The stacked vertebral bones reveal a segmentation that is otherwise minimally evident in human anatomy. This metameric arrangement into individual vertebrae also dictates that communication of the peripheral nervous system with the spinal cord must be likewise segmented, despite the lack of any organization into subunits within the spinal cord. Typically, there are 7 cervical vertebrae, 12 thoracic, 5 lumbar, 5 sacral, and 4 coccygeal (Fig. 9-1). Common palpable landmarks (Fig. 9-2) may locate particular levels, including the most prominent spinous process (vertebra prominens), which usually indicates the seventh cervical. The seventh thoracic spine is usually opposite the inferior angle of the scapula, and the line connecting the iliac crests (Tuffier line) crosses the vertebral column most often at the L4–L5 disc. However, clinical ability to estimate segmental level from palpable landmarks is often overestimated, and these indices are subject to natural variability in anatomic parameters (Fig. 9-3) (1). For instance, the first thoracic vertebral spine may be more prominent than the seventh cervical. Also, the Tuffier line may cross the vertebral column as high as the L3–L4 disc or as low as the L5–S1 disc (2,3,4,5). For this reason, and because of a variable amount of subcutaneous fat, the accuracy of predicting the vertebral level of needle insertion is about 50% at best when unaided by radiologic imaging. Most often, the error occurs when the clinician selects a level one or two segments higher than the intended level in the lower thoracic and lumbar regions, or selects a space too low in the upper thoracic and cervical region (6,7,8,9). Errors are comparable in the sitting and lying positions, are greater with obese subjects, and are greater using the vertebra prominens as a reference point compared to the Tuffier line (10,11).

The numerical designations of the vertebra may differ between individuals because of anomalous patterns of vertebral segmentation, which are predominantly found in the lumbosacral spine (12). This parallels the phylogenetic stability of mammalian cervical and thoracic segmentation and the marked variability in the number of lumbar vertebrae, even among primates (13). The last lumbar or first sacral vertebrae is often indeterminate in configuration, with fusion of L5 to S1 in 6.2% (sacralization of L5, bilateral in 1.5%), or incomplete fusion of S1 to S2 in 5.3% (lumbarization of S1, bilateral in 4.1%) (12).

Although the vertebrae differ in design at the various levels of the vertebral column, common elements can be defined. Each consists of posterior elements forming a vertebral arch and the body (or soma) anteriorly (Fig. 9-4A). The lumbar vertebral bodies are distinctly narrowed into an hourglass shape, with a diameter 15% less at the middle than at the endplates (14). Stout pedicles arise on the posterolateral aspects of the body and fuse with the platelike laminae to encircle the vertebral foramen. Together, the vertebral foramina of the adjacent vertebrae in sequence form the vertebral (or spinal) canal. In the cervical and lumbar regions (Fig. 9-4B), the canal approaches a triangular shape, but it is circular and smaller in the thoracic vertebrae. The width of the canal measured between the pedicles is narrowest in the thoracic region (about 17 mm), but widens in both the cervical (25 mm) and lumbar regions (about 22 mm at L1 to about 27 mm at L5) (15,16,17,18,19,20). In contrast, the anterior/posterior diameter of the vertebral canal is fairly constant (about 16 mm) throughout the vertebral column (21).

Between the pedicles of each adjacent pair of vertebrae, there remains a gap called the intervertebral foramen (or nerve root canal; Fig. 9-5) that provides a passage between the paravertebral space and the vertebral canal, and conveys the segmental nerves as well as other neural and vascular structures. The transverse process is based at the junction of the pedicle and lamina and passes laterally. The spinous process projects posteriorly from the midline junction of the laminae, is often bifid in the cervical column, and may not lie in the midline at other levels.

The pattern described here varies somewhat in different regions of the vertebral column. Vertebral bodies are relatively massive at the lumbar levels, but less so in the thoracic column and especially in the cervical region. Whereas the transverse processes are attached to the pedicles and laminae in the lumbar and thoracic zones, they take their origin from the vertebral body in the cervical vertebrae. The spinous processes are steeply angled caudally in the thoracic region, but are nearly
perpendicular to the axis of the column in cervical and lumbar vertebrae.

