Vertebral Body

Because of graded variation in morphology between the upper thoracic spine and the lumbar spine, it is difficult to make accurate, dichotomous generalizations about the structure of thoracic and lumbar vertebrae. The 11th and 12th vertebral bodies, in particular, possess a mixture of thoracic and lumbar morphology. Generally, when viewed in cross section, thoracic vertebrae possess a heart shape, while lumbar vertebrae are more kidney shaped; a concavity along the dorsal aspect of each marks the ventral portion of the spinal canal. On the left side of thoracic vertebrae, a shallow depression marking the course of the aorta may be visible.

Sagittal vertebral morphology also changes along the rostral-caudal axis (Fig. 32-2). Thoracic vertebral bodies are wedge shaped, ventral height being shorter than dorsal height. This results in the normal thoracic kyphosis. In the lumbar spine, ventral and dorsal heights are generally comparable, and it is the disc shape, rather than vertebral body shape, that is the principal contributor to lumbar lordosis. At L4 and L5, some reverse wedging of the vertebral body may occur, with increased ventral height further contributing to the lower lumbar lordosis. The vertebral body gradually increases in cross-sectional area and height from the thoracic spine to the midlumbar spine (Figs. 32-3 and 32-4). From L2 to L5, vertebral height is usually stable and may decrease slightly. Changes in cross-sectional area are reflected in compression strength (Fig. 32-5).

FIGURE 32-2 Lateral and superior views of T1 (A), T6 (B), T11 (C), T12 (D).

FIGURE 32-3 Vertebral body diameter by level.

(Benzel EC: Biomechanics of spine stabilization, New York, 2001, Thieme, p 1, reprinted with permission.)

FIGURE 32-4 Vertebral body height by level.

(Benzel EC: Biomechanics of spine stabilization, New York, 2001, Thieme, p 1, reprinted with permission.)

FIGURE 32-5 Vertebral body compression strength by level.

(Benzel EC: Biomechanics of spine stabilization, New York, 2001, Thieme, p 1, reprinted with permission.)

Intervertebral Disc and Vertebral End Plate

The general function of the intervertebral disc is twofold: (1) It deforms to accommodate compressive loads, a role assumed by the nucleus pulposus, and (2) it resists tensile and torsional stresses, a role assumed by the anulus fibrosus (Fig. 32-6). At a microscopic level, the nucleus pulposus consists of a semifluid, gel-like substance embedded in a fine meshwork of fibrous strands. This structure results in a viscoelastic property that allows the disc to withstand and absorb axial stress. The anulus fibrosus has a lamellated boundary of intersecting fibrous strands. Anular fibers known as Sharpey fibers penetrate the dense cortical bone that makes up the outer ring of the vertebral end plate. The outermost fibers blend with overlying periosteum and longitudinal ligaments. Viewed in histologic cross section, the transition from nucleus pulposus to anulus fibrosus is a gradual one.

FIGURE 32-6 The intervertebral disc and associated structures: nucleus pulposus, anulus fibrosus, cartilaginous end plate with underlying cancellous bone, and rim apophysis with inserting Sharpey fibers.

(Copyright Cleveland Clinic Foundation.)

The bony vertebral end plate is a concave depression. At its central portion, the cancellous bone of the vertebral body is directly apposed to a cartilaginous plate, which fills the depression up to the level of the apophyseal ring, or marginal ring. The apophyseal ring is composed of cortical bone and is more resistant to compression failure than is the central end plate. Biomechanical studies have shown the strongest region of the lumbar end plate to be the dorsal, lateral aspect of the marginal ring, adjacent to the pedicle.¹

As part of the normal degenerative process, disc bulging and the resultant traction on Sharpey fibers result in bony osteophyte growth along this outer ring. Thus, the degree of concavity of the vertebral body, when viewed in profile, increases with age. Because the stress on Sharpey fibers is greatest along the concavity of a curve, these changes will be most evident along the ventral surface of the thoracic vertebral bodies and the dorsal surface of the lumbar vertebral bodies.

The cross-sectional profile of the intervertebral disc changes along the rostral-caudal axis in accordance with the changing profile of the end plate. In the thoracic spine, the nucleus pulposus is centrally located. In the lumbar spine, it is closer to the dorsal aspect of the disc.

