VERTEBRAL COLUMN

SUMMARY OF PROJECTIONS

Icons in the Essential column indicate projections that are frequently performed in the United States and Canada. Students should be competent in these projections.

Vertebral Column

The vertebral column, or spine, forms the central axis of the skeleton and is centered in the midsagittal plane of the posterior part of the trunk. The vertebral column has many functions: It encloses and protects the spinal cord, acts as a support for the trunk, supports the skull superiorly, and provides for attachment for the deep muscles of the back and the ribs laterally. The upper limbs are supported indirectly via the ribs, which articulate with the sternum. The sternum articulates with the shoulder girdle. The vertebral column articulates with each hip bone at the sacroiliac joints. This articulation supports the vertebral column and transmits the weight of the trunk through the hip joints and to the lower limbs.

The vertebral column is composed of small segments of bone called vertebrae. Disks of fibrocartilage are interposed between the vertebrae and act as cushions. The vertebral column is held together by ligaments, and it is jointed and curved so that it has considerable flexibility and resilience.

In early life, the vertebral column usually consists of 33 small, irregularly shaped bones. These bones are divided into five groups and named according to the region they occupy (Fig. 8-1). The superiormost seven vertebrae occupy the region of the neck and are termed cervical vertebrae. The succeeding 12 bones lie in the dorsal, or posterior, portion of the thorax and are called the thoracic vertebrae. The five vertebrae occupying the region of the loin are termed lumbar vertebrae. The next five vertebrae, located in the pelvic region, are termed sacral vertebrae. The terminal vertebrae, also in the pelvic region, vary from three to five in number in adults and are termed the coccygeal vertebrae.

Fig. 8-1 A, Anterior aspect of vertebral column. B, Lateral aspect of vertebral column, showing regions and curvatures. C, Midsagittal MRI scan of cervical and thoracic spine. Note curves and spinal cord protected by vertebrae.

The 24 vertebral segments in the upper three regions remain distinct throughout life and are termed the true or movable vertebrae. The pelvic segments in the two lower regions are called false or fixed vertebrae because of the change they undergo in adults. The sacral segments usually fuse into one bone called the sacrum, and the coccygeal segments, referred to as the coccyx, also fuse into one bone.

Vertebral Curvature

Viewed from the side, the vertebral column has four curves that arch anteriorly and posteriorly from the midcoronal plane of the body. The cervical, thoracic, lumbar, and pelvic curves are named for the regions they occupy.

In this text, the vertebral curves are discussed in reference to the anatomic position and are referred to as “convex anteriorly” or “concave anteriorly.” Because physicians and surgeons evaluate the spine from the posterior aspect of the body, convex and concave terminology can be the exact opposites. When viewed posteriorly, the normal lumbar curve can correctly be referred to as “concave posteriorly.” Whether the curve is described as “convex anteriorly” or “concave posteriorly,” the curvature of the patient’s spine is the same. The cervical and lumbar curves, which are convex anteriorly, are called lordotic curves. The thoracic and pelvic curves are concave anteriorly and are called kyphotic curves (see Fig. 8-1 B,). The cervical and thoracic curves merge smoothly.

The lumbar and pelvic curves join at an obtuse angle termed the lumbosacral angle. The acuity of the angle in the junction of these curves varies among patients. The thoracic and pelvic curves are called primary curves because they are present at birth. The cervical and lumbar curves are called secondary or compensatory curves because they develop after birth. The cervical curve, which is the least pronounced of the curves, develops when an infant begins to hold the head up at about 3 or 4 months of age and begins to sit alone at about 8 or 9 months of age. The lumbar curve develops when the child begins to walk at about 1 to 1½ years of age. The lumbar and pelvic curves are more pronounced in females, who have a more acute angle at the lumbosacral junction.

Any abnormal increase in the anterior concavity (or posterior convexity) of the thoracic curve is termed kyphosis (Fig. 8-2, B). Any abnormal increase in the anterior convexity (or posterior concavity) of the lumbar or cervical curve is termed lordosis.

Fig. 8-2 A, Scoliosis, lateral curvature of spine. B, Kyphosis, increased convexity of thoracic spine, and lordosis, increased concavity of lumbar spine. C, PA thoracic and lumbar spine showing severe scoliosis.

