Spinal Trauma

Spinal Column

The spinal column is built up from alternating bony vertebrae and fibrocartilaginous disks, which are intimately connected by strong ligaments and supported by powerful musculotendinous masses. The individual bony elements and ligaments are described in Plates 3-2 to 3-10 . There are 33 vertebrae (7 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 4 coccygeal), although the sacral and coccygeal vertebrae are usually fused to form the sacrum and coccyx.

All vertebrae conform to a basic plan, but individual variations occur in the different regions. A typical vertebra consists of an anterior, more-or-less cylindric body and a posterior arch composed of two pedicles and two laminae , the latter united posteriorly to form a spinous process . These processes vary in shape, size, and direction in the various regions of the spine. On each side, the arch also supports a transverse process and superior and inferior articular processes ; the latter form synovial joints with corresponding processes on adjacent vertebrae, and the spinous and transverse processes provide levers for the many muscles attached to them. The increasing size of the vertebral bodies from above downward is related to the increasing weights and stresses borne by successive segments, and the sacral vertebrae are fused to form a solid wedge-shaped base—the keystone in a bridge whose arches curve down toward the hip joints. The intervertebral disks act as elastic buffers to absorb the numerous mechanical shocks sustained by the spinal column.

Only limited movements are possible between adjacent vertebrae, but the sum of these movements confers a considerable range of mobility on the vertebral column as a whole. Flexion, extension, lateral bending, rotation, and circumduction are all possible, and these actions are freer in the cervical and lumbar regions than in the thoracic. Such differences exist because the disks are thicker in the cervical and lumbar areas, the splinting effect produced by the thoracic cage is lacking, the cervical and lumbar spinous processes are shorter and less closely apposed, and the articular processes are shaped and arranged differently.

At birth, the spinal column presents a general posterior convexity, but later, the cervical and lumbar regions become curved in the opposite directions—when the infant reaches the stages of holding up its head (3 to 4 months) and sitting upright (6 to 9 months). The posterior convexities are primary curves associated with the fetal uterine position, whereas the cervical and lumbar anterior secondary curves are compensatory to permit the assumption of the upright position. There may be additional slight lateral deviations due to unequal muscular traction in right-handed and left-handed persons.

Human evolution from a quadrupedal to a bipedal posture was mainly effected by the tilting of the sacrum between the hip bones, by an increase in lumbosacral angulation, and by minor adjustments of the anterior and posterior depths of various vertebrae and disks. An erect posture greatly increases the load borne by the lower spinal joints, and, good as these ancestral adaptations were, some static and dynamic imperfections remain and predispose to strain and backache. The length of the vertebral column averages 72 cm in the adult male and 7 to 10 cm less in the female. The vertebral canal extends through the entire length of the column and provides an excellent protection for the spinal cord, the cauda equina, and their coverings. The spinal vessels and nerves pass through intervertebral foramina formed by notches on the superior and inferior borders of the pedicles of adjacent vertebrae, bounded anteriorly by the corresponding intervertebral disks, and posteriorly, by the joints between the articular processes of adjoining vertebrae. Pathologic or traumatic conditions affecting any of these structures may produce pressure on the nerves or vessels they transmit.

Atlas and Axis

The atlas and axis are the first and second cervical vertebrae, and both are atypical. They are linked together and to the skull and other cervical vertebrae by a layered pattern of craniocervical ligaments (see Plates 3-4 and 3-5 ).

The atlas (named after the mythical giant who carried the earth on his shoulders) supports the globe of the skull. It lacks a body and forms a ring consisting of shorter anterior and longer posterior arches, with two lateral masses. The enclosed vertebral foramen is relatively large.

The anterior arch is slightly curved, with an anterior midline tubercle and a posterior midline facet for articulation with the dens of the axis. The lateral masses bear superior and inferior articular facets and transverse processes. The superior articular facets are concave and ovoid (often waisted, or reniform) and are directed upward and inward as shallow cups, or foveae, for the reception of the occipital condyles. Nodding movements of the head mainly occur at these atlanto-occipital joints. The inferior articular facets are almost circular, gently concave, and face downward and slightly medially and backward; they articulate with the superior articular facets on the axis. The transverse processes are each pierced by a foramen for the vertebral artery, and project so far laterally that they can be easily palpated by pressing inward between the mandibular angles and the mastoid processes. They provide attachments and levers for some of the muscles involved in head rotation. On the anteromedial aspect of each lateral mass is a small tubercle for the attachment of the transverse ligament of the atlas.

The posterior arch is more curved than the anterior and has a small posterior tubercle , which is a rudimentary spinous process. Just behind each superior articular facet is a shallow groove for the vertebral artery and first cervical spinal nerve, the nerve lying between the artery and the bone.