Anterior Longitudinal Ligament

The anterior longitudinal ligament (ALL) is a strong, broad ligament that spans the ventral surface of all the vertebral bodies. Its width increases along the rostral-caudal axis; at the lower lumbar levels, it encompasses almost half of the total circumference of the vertebral body. The ALL has multiple layers. The innermost layer inserts on each vertebral body and is only loosely adherent to the anulus fibrosus of the intervertebral disc. The middle layer bridges two or three vertebral bodies. The outer layer bridges up to five levels at a time.

Because of relative strength, the ALL is an important contributor to spine stability, particularly in the lumbar spine. It resists hyperextension and, to a lesser degree, translational motion.

Posterior Longitudinal Ligament

The posterior longitudinal ligament (PLL) also spans the full rostral-caudal axis of the spine but is less substantial than the ALL. It is located along the dorsal surface of the vertebral bodies, within the spinal canal (Fig. 32-7). At the midbody level, it is relatively narrow, but it widens considerably at the level of the disc before narrowing again as it transitions to the level below. It is adherent at the level of the end plate and anulus but elevated from the concave dorsal surface of the midvertebral body. Along the lateral margins of the PLL, there are often areas of adhesion with the underlying dura. Although the PLL’s contribution to spine stability is modest, it serves to direct disc herniations dorsolaterally, away from the central portion of the spinal canal.

FIGURE 32-7 A, Ventral spinal canal with posterior longitudinal ligament. B, Dorsal spinal canal with ligamentum flavum.

(Copyright Cleveland Clinic Foundation.)

Pedicle

With the increasing popularity of pedicle screws, particularly in the thoracic spine, pedicle anatomy has become the subject of extensive investigation. Nonetheless, subtle variations from one level to the next and significant interindividual variability require the surgeon to be familiar not only with anatomic principles but also with the specific anatomy of the patient undergoing the operation.

Throughout the thoracic spine, the rostral surface of the pedicle is typically flush with the apophyseal ring of the superior end plate. In the upper and midthoracic regions, the caudal pedicle surface inserts at approximately the midplane of the vertebral body. The intervertebral foramen at these levels is formed almost exclusively by the inferior vertebral notch; the superior vertebral notch is small or nonexistent. By the lower thoracic spine, a relative increase in pedicle height has resulted in the caudal pedicle surface inserting somewhat lower, in plane with the lower one third of the vertebral body. In the lumbar spine, the pedicle is positioned progressively lower on the vertebral body and the superior vertebral notch is more pronounced.

Sagittal angulation of the pedicle with respect to the vertebral body also differs between the thoracic and lumbar spine. In the thoracic spine, the pedicle angles downward to meet the vertebral body, while in the lumbar spine, the pedicle is approximately coplanar with the vertebral body in a sagittal view. In thoracic pedicle screw placement, the so-called anatomic trajectory follows this pedicular axis; however, many surgeons employ a “straight-on trajectory,” in which the sagittal angulation of the screw is coplanar with the vertebral end plate rather than the pedicle itself. This oblique passage is possible because of the excess of pedicle height relative to pedicle width.

Transverse pedicle width, rather than pedicle height, is the dimension that limits the size of a pedicle screw that can be placed at a given level. In the thoracic spine, pedicle height is commonly double that of pedicle width. Pedicle width is smallest in the region of T4-6, but increases only minimally until one arrives at the thoracolumbar junction. A graphical depiction of changes in these two dimensions across the neuraxis is shown in Figures 32-8 and 32-9.

FIGURE 32-8 Transverse pedicle width by level.

(Benzel EC: Biomechanics of spine stabilization, New York, 2001, Thieme, p 6, reprinted with permission.)

FIGURE 32-9 Sagittal pedicle width by level.

(Benzel EC: Biomechanics of spine stabilization, New York, 2001, Thieme, p 6, reprinted with permission.)

For the same reason that pedicle width constrains screw placement more than pedicle height does, transverse pedicle angle is a more relevant anatomic variable than is sagittal angle in the placement of thoracic pedicle screws. If a screw’s medial-lateral trajectory differs from that of the pedicle by even a relatively small amount, medial or lateral breach may result. Transverse pedicle angle declines fairly steadily as one proceeds down the thoracic spine until a nearly “straight-ahead” trajectory is encountered in the lower thoracic vertebrae; it then increases steeply across the lumbar levels, such that the L5 pedicle has a transverse angle of 25 to 30 degrees (Fig. 32-10).

