex bony and ligamentous articulations of this region that facilitate its unique biomechanical properties, accounting for 25% of flexion and extension and 50% of rotation of the neck. The occiput is the skull’s most inferior bone, and it retains a cupped shape posteriorly that gives way to the triangular foramen magnum inferiorly. This foramen harbors the cervical spine cord as it ascends and transitions into the medulla and upper brainstem. At the anterolateral border of the foramen magnum are the occipital condyles, which are articulation points for the atlas (C1). These articulations are relatively flat so as to limit axial rotation of the atlantooccipital joint.
The atlas has two superior protuberances, known as its lateral masses, that articulate with the occipital condyles. The atlas is the only vertebral body to lack a spinous process, and in rare cases it may entirely lack a posterior arch. The atlas is a ring-shaped structure that lacks a vertebral body and does not make contact with an intervertebral disc. Embryologically, the body of the atlas becomes the dens (odontoid process) of the axis (C2). The dens articulates with the atlas anteriorly as it projects upward from the axis. The other points of articulation between the atlas and axis are the synovial joints of the articulating processes. The C1-2 joints are biconcave, as opposed to the flatter articulation of the occiput and C1. Whereas the occiput-C1 joint is designed mainly to flex and extend, the C1-2 joint is designed to provide axial rotation.
The ligamentous complex of the craniovertebral junction is a sophisticated network that plays a crucial role in maintaining the physical stability of the vertebra that protects the body’s most critical neurologic structures. Of the eight ligaments that support the craniovertebral junction, several have notable clinical relevance. The tectorial membrane is contiguous with the cranial dura mater and inserts onto the clivus. It originates as a superior continuation of the posterior longitudinal ligament and is thought to prevent anterior spinal cord compression by the clivus and possibly by the dens. The left and right alar ligaments function to limit axial rotation and connect the anterior and superior aspects of the dens to the occiput. The cruciate ligament is com- posed of vertical and horizontal components. The horizontal component is known as the transverse ligament, and it connects the two medial walls of the atlas, snugly securing the dens as it articulates with the anterior ring of the atlas. It acts to effectively limit the movement of C1 on C2 in the anteroposterior plane. The vertical components of the cruciate ligament consist of inferior and superior bands, but their function is not as well understood.
The suboccipital muscles extend the head and rotate it and the atlas ipsilaterally. At the suboccipital region there exist the rectus capitis posterior major and rectus capitis posterior minor muscles and the obliquus capitis inferior and obliquus capitis superior muscles of the head (see Plate 1-5). These muscles are directly deep to the semispinalis capitis muscle, and three of them bound the suboccipital triangle.
The rectus capitis posterior major muscle arises from the spinous process of the axis. Broadening as it ascends, it inserts into the middle of the inferior nuchal line of the occipital bone and into the bone below this line. The obliquus capitis inferior muscle arises from the spinous process of the axis and, passing horizontally, ends in the transverse process of the atlas. The obliquus capitis superior muscle arises from the transverse process of the atlas. Passing upward and medially, this muscle inserts into the occipital bone above the inferior nuchal line, where it overlaps the insertion of the rectus capitis posterior major. The rectus capitis posterior minor muscle lies medial to the rectus capitis posterior major muscle. It originates from the posterior tubercle of the atlas and, widening as it ascends, inserts into the medial part of the inferior nuchal line and into the occipital bone.
It is the area between the two oblique muscles and the rectus capitis major muscle that is defined as the suboccipital triangle (see Plate 1-5). Its floor is the posterior atlantooccipital membrane, which is attached to the posterior margin of the posterior arch of the atlas. Deep to this membrane, the vertebral artery occupies the groove on the upper surface of the posterior arch of the atlas as it passes medially toward the foramen magnum.
The suboccipital nerve emerges from between the vertebral artery and the posterior arch of the atlas. It divides in the dense tissue of the suboccipital triangle and branches into the suboccipital muscles. The suboccipital nerve (dorsal ramus of C1) has no cutaneous dis- tribution. The medial branch of the dorsal ramus of C2 is known as the greater occipital nerve (dorsal ramus of C2), which has a distribution as high as the vertex of the scalp. It emerges below the obliquus capitis inferior muscle and turns upward to cross the suboccipital tri- angle and reach the scalp by piercing the semispinalis capitis and trapezius muscles. The lesser occipital nerve of the cervical plexus (ventral ramus of C2) supplies the skin of the scalp behind the ear as well as the skin of the back of the ear itself. The third occipital nerve, the medial branch of the dorsal ramus of C3, distributes in the upper neck and to the scalp, to just beyond the superior nuchal line.
By far the most important of the vascular structures that traverse the craniovertebral junction are the vertebral arteries. The vertebral arteries typically enter the transverse foramina at C6. The path of the vertebral artery is relatively linear until it reaches C2, where the foramina are oriented obliquely when compared with the more horizontal orientation of the more caudal foramina. It continues through the more horizontally oriented transverse foramen of C1 and then arches anteromedially until it lies in the groove of the posterior arch of C1 known as the sulcus arteriosus. It then continues medially and pierces the atlantooccipital mem- brane. The venous drainage of the craniovertebral junction is via the jugular venous feeders and ultimately the subclavian vein. There is often a well-developed venous plexus at the C1-2 junction just lateral to the dura and around the C2 roots that surgeons must contend with when exposing the C1-2 region.