In frontal view, the vertebral column varies in width in several regions (see Fig. 8-1). Generally, the width of the spine gradually increases from the second cervical vertebra to the superior part of the sacrum and then decreases sharply. A slight lateral curvature is sometimes present in the upper thoracic region. The curve is to the right in right-handed persons and to the left in left-handed persons. For this reason, lateral curvature of the vertebral column is believed to be the result of muscle action and to be influenced by occupation. An abnormal lateral curvature of the spine is called scoliosis. This condition also causes the vertebrae to rotate toward the concavity. The vertebral column develops a second or compensatory curve in the opposite direction to keep the head centered over the feet (see Fig. 8-2, A).

Typical Vertebra

A typical vertebra is composed of two main parts—an anterior mass of bone called the body and a posterior ringlike portion called the vertebral arch (Figs. 8-3 and 8-4). The vertebral body and arch enclose a space called the vertebral foramen. In the articulated column, the vertebral foramina form the vertebral canal.

Fig. 8-3 A, Superior aspect of thoracic vertebra, showing structures common to all vertebral regions. B, Axial CT image of lumbar vertebra showing most of anatomy identified in A. Note spinal cord (white) within vertebral foramen. l, lamina; tp, transverse process. (B, Modified from Kelley LL, Petersen CM: Sectional anatomy for imaging professionals, ed 2, St Louis, 2007, Mosby.)

Fig. 8-4 Lateral aspect of thoracic vertebra, showing structures common to all vertebral regions.

The body of the vertebra is approximately cylindric in shape and is composed largely of cancellous bony tissue covered by a layer of compact tissue. From the superior aspect, the posterior surface is flattened, and from the lateral aspect, the anterior and lateral surfaces are concave. The superior and inferior surfaces of the bodies are flattened and covered by a thin plate of articular cartilage.

In the articulated spine, the vertebral bodies are separated by intervertebral disks. These disks account for approximately one fourth of the length of the vertebral column. Each disk has a central mass of soft, pulpy, semigelatinous material called the nucleus pulposus, which is surrounded by an outer fibrocartilaginous disk called the anulus fibrosus. It is common for the pulpy nucleus to rupture or protrude into the vertebral canal, impinging on a spinal nerve. This condition is called herniated nucleus pulposus (HNP), or more commonly slipped disk. HNP most often occurs in the lumbar region as a result of improper body mechanics, and it can cause considerable discomfort and pain. HNP also occurs in the cervical spine as a result of trauma (i.e., whiplash injuries) or degeneration.

The vertebral arch (see Figs. 8-3 and 8-4) is formed by two pedicles and two laminae that support four articular processes, two transverse processes, and one spinous process. The pedicles are short, thick processes that project posteriorly, one from each side, from the superior and lateral parts of the posterior surface of the vertebral body. The superior and inferior surfaces of the pedicles, or roots, are concave. These concavities are called vertebral notches. By articulation with the vertebrae above and below, the notches form intervertebral foramina for the transmission of spinal nerves and blood vessels. The broad, flat laminae are directed posteriorly and medially from the pedicles.

The transverse processes project laterally and slightly posteriorly from the junction of the pedicles and laminae. The spinous process projects posteriorly and inferiorly from the junction of the laminae in the posterior midline. A congenital defect of the vertebral column in which the laminae fail to unite posteriorly at the midline is called spina bifida. In serious cases of spina bifida, the spinal cord may protrude from the affected individual’s body.

Four articular processes, two superior and two inferior, arise from the junction of the pedicles and laminae to articulate with the vertebrae above and below (see Fig. 8-4). The articulating surfaces of the four articular processes are covered with fibrocartilage and are called facets. In a typical vertebra, each superior articular process has an articular facet on its posterior surface, and each inferior articular process has an articular facet on the anterior surface. The planes of the facets vary in direction in the different regions of the vertebral column and often vary within the same vertebra. The articulations between the articular processes of the vertebral arches are referred to as zygapophyseal joints. Some texts refer to these joints as interarticular facet joints.

The movable vertebrae, with the exception of the first and second cervical vertebrae, are similar in general structure. Each group has certain distinguishing characteristics, however, that must be considered in radiography of the vertebral column.