The axis, or second cervical vertebra, has a toothlike process, or dens, projecting upward from its body. The dens is really the divorced body of the atlas that has united with the axis to form a pivot around which the atlas and the superjacent skull can rotate. Its anterior surface has an oval anterior facet for articulation with the facet on the back of the anterior arch of the atlas, and a smaller posterior facet lower down on its posterior surface, which is separated from the transverse ligament of the atlas by a small bursa. The apex of the dens is attached to the lower end of the apical ligament, and the alar ligaments are attached to its sides.

The body of the axis has a lower liplike projection that overlaps the anterosuperior border of the third cervical vertebra. Its anterior surface shows a median ridge separating slight depressions for slips of the longus colli muscles. The posteroinferior border of the body is less prominent, and attached to it are the tectorial membrane and the posterior longitudinal spinal ligament. The pedicles and laminae are stout, and the latter end in a strong, bifid spinous process . The vertebral foramen of the axis is somewhat smaller than that of the atlas. On each side of the body are superior and inferior articular and transverse processes. The articular processes are offset, because the superior pair is anterior in position to the inferior pair. They articulate with the adjoining processes of the atlas and third cervical vertebra. The transverse processes are smaller and shorter than those of the atlas, and their foramina are inclined superolaterally to allow the contained vertebral arteries and nerves to pass easily into the more widely spaced transverse foramina of the atlas.

Cervical Vertebrae

The first two cervical vertebrae, the atlas and the axis, are illustrated in Plate 3-2 . The other five (C3 to C7) show the general vertebral features, but cervical vertebrae are easily distinguishable by the presence of foramina in their transverse processes, which (except in the case of the seventh vertebra) transmit the vertebral vessels and nerves (see Plate 3-2 ).

The cervical vertebral bodies are smaller than those of the other movable vertebrae and increase in size from above downward; they are broader in the transverse diameter than anteroposteriorly. The superior body surfaces are concave from side to side and slightly convex from front to back, whereas the inferior surfaces are reciprocally curved or saddle shaped. The lateral edges of the superior body surface are raised, whereas those of the lower surface are beveled, and small clefts exist between them. Some claim these are miniature synovial joints, but others believe they are merely spaces in the lateral parts of the corresponding intervertebral disks.

The vertebral foramina are comparatively large in order to accommodate the cervical enlargement of the spinal cord; they are bounded by the bodies, pedicles, and laminae of the vertebrae. The pedicles project posterolaterally from the bodies and are grooved by superior and inferior vertebral notches of almost equal depth, which form the intervertebral foramina by connecting with similar notches on adjacent vertebrae. The medially directed laminae are thin and relatively long and fuse posteriorly to form short, bifid spinous processes . Projecting laterally from the junction of the pedicles and laminae are articular pillars supporting superior and inferior articular facets .

Each transverse process is pierced by a foramen, bounded by narrow bony bars ending in anterior and posterior tubercles; these are interconnected lateral to the foramen by the so-called costotransverse bar . Only the medial part of the posterior bar represents the true transverse process; the anterior and costotransverse bars and the lateral portion of the posterior bar constitute the costal element. These elements, especially in the seventh and/or sixth cervical vertebrae, may develop abnormally to form cervical ribs. The upper surfaces of the costotransverse bars are grooved and lodge the anterior primary rami of the spinal nerves. The anterior tubercles of the sixth cervical vertebra are large and are termed the carotid tubercles because the common carotid arteries lie just anteriorly and can be compressed against them.

The seventh cervical vertebra is called the vertebra prominens because its spinous process is long and ends in a tubercle that is easily palpable at the lower end of the nuchal furrow; the spinous process of the first thoracic vertebra is just as prominent. The seventh cervical vertebra sometimes lacks a transverse foramen on one or both sides; when present, the foramina transmit only small accessory vertebral veins.

External Craniocervical Ligaments

The ligaments uniting the cranium, atlas, and axis allow free, yet safe, movement of the head, and extra security is provided by the ligamentous action of the surrounding muscles. Ligaments best seen from the external aspect are shown in the illustration.

The anterior atlanto-occipital membrane is a wide, dense, fibroelastic band extending between the anterior margin of the foramen magnum and the upper border of the anterior arch of the atlas. Laterally, it is continuous with the articular capsules of the atlanto-occipital joints. In the midline, it is reinforced by the upward continuation of the anterior longitudinal ligament.

