Vertebrae and Joints
The seven cervical vertebrae are relatively small and enclose a wide vertebral canal with adequate space for the cervical part of the spinal cord. Each transverse process is perforated by a foramen transversarium transmitting the vertebral vessels. The spinous processes all give attachment to a strong midline elastic ligament, the ligamentum nuchae.
Four of the cervical vertebrae (numbers 3–6) have a typical appearance whereas the first, second and seventh are modified. The typical vertebrae (Fig. 8.11) possess short bifid spines and their transverse processes have anterior and posterior tubercles. Often the upper and lower surfaces of the vertebral bodies are not flat but curve upwards at their lateral edges. The facets on the superior articular processes face obliquely backwards and upwards and therefore rotation and lateral flexion always occur together.
The first cervical vertebra, the atlas (Fig. 8.12), has anterior and posterior arches, relatively large transverse processes and two lateral masses. The atlas has no body and its spinous process is represented by a tubercle. On the superior surface of each lateral mass is a concave facet, which articulates with the convex occipital condyle of the skull. The atlantooccipital joints permit flexion and extension (nodding movements).
The second cervical vertebra, the axis (Fig. 8.13), possesses some of the features of a typical cervical vertebra but it has a unique vertical projection, the dens (odontoid process). This projects superiorly from the upper surface of its body and represents the body of the atlas. The dens articulates by a synovial joint with a facet on the posterior surface of the anterior arch of the atlas, where it is retained by the alar, apical and transverse ligaments (Figs 8.15 & 8.16). The planes of the lateral atlantoaxial joints and the pivot joint of the dens (Fig. 8.14) allow rotation of the head as in looking from side to side.
The seventh cervical vertebra (Fig. 8.17) possesses a long, non-bifid spine, which provides the inferior attachment for the ligamentum nuchae. The spinous process is easily palpable and hence, the vertebra is called the vertebra prominens. The foramina transversaria of this vertebra are traversed by the vertebral veins but not by the arteries. The costal element of the seventh vertebra, represented by the anterior tubercle and the bar of bone in front of the transverse foramen, may form a cervical rib. The subclavian artery and the first thoracic nerve root may be stretched and distorted as they pass over (superior to) a cervical rib, leading to arterial damage and pain referred along the medial side of the upper limb.
The joints of the whole cervical column allow movements of extension, rotation and lateral flexion. These movements are brought about by the prevertebral and postvertebral muscles (Fig. 8.18), assisted by sternocleidomastoid and trapezius. The prevertebral muscles comprise the scalene group (p. 331) and the longus colli group. The latter passes from the base of the skull down the anterior surface of the vertebral column into the thorax. The prevertebral muscles are much smaller than the postvertebral group which has an antigravity action in keeping the head upright.
Arthritis involving joints of the cervical spine is often associated with the formation of bony outgrowths (osteophytes), which may compress the nerve roots that contribute to the brachial plexus (p. 80). Injuries to the cervical column, particularly involving fracture or dislocation of vertebrae, may result in spinal cord injury leading to quadriplegia or death. The atlantoaxial joint is particularly liable to disruption in hyperextension injuries.
The bodies of the 12 thoracic vertebrae increase in size from above downwards. The bodies bear characteristic costal facets (Fig. 8.19), which form synovial joints with the heads of the ribs. Typically a vertebral body possesses one pair of facets (superior and inferior costal facets) on each side adjacent to the attachment of the pedicle. The upper facet receives the rib whose number corresponds to the vertebra, while the lower facet articulates with the rib below. However the tenth, eleventh and twelfth vertebrae possess facets on each side, which are for articulation with their own ribs. The vertebral canal is smaller than in any other region.
The transverse processes project laterally and backwards and typically each bears near its tip a facet for the tubercle of the corresponding rib. The spinous processes are long and slope steeply downwards. The plane of the joints between the facets on the articular processes is almost vertical and permits rotation. However, all movements in the thoracic region are restricted by the rib cage.
The upper four lumbar vertebrae are very similar. The vertebral foramina are moderate in size (Fig. 8.20) but the bodies are comparatively large, with concave sides. The transverse processes taper and are directed laterally and slightly backwards. The spinous processes are deep and rectangular. Facets on the superior articular processes face medially and ‘grasp’ the laterally directed inferior facets of the vertebra above, permitting wide ranges of flexion, extension and lateral flexion but severely restricting rotation.
The fifth lumbar vertebra has shorter transverse processes and a less angular spinous process. Its inferior articular facets are widely separated and face anteriorly. They articulate with the sacrum (Fig. 8.21) and prevent forward displacement of the vertebra. A fracture or developmental defect between the superior and inferior articular processes of the fifth lumbar vertebra will allow its body to slip anteriorly, a condition called spondylolisthesis, which may stretch or compress the cauda equina (p. 411). One or both transverse processes may be fused with the upper part of the sacrum (sacralization of the fifth lumbar vertebra), which can cause difficulty in the interpretation of radiographs.