Cervical Vertebrae

The first two cervical vertebrae are atypical in that they are structurally modified to join the skull. The seventh vertebra is also atypical and slightly modified to join the thoracic spine. Atypical and typical vertebrae are described in the following sections.

ATLAS

The atlas, the first cervical vertebra (C1), is a ringlike structure with no body and a very short spinous process (Fig. 8-5). The atlas consists of an anterior arch, a posterior arch, two lateral masses, and two transverse processes. The anterior and posterior arches extend between the lateral masses. The ring formed by the arches is divided into anterior and posterior portions by a ligament called the transverse atlantal ligament. The anterior portion of the ring receives the dens (odontoid process) of the axis, and the posterior portion transmits the proximal spinal cord.

Fig. 8-5 Superior aspect of atlas (C1).

The transverse processes of the atlas are longer than those of the other cervical vertebrae, and they project laterally and slightly inferiorly from the lateral masses. Each lateral mass bears a superior and an inferior articular process. The superior processes lie in a horizontal plane, are large and deeply concave, and are shaped to articulate with the occipital condyles of the occipital bone of the cranium.

AXIS

The axis, the second cervical vertebra (C2) (Figs. 8-6 and 8-7), has a strong conical process arising from the upper surface of its body. This process, called the dens or odontoid process, is received into the anterior portion of the atlantal ring to act as the pivot or body for the atlas. At each side of the dens on the superior surface of the vertebral body are the superior articular processes, which are adapted to join with the inferior articular processes of the atlas. These joints, which differ in position and direction from the other cervical zygapophyseal joints, are clearly visualized in an AP projection if the patient is properly positioned. The inferior articular processes of the axis have the same direction as the processes of the succeeding cervical vertebrae. The laminae of the axis are broad and thick. The spinous process is horizontal in position. Fig. 8-8 shows the relationship of C1 and C2 with the occipital condyles.

Fig. 8-6 Anterior aspect of axis (C2).

Fig. 8-7 Lateral aspect of axis (C2).

Fig. 8-8 Coronal MRI shows atlas, axis, and occipital bone of skull and their relationship. (Courtesy Siemens Medical Systems, Iselin, NJ.)

SEVENTH VERTEBRA

The seventh cervical vertebra (C7), termed the vertebra prominens, has a long, prominent spinous process that projects almost horizontally to the posterior. The spinous process of this vertebra is easily palpable at the posterior base of the neck. It is convenient to use this process as a guide in localizing other vertebrae.

TYPICAL CERVICAL VERTEBRA

The typical cervical vertebrae (C3-6) have a small, transversely located, oblong body with slightly elongated anteroinferior borders (Fig. 8-9). The result is anteroposterior overlapping of the bodies in the articulated column. The transverse processes of the typical cervical vertebra arise partly from the sides of the body and partly from the vertebral arch. These processes are short and wide, are perforated by the transverse foramina for the transmission of the vertebral artery and vein, and present a deep concavity on their upper surfaces for the passage of the spinal nerves. All cervical vertebrae contain three foramina: the right and left transverse foramina and the vertebral foramen.

Fig. 8-9 A, Superior aspect of typical cervical vertebra. B, Lateral aspect of typical cervical vertebra.

The pedicles of the typical cervical vertebra project laterally and posteriorly from the body, and their superior and inferior vertebral notches are nearly equal in depth. The laminae are narrow and thin. The spinous processes are short, have double pointed (bifid) tips, and are directed posteriorly and slightly inferiorly. Their palpable tips lie at the level of the interspace below the body of the vertebra from which they arise.

The superior and inferior articular processes are located posterior to the transverse processes at the point where the pedicles and laminae unite. Together the processes form short, thick columns of bone called articular pillars. The fibrocartilaginous articulating surfaces of the articular pillars contain facets. The zygapophyseal facet joints of the second through seventh cervical vertebrae lie at right angles to the midsagittal plane and are clearly shown in a lateral projection (Fig. 8-10, A).

Fig. 8-10 A, Direction of cervical zygapophyseal joints. B, Direction of cervical intervertebral foramina.