FIGURE 32-10 Transverse pedicle angle by level.

(Benzel EC: Biomechanics of spine stabilization, New York, 2001, Thieme, p 6, reprinted with permission.)

Transpedicular instrumentation in the thoracic spine is more challenging than that in the lumbar spine because of the presence of the adjacent spinal cord, the smaller pedicle diameter, and the relative proximity to neural structures (see Figs. 32-8, 32-9, and 32-14). In the lumbar spine, there is approximately 1.5 mm of epidural space between the medial pedicle wall and the thecal sac.² In the thoracic spine, the medial pedicle border is contiguous with the edge of the thecal sac.³ In the lumbar spine, the distance from the upper edge of the pedicle to the nerve root above is approximately 5 mm, and the distance from the lower edge of the pedicle to the nerve root below, approximately 1.5 mm.² In the thoracic spine, the distance from the upper edge of the pedicle to the nerve root above is approximately 2 to 4 mm and the distance from the lower edge of the thoracic pedicle to the nerve root below is approximately 2 to 3 mm.³

Facet and Pars Interarticularis

The facet joint, or zygapophyseal joint, is formed by an inferior articulating process emanating from the level above and a superior articulating process emanating from the level below. The facet joint is a synovial joint, possessing a true joint capsule. This capsule has two layers: an outer layer, composed of parallel bundles of collagenous fibers, and an inner elastic layer, similar in composition to the ligamentum flavum.⁴ Gliding articulation at the facet joint is limited by the capsular fibers, which are relatively lax in the cervical region but increase in tautness along the rostral caudal axis.

The facet joint functions primarily as a motion-limiting structure; only in extension does it function in an axial load-bearing capacity. The way in which a facet joint constrains motion depends on the alignment of its articulating surfaces (see Fig. 32-2). Lumbar facets occupy a plane that is intermediate between the sagittal and coronal planes. In the lumbar spine, there is a progression from relatively sagittal (25 degrees from sagittal) at L1-2 to relatively coronal (50 degrees from sagittal) at L5-S1. To the surgeon approaching the dorsal lumbar spine, the entry into the facet joint will be found progressively more laterally as one descends toward the lumbosacral junction. This more coronal orientation at the lumbosacral junction is an important element in preventing spondylolisthesis. Thoracic facets have an alignment that is oblique to all three cardinal planes; the articulating surface faces dorsally and slightly superolaterally. Thus, the superior articulating process is positioned relatively medial to the inferior articulating process; this is in contrast to the lumbar spine, where the superior articulating process occupies a lateral position. The transition from thoracic facet orientation to upper lumbar orientation occurs abruptly at the thoracolumbar junction.

Coincident with this transition in facet angulation in the lower thoracic spine is a change in overall facet morphology. Viewed from the back, thoracic facets have a flat, monotonous, shingle-style arrangement; the facet complex resides in a trough between the dorsally directed lamina and the dorsally directed transverse process. Lumbar facets are more protuberant, pedunculated structures that occupy an elevated position relative to the lamina and transverse processes. The transition between the two occurs in the lower thoracic spine. In the lumbar spine, the mammillary process is visible as a slight bony prominence at the junction of the superior articulating process and transverse process; it serves as a site of attachment for deep paraspinous musculature and is an important landmark for pedicle screw insertion.

Transverse Process

The transverse processes of the thoracic spine and the lumbar spine are relatively thin, consisting of both cortical and cancellous bone. In the thoracic spine, the transverse process projects dorsolaterally and articulates at its tip with the like-numbered rib. In the lumbar spine, the transverse processes project straight laterally and are easily fractured during wide dorsal exposure for dorsolateral fusion. The transverse processes of T11 and T12 are hypoplastic relative to their neighbors and do not articulate with their corresponding ribs. Commonly, the T11 transverse process is a dorsolaterally directed bony stump. In lieu of a T12 transverse process, there are only three small tubercles: a superior tubercle, which is equivalent to the lumbar mammillary process; an inferior tubercle, which is equivalent to the lumbar accessory process; and a lateral tubercle, which represents a very small equivalent of a transverse process. Variation in transverse process morphology can be seen in Figure 32-2.