The sacral vertebral column is a special case. In childhood, the sacral vertebrae are connected by cartilage, but progressively fuse into a single structure after puberty. In the adult, only a narrow residue of the sacral discs persists. The fusion of adjacent vertebrae eliminates the intervertebral foramina. Instead, the concave anterior surface of the sacrum features four pairs of large anterior sacral foramina that provide passage from the midline sacral canal for the anterior rami of the upper four sacral nerves. In contrast with their posterior counterparts, which provide an exit for the posterior primary rami at each level, the anterior foramina are unsealed and provide a ready passage for escape of local anesthetic solution injected into the sacral canal. On the posterior surface of the sacrum, there is a median crest with three or more, but commonly four, variably prominent tubercles, representing the sacral spinous processes. Lateral to this crest and medial to the four posterior sacral foramina is the intermediate sacral crest with a row of four tubercles, representing the upper four sacral articular processes. The remnants of the S5 inferior articular processes are free and prominent, and flank the sacral hiatus, constituting the sacral cornua.

The sacrum is the least predictable portion of vertebral anatomy. Specifically, the volume of the adult sacral canal may vary from 12 mL to 65 mL (22). Typically, fusion of the posterior roof of the sacral vertebral canal is complete down to the S5 level, where the sacral hiatus persists as an opening bordered by the sacral cornua on either side and is covered by the thick, fibrous posterior sacrococcygeal ligament (Fig. 9-6). However, either an entire lack of any posterior bony roof or virtually complete closure of the sacral vertebral canal are found in about 8% of subjects (Fig. 9-7) (23). In 5% of adult sacral bones, the anterior–posterior diameter of the canal at the hiatus is 2 mm or less (22), making access by needle placement very difficult or impossible in some subjects. In children, however, the sacral hiatus is predictably open and access is readily performed.

The coccyx is a small triangular bone consisting of three to five fused rudimentary vertebrae; it attaches by means of its upper articular surface to the lower articular surface of the sacrum. It has two prominent coccygeal cornua that abut their sacral counterparts. The bone tends to be angulated forward from the sacrococcygeal junction, with its pelvic surface facing anteriorly and upward.

The ligamentous system of the vertebral column must bond the adjacent segments adequately while allowing for a degree of bending motion. The joints between the successive vertebral bones (Fig. 9-8) control this motion.

The adjacent vertebral bodies are joined at their endplates by the discs, which are composed of fibrocartilaginous joints with an avascular gelatinous core—the nucleus pulposus—surrounded by a reinforcing annular ligament made of collagenous lamellae. These serve the purposes of bearing weight while permitting flexibility. In the cervical region, the lateral upper edge of the vertebral bodies extends as the uncinate process, which articulates with the body of the next cephalad vertebra as the uncovertebral joint (of Luschka). Arthritic degeneration and expansion of this joint may encroach upon the cervical nerve root canals.

The posterior elements of adjacent vertebrae articulate by true diarthrodial joints, the zygapophyseal (or facet) joints. The inferior articular process projecting caudally overlaps the superior articular process from the next most caudal vertebra. In the cervical and lumbar column, the facet joints are posterior to the transverse processes, whereas the thoracic facets are anterior to the transverse processes. Joint surfaces are angled midway between the axial and coronal planes in the cervical region, but are much more vertically deployed at thoracic levels, and are almost in a coronal plan (24). At lumbar levels, the facet joints are distinctly curved, such that the posterior portion is in the sagittal plane whereas the anterior portion is almost in the coronal plane. The transition from the characteristic thoracic to lumbar type articulation is gradual in half of subjects, with intermediate articular orientation at T11–T12 (25). In others, the change is abrupt, with a coronal superior facet and a curved/sagittal inferior facet on a single vertebra at T12 (29%), T11 (16%), or L1 (0.5%), with frequent asymmetry.

The orientation of the facets guides and constrains movements between the two vertebrae. Specifically, minimal rotation is allowed at lumbar levels because of the sagittally opposing

portions, whereas thoracic facets permit rotation but limit flexion. In the cervical column, movement in all planes is less restricted and includes translation (anterior/posterior sliding) between adjacent vertebrae controlled by the uncinate processes as lateral guide rails (24). Movement in the sagittal plane (flexion and extension) is maximal in the cervical region and at L5–S1. In addition to restricting motion, the facets bear weight in the cervical region (26) and at the lumbosacral joint (27). Lumbar posterior joints may transmit load in part through contact of the tip of the inferior articular process with the lamina of the vertebra below (28).

The capsule of the facets is loose and redundant at the inferior and superior ends of the joint, and injection of the joints is often accompanied by leakage of the solution into the epidural space from medial disruption of the capsule (29,30,31). Rudimentary fibroadipose menisci and synovial folds cushion the superior and inferior poles of the lumbar zygapophyseal joints (32), but these typically disappear with age and the cartilage on the joint surfaces thins (33). Although small myelinated nerves innervate the facet menisci (34), no clear evidence implicates them in chronic back pain.