The intervertebral foramina of the cervical region are directed anteriorly at a 45-degree angle from the midsagittal plane of the body (Fig. 8-11; see Fig. 8-10, B). The foramina are also directed at a 15-degree inferior angle to the horizontal plane of the body. Accurate radiographic demonstration of these foramina requires a 15-degree longitudinal angulation of the central ray and a 45-degree medial rotation of the patient (or a 45-degree medial angulation of the central ray). A lateral projection is necessary to show the cervical zygapophyseal joints. The positioning rotations required for showing the intervertebral foramina and zygapophyseal joints of the cervical spine are summarized in Table 8-1. A full view of the cervical spine is shown in Fig. 8-12 along with surrounding tissues.

TABLE 8-1

Positioning rotations needed to show intervertebral foramina and zygapophyseal joints

^*From the anatomic position.

Fig. 8-11 Anterior oblique of cervical vertebrae, showing intervertebral transverse foramina and zygapophyseal joints.

Fig. 8-12 A, MRI sagittal plane of cervical spine. Note position of spinal cord (arrow) in relation to vertebral bodies. B, MRI sagittal plane showing anterior displacement of C4 on C5. Narrowed spinal canal compresses spinal cord causing paralysis. C, Axial CT of typical cervical vertebra. (B, Modified from Jackson SA, Thomas RM: Cross-sectional imaging made easy, New York, 2004, Churchill Livingstone; C, modified from Kelley LL, Petersen CM: Sectional anatomy for imaging professionals, ed 2, St Louis, 2007, Mosby.)

Thoracic Vertebrae

The bodies of the thoracic vertebrae increase in size from the 1st to the 12th vertebrae. They also vary in form, with the superior thoracic bodies resembling cervical bodies and the inferior thoracic bodies resembling lumbar bodies. The bodies of the typical (third through ninth) thoracic vertebrae are approximately triangular in form (Figs. 8-13 and 8-14). These vertebral bodies are deeper posteriorly than anteriorly, and their posterior surface is concave from side to side.

Fig. 8-13 Superior aspect of thoracic vertebra.

Fig. 8-14 Lateral aspect of thoracic vertebra.

The posterolateral margins of each thoracic body have costal facets for articulation with the heads of the ribs (Fig. 8-15). The body of the first thoracic vertebra presents a whole costal facet near its superior border for articulation with the head of the first rib and presents a demifacet (half-facet) on its inferior border for articulation with the head of the second rib. The bodies of the second through eighth thoracic vertebrae contain demifacets superiorly and inferiorly. The ninth thoracic vertebra has only a superior demifacet. Finally, the 10th, 11th, and 12th thoracic vertebral bodies have a single whole facet at the superior margin for articulation with the 11th and 12th ribs (Table 8-2).

TABLE 8-2

Costal facets and demifacets

^*On each side of a vertebral body.

Fig. 8-15 Thoracic spine. A, Posterior oblique aspect showing zygapophyseal joints, intervertebral foramina, and facets and demifacets (see Table 8-2). B, Posterior aspect showing attachment of ribs and joints.

The transverse processes of the thoracic vertebrae project obliquely, laterally, and posteriorly. With the exception of the 11th and 12th pairs, each process has on the anterior surface of its extremity a small concave facet for articulation with the tubercle of a rib. The laminae are broad and thick, and they overlap the subjacent lamina. The spinous processes are long. From the fifth to the ninth vertebrae, the spinous processes project sharply inferiorly and overlap each other, but they are less vertical above and below this region. The palpable tip of each spinous process of the fifth to ninth thoracic vertebrae corresponds in position to the interspace below the vertebra from which it projects.

The zygapophyseal joints of the thoracic region (except the inferior articular processes of the 12th vertebra) angle anteriorly approximately 15 to 20 degrees to form an angle of 70 to 75 degrees (open anteriorly) to the midsagittal plane of the body (Fig. 8-16, A; see Fig. 8-15). To show the zygapophyseal joints of the thoracic region radiographically, the patient’s body must be rotated 70 to 75 degrees from the anatomic position or 15 to 20 degrees from the lateral position.

Fig. 8-16 A, Direction of thoracic zygapophyseal joints. B, Direction of thoracic intervertebral foramina.