Sacral and coccygeal vertebrae
The sacrum is a triangular bone formed by the fusion of five vertebrae (Figs 8.21 & 8.22). The upper surface of the sacrum resembles that of a lumbar vertebra and carries the lumbosacral disc. Below the apex of the sacrum lies the coccyx (Fig. 8.23), which may be a small single bone or up to four rudimentary vertebrae. The coccyx and the sacrum usually articu- late via a small intervertebral disc, although they may be fused. The sacrum slopes backwards and downwards and is concave anteriorly. The bone in the female has relatively small joint surfaces and larger alae, while in the male the larger sacral promontory often creates a heart-shaped pelvic inlet (p. 214). The fused pedicles and laminae enclose the sacral canal, triangular in cross-section, which opens posteroinfe- riorly at the V-shaped sacral hiatus. The canal contains the lower part of the cauda equina, comprising the roots of the sacral and coccygeal nerves. The anterior rami of the upper four sacral nerves pass into the pelvis via the anterior sacral foramina and contribute to the sacral plexus. The posterior rami traverse the posterior sacral foramina (Fig. 8.22). Lateral to the foramina are the lateral masses, each of which bears an auricular surface for articulation with the ilium (Fig. 8.23). Anaesthetic may be injected through the sacral hiatus and the caudal canal into the epidural space to anaesthetize the cauda equina.
The sacroiliac joint is synovial but allows very little movement because of the irregularity of the articulating surfaces and the thick posterior interosseous ligament. Each joint is further supported by the anterior and posterior sacroiliac ligaments and the iliolumbar, sacrospinous and sacrotuberous ligaments. Body weight, acting downwards through the lumbosacral disc, tends to rotate the lower part of the sacrum backwards, a movement prevented by the sacrospinous and sacrotuberous ligaments (Fig. 8.25).
The different features of vertebrae from the regions of the column are summarised in Table 8.2.
The plane synovial joints between the facets of adjacent superior and inferior articular processes are called zygapophysial or facet joints. The joints in the different regions of the vertebral column allow different movements, determined by the orientations of the articular processes.
Intervertebral discs connect adjacent vertebral bodies (Fig. 8.24) and act as fibrocartilaginous joints along the whole length of the vertebral column. Like the vertebral bodies, the discs gradually increase in size from above downwards, the largest being the lumbosacral disc between the fifth lumbar vertebra and the sacrum (Fig. 8.25). The discs contribute about one-fifth of the length of the vertebral column. Each disc consists of a laminated anulus fibrosus surrounding a gelatinous nucleus pulposus (Fig. 8.28). The nucleus pulposus lies closer to the posterior surface of the disc and thus is more liable to posterior herniation when the disc is damaged. This herniation, often called a slipped disc, may occur near the midline and compress the spinal cord or cauda equina. Posterolateral herniation may compress nerves near the intervertebral foramen (p. 396) and cause muscle weakness and referred pain. Usually, the herniation affects nerve roots passing through the intervertebral foramen below the affected disc. In the cervical region, herniation most commonly occurs between vertebrae C6–C7, affecting nerve C7, and between vertebrae C7–T1, affecting nerve C8. Compression of nerve C7 may produce pain in the dermatome C7 (p. 74) and weakness of extension of the elbow and wrist joints. Compression of nerve C8 may produce pain in the dermatome C8 (p. 74) and weakness of finger movements. In the lumbar spine, herniation most commonly occurs between vertebrae L4–L5, affecting nerve L5, and between vertebrae L5–S1, affecting nerve S1. Compression of nerve L5 may produce pain in the L5 dermatome (p. 258) and weakness of ankle dorsiflexion and extension of the great toe. Compression of nerve S1 may produce pain in the S1 dermatome (p. 258) and weakness of plantar flexion. Pain referred from the back into the lower limb is often called sciatica.
The intervertebral discs are reinforced by posterior and anterior longitudinal ligaments (Figs 8.26 & 8.27). These ligaments attach to vertebral bodies and intervertebral discs and anchor inferiorly to the sacrum and superiorly to the cervical vertebrae or skull. Whiplash injuries involving excessive extension-flexion are caused by rear-end car crashes. There may be damage to the joints and ligaments of the cervical spine, including the anterior longitudinal ligament, resulting in cervical pain and restricted movement.
Other ligaments interconnect the laminae, spinous processes and transverse processes of adjacent vertebrae. Ligamenta flava interconnect the laminae within the vertebral canal. The high content of elastic tissue gives these ligaments their yellow appearance and they assist return of the vertebral column to the erect position following flexion. Supraspinous and interspinous ligaments connect adjacent spinous processes of thoracic and lumbar vertebrae. It is through these ligaments that a needle is inserted to withdraw cerebrospinal fluid during lumbar puncture. The supraspinous and interspinous ligaments are replaced in the cervical region by the ligamentum nuchae, which attaches to the skull at the external occipital protuberance and crest and to the spinous processes of all the cervical vertebrae. Intertransverse ligaments connect the transverse processes of adjacent vertebrae. The lumbosacral joint is reinforced by the iliolumbar ligament, which attaches the transverse process of the fifth lumbar vertebra to the iliac crest (Fig. 8.28).