Facet arthritis with periarticular exostoses is another etiology of cord and nerve compression (35), and pathologic medial expansion of facets may interfere with epidural or spinal needle placement as well.

The supraspinous ligament joins the tips of the spinous processes as a heavy band (Fig. 9-8), but thins and vanishes in the lower lumbar region (36), which allows greater flexion
at the L5–S1 joint. Between the spinous processes, the interspinous ligament forms a narrow web. It may have a slit-like midline cavity filled with fat (36), which may mistakenly give a positive loss-of-resistance sign during epidural needle insertion. The supraspinous and intraspinous ligaments are largely composed of collagen so that a needle passing through them generates a characteristic snapping sensation as the fibers are parted. In contrast, the ligamentum flavum is 80% elastin, with a dense and homogenous texture that is readily appreciated as a needle passes through it. In normal posture, the ligamentum flavum is under tension, so that it retracts to half its length when cut. It spans from the anterior surface of the upper lamina of an adjacent pair of vertebrae to the posterior aspect of the lower lamina. The right and left halves meet at an angle of less than 90 degrees, and a gap may be present in the midline (37). Its lateral edges wrap anteriorly around the medial margin of the facet joints, reinforcing the joint capsule. Bone may grow into the margins of the ligamentum flavum even in young individuals, especially at mid and lower thoracic levels (38,39,40,41), which may result from the increased rotatory strains at these levels (40). These fine bone spurs in the ligamentum flavum may impede the progress of a spinal or epidural needle and may be mistaken for fracture fragments on x-ray images.

With age, the intervertebral discs become desiccated and lose height, which in turn causes increased overlap of the facets. This decreases the longitudinal dimensions of the intervertebral foramen and foreshortens the ligamentum flavum, causing it to buckle into the foramen, further contributing to foraminal narrowing.

The broad anterior longitudinal ligament reinforces the anterior aspect of the discs and binds the vertebral bodies together. Posterior to the vertebral bodies, the posterior longitudinal ligament likewise constrains the relative motion of the vertebral bodies. It runs along the anterior surface of the dural sac as a tight band, but broadens and merges tightly with the discs at each level (Fig. 9-9). This reinforcement typically prevents herniations of the disc in the midline. At cervical and thoracic levels, ossification may result in spinal stenosis (41).

Everything outside the dural sac but within the vertebral canal can be considered to constitute the epidural space. Its outer boundaries are thus the walls of the vertebral canal, including the vertebral bodies and discs anteriorly, pedicles laterally, and laminae and ligamenta flava posteriorly. It is commonly said that the epidural space is a “potential space,” but it naturally has contents, specifically fat, vessels, and nerves. There is no clear physiologic purpose in having fat within the spinal canal. Indeed, the cranial epidural space is entirely empty. Whereas the brain is protected by a rigid case, the spinal cord must exist within the flexible vertebral column. Perhaps the epidural fat, which is nearly fluid in texture and which has nonadherent surfaces to permit gliding movement of the neural structures, provides a padding effect.

The distance from the skin to the lumbar epidural space in the midline is on average about 5 cm, but may be as small as 3 cm and rarely greater than 8 cm. Distance loosely correlates with weight, and it is somewhat greater at L3–L4 than at other lumbar levels (42,43,44,45,46,47). A practitioner must be cautious when using these dimensions clinically, however, since substantial variability makes it unreliable to expect the actual depth to be close to the published mean.

The distribution of epidural contents is highly nonuniform. Investigations by cryomicrotome sectioning (Figs. 9-10,9-11,9-12,9-13,9-14) (37) and in vivo imaging avoid the dissection artifact. These methods show that the undisturbed lumbar vertebral canal is nearly filled by the dural sac. Thus, the epidural space is empty in large areas, where the dura contacts bone and ligament. It is important to recognize, however, that the dura is not adherent to the canal wall in these empty areas, and solutions and catheters may pass through them. Separated by these empty areas, the epidural contents occur as a series of metamerically and circumferentially discontinuous compartments (Fig. 9-15). In contrast to this general pattern, the dural sac inferior to the L4–L5 disc and in the sacral canal tapers to a smaller diameter and does not fill the canal as completely,
resulting in a proportionate increase in epidural fat. This may contribute to difficulty in delivering local anesthetic to the L5 and sacral nerve roots during epidural anesthesia, since solution is not confined in close proximity with neural structures at these levels.

Because of their distinct designs, the various epidural compartments are considered separately in detail.

The spinal cord, nerve roots, and CSF are enclosed in membranes termed meninges (Fig. 9-31).