The intervertebral foramina of the thoracic region are perpendicular to the midsagittal plane of the body (see Figs. 8-15 and 8-16, B). These foramina are clearly shown radiographically with the patient in a true lateral position (see Table 8-1). During inspiration, the ribs are elevated. The arms must also be raised enough to elevate the ribs, which otherwise cross the intervertebral foramina. A full view of the thoracic vertebrae is seen in Fig. 8-17 along with surrounding tissues.

Fig. 8-17 MRI sagittal plane of thoracic vertebrae region showing vertebral bodies and relationship to spinal cord.

Lumbar Vertebrae

The lumbar vertebrae have large, bean-shaped bodies that increase in size from the first to the fifth vertebra in this region. The lumbar bodies are deeper anteriorly than posteriorly, and their superior and inferior surfaces are flattened or slightly concave (Fig. 8-18, A). At their posterior surface, these vertebrae are flattened anteriorly to posteriorly, and they are transversely concave. The anterior and lateral surfaces are concave from the top to the bottom (Fig. 8-18, B).

Fig. 8-18 A, Superior aspect of lumbar vertebra. B, Lateral aspect of lumbar vertebra.

The transverse processes of lumbar vertebrae are smaller than those of the thoracic vertebrae. The superior three pairs are directed almost exactly laterally, whereas the inferior two pairs are inclined slightly superiorly. The lumbar pedicles are strong and are directed posteriorly; the laminae are thick. The spinous processes are large, thick, and blunt, and they have an almost horizontal projection posteriorly. The palpable tip of each spinous process corresponds in position with the interspace below the vertebra from which it projects. The mamillary process is a smoothly rounded projection on the back of each superior articular process. The accessory process is at the back of the root of the transverse process.

The body of the fifth lumbar segment is considerably deeper in front than behind, which gives it a wedge shape that adapts it for articulation with the sacrum. The intervertebral disk of this joint is also more wedge-shaped than the disks in the interspaces above the lumbar region. The spinous process of the fifth lumbar vertebra is smaller and shorter, and the transverse processes are much thicker than those of the upper lumbar vertebrae.

The laminae lie posterior to the pedicles and transverse processes. The part of the lamina between the superior and inferior articular processes is called the pars interarticularis (Fig. 8-19).

Fig. 8-19 Axial CT image of L5 showing fractures of right and left pars interarticularis (arrows).

The zygapophyseal joints of the lumbar region (Figs. 8-20 and 8-21, A) are inclined posteriorly from the coronal plane, forming an average angle (open posteriorly) of 30 to 60 degrees to the midsagittal plane of the body.

Fig. 8-20 Right posterior oblique view of lumbar vertebrae, showing zygapophyseal joints.

Fig. 8-21 A, Direction of lumbar zygapophyseal joints. B, Superior aspect showing orientation of lumbar intervertebral foramina. C, Axial CT image of lumbar spine showing angles of zygapophyseal joints (arrows).

The average angle increases from cephalad to caudad with L1-2 at 15 degrees, L2-3 at 30 degrees, and L3-4 through L5-S1 at 45 degrees. Table 8-3 shows, however, that these joint angles may vary widely at each level. Numerous upper joints have no angle, and many lower joints have an angle of 60 degrees or more. Although the customary 45-degree oblique body position shows most clinically significant lumbar zygapophyseal joints (L3 through S1), 25% of L1-2 and L2-3 joints are shown on an AP projection, and a small percentage of L4-5 and L5-S1 joints are seen on a lateral projection.

TABLE 8-3

Lumbar zygapophyseal joint angle ^*

^*In relation to the sagittal plane.

^†Joint space oriented parallel to sagittal plane.

^‡Joint space perpendicular to sagittal plane.

From Bogduk N, Twomey L: Clinical anatomy of the lumbar spine, ed 3, London, 1997, Churchill Livingstone.

The intervertebral foramina of the lumbar region are situated at right angles to the midsagittal plane of the body except for the fifth, which turns slightly anteriorly (Fig. 8-21, B). The superior four pairs of foramina are shown radiographically with the patient in a true lateral position; the last pair requires slight obliquity of the body (see Table 8-1).

Spondylolysis is an acquired bony defect occurring in the pars interarticularis, the area of the lamina between the two articular processes. The defect may occur on one or both sides of the vertebra, resulting in a condition termed spondylolisthesis. This condition is characterized by the anterior displacement of one vertebra over another, generally the fifth lumbar over the sacrum. Spondylolisthesis almost exclusively involves the lumbar spine (Fig. 8-22).