The posterior atlanto-occipital membrane is broader and thinner than the anterior one and connects the posterior margin of the foramen magnum with the upper border of the posterior arch of the atlas. On each side, it arches over the groove for the vertebral artery, leaving an opening for the upward passage of the artery and the outward passage of the first cervical spinal nerve.

Articular capsules surround the joints between the occipital condyles and the superior atlantal facets. The capsules are rather loose, allowing nodding movements of the head, and are thin medially; laterally, they are thickened and form the lateral atlanto-occipital ligaments , which limit lateral tilting of the head.

The anterior longitudinal ligament extends from the base of the skull to the sacrum. Its uppermost part reinforces the anterior atlanto-occipital membrane in the midline. The part between the anterior tubercle of the atlas and the anterior median ridge on the axis may have lateral extensions—the atlantoaxial (epistrophic) ligaments .

The ligamentum nuchae is a dense fibroelastic membrane stretching from the external occipital protuberance and crest to the posterior tubercle of the atlas and the spinous processes of all the other cervical vertebrae. It provides areas for muscular attachments and forms a midline septum between the posterior cervical muscles. The ligamentum nuchae is better developed in quadrupeds than in humans.

The ligamenta flava contain a high proportion of yellow elastic fibers and connect the laminae of adjacent vertebrae. They are present between the posterior arch of the atlas and the laminae of the axis but absent between the atlas and skull.

Intervertebral disks are lacking between the occiput and atlas and between the atlas and axis.

Internal Craniocervical Ligaments

The ligaments on the posterior aspects of the vertebral bodies contribute added strength to the craniocervical region, and some are specifically arranged to check excessive movements, such as rotation at the median and lateral atlantoaxial joints.

The broad, strong tectorial membrane lies within the vertebral canal. It prolongs the posterior longitudinal ligament upward from the posterior surface of the body of the axis to the anterior and anterolateral margins of the foramen magnum, where it blends with the dura mater. It covers the dens and its ligaments and gives added protection to the junctional area between the medulla oblongata and spinal cord.

The median atlantoaxial pivot joint lies between the dens of the axis and the ring formed by the anterior arch and transverse ligament of the atlas (see Plate 3-2 ). Two small synovial cavities surrounded by thin articular capsules are present between the dens and the anterior arch in front, and the transverse ligament of the atlas behind.

The transverse ligament of the atlas is a strong band passing horizontally behind the dens and attached on each side to a tubercle on the medial side of the lateral mass of the atlas. From its midpoint, bands pass vertically upward and downward to become fixed, respectively, to the basilar part of the occipital bone between the tectorial membrane and the apical ligament of the dens and to the posterior surface of the body of the axis: the superior and inferior longitudinal fascicles . These transverse and vertical bands together form the cruciform ligament.

The apical ligament is a slender cord connecting the apex of the dens to the anterior midpoint of the foramen magnum, lying between the anterior atlanto-occipital membrane and the upper limb of the cruciform ligament.

The alar ligaments are two fibrous bands stretching upward and outward from the superolateral aspects of the dens to the medial sides of the occipital condyles. They check excessive rotation at the median atlanto-occipital joint.

Lateral atlantoaxial joints are formed between the almost-flat inferior articular facets on the lateral masses of the atlas and the superior articular facets of the axis. They are synovial joints with thin, loose articular capsules. An accessory ligament extends from near the base of the dens to the lateral mass of the atlas, close to the attachment of the transverse ligament. It assists the alar ligaments in restricting atlantoaxial rotation.

Thoracic Vertebrae

The 12 thoracic vertebrae are intermediate in size between the smaller cervical and larger lumbar vertebrae. The vertebral bodies are heart shaped and are slightly deeper posteriorly than anteriorly. They are easily recognized by costal facets on both sides of the bodies and on all the transverse processes (except those of the eleventh and twelfth thoracic vertebrae), which articulate, respectively, with facets on the heads and tubercles of the corresponding ribs.

The vertebral foramina are smaller and more rounded than those in the cervical region, and so conform to the reduced size and more circular shape of the spinal cord in the thoracic region. They are bounded by the posterior surfaces of the vertebral bodies and by the pedicles and laminae forming the vertebral arches. The stout pedicles are directed backward; they have very shallow superior and much deeper inferior vertebral notches. The laminae are short, relatively thick, and partly overlap each other from above downward. The typical thoracic superior articular processes project upward from the junctions of the pedicles and laminae, and their facets slant backward and slightly upward and outward. The inferior articular processes project downward from the anterior parts of the laminae, and their facets face forward and slightly downward and inward. The processes and facets in the cervicothoracic and thoracolumbar junctional areas show gradual transitional changes.