Rib Articulations of the Thoracic Spine

The relationship of each rib to its corresponding vertebral body changes along the rostral-caudal axis of the thoracic spine (see Fig. 32-1). The 1st rib articulates exclusively with T1 via a single, complete facet located at the lateral aspect of the vertebral body. The 2nd through 9th ribs articulate with their like-numbered vertebrae, as well as the vertebra below, via paired demifacets. These demifacets are also located on the lateral aspect of the vertebral body but in a progressively dorsal position. By the lower thoracic spine (T10-12), two important changes have occurred, First, the pattern of articulation has returned to that of a single, complete, unpaired facet; second, the gradual dorsal movement of the articulation has resulted in the joint being located on the lateral surface of the pedicle, not the vertebral body. However, the size of the rib head and its joint capsule are such that ventrolateral access to the disc space or neural foramen inevitably requires removal of some or all of the adjacent rib head, regardless of level (Fig. 32-11). Because the parietal pleura obscures much of the relevant anatomy, palpation of the rib head is an important orienting maneuver during transthoracic approaches to the spine; often the only recognizable structure that is visible through the pleura is the sympathetic chain, coursing over the rib heads (Fig. 32-12).

FIGURE 32-11 Rib removal is required for access to the thoracic neural foramen and intervertebral disc.

FIGURE 32-12 Lateral view of thoracic spine with segmental vessels and sympathetic chain located deep to the parietal pleura.

A second site of rib articulation, the costotransverse joint, is located at the ventral aspect of the tip of the transverse processes of T1-10. Ribs 11 and 12 fail to make contact with their respective transverse processes, a fact that can be appreciated during dorsolateral approaches at the thoracolumbar junction.

Lamina and Spinous Process

In general, the laminae are arranged in a shingle-like array, in which the lamina above overlaps the one below (see Fig. 32-1). During laminectomy, the deeper position of the more rostral portion of the lamina is readily appreciated. In the thoracic spine and the lumbar spine, the pars interarticularis consists of the lateralmost portion of the lamina. This is the portion of lamina that resides between, and functionally connects, the superior articulating process and the inferior articulating process. The pars interarticularis is important for spine stability; without it, the stabilizing effect of the facet joint below is lost.

When viewed from a dorsal approach, laminae of the upper thoracic region are horizontally oriented and relatively narrow in the rostral-caudal axis. Though the spinous process has a caudal angulation, this angulation is relatively minor (approximately 40 degrees from horizontal); hence, the relative prominence of the upper thoracic spine to dorsal palpation. As one moves into the middle thoracic region, the laminae become more V-shaped, giving rise to a more caudally directed spinous process. The laminae also become broader along their rostral-caudal axis. In both the upper and middle thoracic spine, the interlaminar space is small, if not nonexistent, when the patient is in a nonflexed, prone position. In the lowest two thoracic vertebrae, this V-shaped configuration is diminished; the laminae again become relatively horizontal, and the caudally directed angulation of the spinous process is less. Concomitant with this is an elongation of the superior and inferior articulating processes in the rostral-caudal axis. The result of these changes is the development of an interlaminar space between the paired facet complexes.

In the lumbar spine, the elongation of the articulating processes, combined with the nature of their confluence with the laminae, gives the dorsal elements the appearance of an H. The interlaminar space becomes more prominent with further caudal descent. At the same time, the spinous processes transition from a slight caudal angulation to completely horizontal at L5. As a result of these changes, the interlaminar space at L5-S1 is sufficiently large to permit inadvertent passage of tissue dilator during minimally invasive procedures.

Laminar anatomy is commonly appreciated during dorsal exposure for decompression or fusion procedures. Particularly for dorsal fusion procedures, understanding the relationships between the lamina, the pars interarticularis, the facet joints, and the transverse process is essential to obtaining good exposure without unnecessary destabilization of the facet complexes. The surgeon dissecting the paraspinous muscles off the spinous process arrives at the midportion of the lamina, the cross-bar of the H. As the surgeon proceeds laterally, the lateral edge of the lamina is encountered; this is the lateral boundary of the pars interarticularis. Just above and below this bony edge, the humps of the facet articulations with the levels above and below can be palpated. By following this edge of the pars interarticularis rostrally and slightly laterally, taking care to remain lateral to the facet joint capsule, the surgeon can expose the outer edge of the superior articulating process and the transverse process, without destabilizing the supra-adjacent facet articulation. The pars interarticularis tends to be thinner at its rostral-medial portion and thicker in its caudal-lateral portion.