Fig. 8-22 Lateral lumbar spine showing spondylolisthesis. A, A 53-year-old man presenting with pain in the legs and difficulty standing for more than 5 minutes without pain. L4 is anteriorly displaced 20% over L5. B, Surgery performed to stabilize spondylolisthesis. The patient recovered fully from pain.

Spondylolisthesis is of radiologic importance because oblique-position radiographs show the “neck” area of the “Scottie dog” (i.e., the pars interarticularis). (Oblique positions involving the lumbar spine, including the Scottie dog, are presented later in this chapter, starting with Fig. 8-95.) A full view of the lumbar vertebrae is seen in Fig. 8-23 along with surrounding tissues.

Fig. 8-23 MRI sagittal plane of lumbar spine. Note intervertebral disks between vertebral bodies.

Fig. 8-95 AP oblique lumbar spine: RPO for right zygapophyseal joints.

Sacrum

The sacrum is formed by fusion of the five sacral vertebral segments into a curved, triangular bone (Figs. 8-24 and 8-25). The sacrum is wedged between the iliac bones of the pelvis, with its broad base directed obliquely, superiorly, and anteriorly and its apex directed posteriorly and inferiorly. Although the size and degree of curvature of the sacrum vary considerably in different patients, the bone is normally longer, narrower, more evenly curved, and more vertical in position in males than in females. The female sacrum is more acutely curved, with its greatest curvature in the lower half of the bone; it also lies in a more oblique plane, which results in a sharper angle at the junction of the lumbar and pelvic curves.

Fig. 8-24 A, Anterior aspect of sacrum and coccyx. B, Lateral aspect of sacrum and coccyx. C, Sagittal MRI scan of sacrum and coccyx.

Fig. 8-25 A, Coronal CT of sacrum. Note sacroiliac (SI) joints. B, Axial CT of sacrum. Note angle of SI joints and sacral nerves in sacral canal. (Modified from Kelley LL, Petersen CM: Sectional anatomy for imaging professionals, ed 2, St Louis, 2007, Mosby.)

The superior portion of the first sacral segment remains distinct and resembles the vertebrae of the lumbar region (Fig. 8-26). The superior surface of the base of the sacrum corresponds in size and shape to the inferior surface of the last lumbar segment, with which it articulates to form the lumbosacral junction. The concavities on the upper surface of the pedicles of the first sacral segment and the corresponding concavities on the lower surface of the pedicles of the last lumbar segment form the last pair of intervertebral foramina. The superior articular processes of the first sacral segment articulate with the inferior articular processes of the last lumbar vertebra to form the last pair of zygapophyseal joints.

Fig. 8-26 A, Sagittal section of sacrum. B, Base of sacrum.

At its superior anterior margin, the base of the sacrum has a prominent ridge termed the sacral promontory. Directly behind the bodies of the sacral segments is the sacral canal, which is the continuation of the vertebral canal. The sacral canal is contained within the bone and transmits the sacral nerves. The anterior and posterior walls of the sacral canal are each perforated by four pairs of pelvic sacral foramina for the passage of the sacral nerves and blood vessels.

On each side of the sacral base is a large, winglike lateral mass called the ala (see Fig. 8-26, B). At the superoanterior part of the lateral surface of each ala is the auricular surface, a large articular process for articulation with similarly shaped processes on the iliac bones of the pelvis.

The inferior surface of the apex of the sacrum (Fig. 8-27) has an oval facet for articulation with the coccyx and the sacral cornua, two processes that project inferiorly from the posterolateral aspect of the last sacral segment to join the coccygeal cornua.

Fig. 8-27 Transverse sections of sacrum. A, Section through superior sacral portion. B, Section through inferior sacral portion.

Coccyx

The coccyx is composed of three to five (usually four) rudimentary vertebrae that have a tendency to fuse into one bone in the adult (see Fig. 8-24). The coccyx diminishes in size from its base inferiorly to its apex. From its articulation with the sacrum it curves inferiorly and anteriorly, often deviating from the midline of the body. The coccygeal cornua project superiorly from the posterolateral aspect of the first coccygeal segment to join the sacral cornua.