Most of the thoracic spinous processes are long and are inclined downward and backward. Those of the upper and lower thoracic vertebrae are more horizontal. The transverse processes are also relatively long and extend posterolaterally from the junctions of the pedicles and laminae. Except for those of the lowest two or, occasionally, three thoracic vertebrae, the transverse processes have small oval facets near their tips, which articulate with similar facets on the corresponding rib tubercles.

Adjacent vertebral bodies are connected by intervertebral disks and by anterior and posterior longitudinal ligaments ; the transverse processes, by intertransverse ligaments ; the laminae, by ligamenta flava ; and the spinous processes, by supraspinal and interspinal ligaments . The joints between the articular processes are surrounded by fibrous articular capsules.

Costovertebral Joints. The ribs are connected to the vertebral bodies and transverse processes by various ligaments. The costocentral joints between the bodies and rib heads have articular capsules , and the second to tenth costal heads, each of which articulates with two vertebrae, are connected to the corresponding intervertebral disks by intra-articular ligaments. Radiate (stellate) ligaments unite the anterior aspects of the rib heads with the sides of the vertebral bodies above and below, and with the intervening disks.

The costotransverse joints between the facets on the transverse processes and on the tubercles of the ribs are also surrounded by articular capsules. They are reinforced by a (middle) costotransverse ligament between the rib neck and the adjoining transverse process, a superior costotransverse ligament between the rib neck and the transverse process of the vertebra above, and a lateral costotransverse ligament interconnecting the end of a transverse process to the nonarticular part of the related costal tubercle.

Lumbar Vertebrae and Intervertebral Disk

The five lumbar vertebrae are the largest separate vertebrae and are distinguished by the absence of transverse foramina and costal facets. The vertebral bodies are wider from side to side than from front to back, and the upper and lower surfaces are kidney shaped and almost parallel, except in the case of the fifth vertebral body, which is slightly wedge shaped. The triangular vertebral foramina are larger in the thoracic vertebrae and smaller in the cervical vertebrae.

The pedicles are short and strong and arise from the upper and posterolateral aspects of the bodies; the superior vertebral notches are therefore less deep than the inferior notches. The laminae are short, broad plates that meet in the midline to form the quadrangular and almost horizontal spinous processes . The intervals between adjacent laminae and spinous processes are relatively wide.

The articular processes project vertically upward and downward from the junctional areas between the pedicles and the laminae. The superior facets are gently concave and face posteromedially to embrace the inferior facets of the vertebra above, which are curved and disposed in a reciprocal fashion. This arrangement permits some flexion and extension but very little rotation. The transverse processes of the upper three lumbar vertebrae are long and slender, whereas those of the fourth, and especially of the fifth, are more pyramidal.

Near the roots of each transverse process are small accessory processes ; other small, rounded mammillary processes protrude from the posterior margins of the superior articular processes. The former may represent the true transverse processes (or their tips) because many of the so-called transverse processes are really costal elements. In the first lumbar vertebra, these elements occasionally develop into lumbar ribs.

The fifth lumbar vertebra is atypical. It is the largest, its body is deeper anteriorly, its inferior articular facets face almost forward and are set more widely apart, and the roots of its stumpy transverse processes are continuous with the posterolateral parts of the body and with the entire lateral surfaces of the pedicles.

The intervertebral disks are interposed between the adjacent vertebral bodies from the axis to the sacrum and are immensely strong fibrocartilaginous structures that provide powerful bonds and elastic buffers. They consist of outer concentric layers of fibrous tissue—the annulus fibrosus (the fibers in adjacent layers are arranged obliquely but in opposite directions, to assist in resisting torsion)—and a central springy, pulpy zone, the nucleus pulposus. The blood and nerve supplies to the disks are inconspicuous. If the annular fibers give way as a result of injury or disease, the enclosed turgid nucleus pulposus may prolapse and press on related nervous and vascular structures.

In health and maturity, the intervertebral disks account for almost 25% of the length of the vertebral column; they are thinnest in the upper thoracic region and thickest in the lumbar region. In vertical section, the lumbar disks are rather wedge shaped, with the thicker edge anteriorly. The forward convexity of the lumbar spine is due more to the shape of the disks than to disparities between the anterior and posterior depths of the lumbar vertebrae. The more defined wedge shape of the lumbosacral disk helps to minimize the effects of the marked lumbosacral angulation.

As age advances, the nucleus pulposus undergoes changes: its water content decreases, its mucoid matrix is gradually replaced by fibrocartilage, and it ultimately comes to resemble the annulus fibrosus. The resultant loss of depth in each disk is small, but overall, it may amount to a decrease of 2 to 3 cm in the height of the spinal column.