Summary of Key Points
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Transverse pedicle width is the limiting variable in pedicle screw diameter. Width is usually smallest around T4-6, but it does not increase much until lower thoracic segments and the thoracolumbar junction.
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Lateral to medial angulation of the pedicle is greatest in the upper thoracic and lower lumbar regions. At the thoracolumbar junction, there is little medial angulation and screw trajectory is nearly straight ahead.
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The pars interarticularis is a structure critical for spinal stability. It connects the pedicle and superior articulating process with the inferior articulating process and lamina. Full thickness violation of the pars interarticularis risks spinal instability and anterolisthesis.
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T11 and T12 typically have a dorsal bony morphology that is markedly different from other thoracic segments. Understanding of this morphology is critical for accurate identification of pedicle position during placement of spinal instrumentation.
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The mammillary process, located at the junction of the transverse process and the superior articulating process of each lumbar vertebrae, represents an insertion site of the deep paraspinous musculature. It is an important landmark for pedicle screw insertion in the lumbar spine, though it is not always visible in each patient.
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The lateral recess is a space defined medially by the edge of the thecal sac and laterally by the medial pedicular plane at the level of the mid- and lower-vertebral body. In the lumbar spine, it is located ventral to the ligamentum flavum and facet complex, dorsal to the vertebral body, and just rostral and medial to the neural foramen. Lateral recess stenosis results from spondylotic redundancy of the ligamentum flavum and facet hypertrophy.
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Foraminal stenosis commonly results from loss of disc height with secondary impaction of the superior articular process into the neural foramen.
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In the thoracic and lumbar spine, unlike the cervical spine, nerve roots exit below their like-named pedicle. The exiting root leaves the spinal canal at some distance above the intervertebral disc. It is only once the root has exited the bony foramen that it crosses the plane of the disc. This is critical to understanding the different dermatomal/myotomal manifestations of posterolateral disc herniation versus far lateral disc herniation.
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In terms of arterial supply, the thoracic spinal cord has two areas of surgical vulnerability: the midthoracic region, which is a watershed zone vulnerable to hypotension and global ischemia, and the lower thoracic region, which receives a disproportionate portion of its arterial supply from a single great radiculomedullary artery, the artery of Adamkiewicz.
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Variations in rib number and the presence of transitional lumbosacral vertebrae are common anatomic variations that may lead to wrong-level surgery if not recognized. Consistent use of a single localization scheme in preoperative and intraoperative imaging will reduce the probability of wrong-level error.
The typical human spine consists of 33 vertebrae: 7 cervical, 12 thoracic, 5 lumbar, and 5 fused sacral vertebrae. At the caudal portion of the sacrum, 4 or 5 ossicles comprise the coccyx. Thoracic vertebrae are defined by their articulations with ribs, though variation in rib number is commonplace.
The complexity of the thoracic and lumbar spines is rooted in the gradual transformation of anatomic structures as one descends along the rostral-caudal axis. The safe application of anatomic knowledge to surgical procedures requires surgeons to be at least as familiar with the differences within regional groupings as they are with the commonalities that typically characterize those groupings.
Spinal Alignment
The acquisition of upright posture and bipedal locomotion during human evolution imposed specific demands on the thoracic and lumbar spine. These demands are reflected in its overall structure ( Fig. 9-1 ). Upright posture requires added load-bearing capacity: along the rostral-caudal axis, there is a substantial increase in the robustness of not only the vertebral bodies, but also other stabilizing structures, such as the articular and spinous processes. Bipedalism requires spinal balance (i.e., the head is maintained overtop of the femoral heads); this is reflected in normal spinal curvature. Unlike our quadruped ancestors, who possessed a single, broad spinal curvature, humans exhibit a combination of thoracic kyphosis and lumbar lordosis. These curves are formed by relatively small variations in intervertebral disc and vertebral body morphology. In the absence of spine deformity, the thoracic and lumbar curvatures are balanced, and the midportion of the C7 vertebral body sits atop the L5-S1 pivot point, located at the dorsal aspect of the L5-S1 intervertebral disc. With normal spinal balance, the degree of thoracic and lumbar curvature may vary considerably without adverse effect. However, deviations from normal sagittal alignment are mechanically unfavorable, imposing stress on axial musculature and resulting in pain and acceleration of the degenerative process.
Vertebrae and Ligaments
Vertebral Body
Because of graded variation in morphology between the upper thoracic and lumbar spine, it is difficult 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, whereas 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. 9-2 ). Thoracic vertebrae bodies are wedge shaped, with 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 disc shape, rather than vertebral body shape, which 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 to the midlumbar spine ( Figs. 9-3 and 9-4 ). From L2 to L5, vertebral height is usually stable and may decrease slightly. Changes in cross-sectional area are reflected in compression strength ( Fig. 9-5 ).
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 pulposis, and (2) it resists tensile and torsional stresses, a role assumed by the annulus fibrosis ( Fig. 9-6 ). At a microscopic level, the nucleus pulposis 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 annulus fibrosis has a lamellated boundary of intersecting fibrous strands. Annular 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 pulposis to annulus fibrosis is gradual.
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 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.
As part of the normal degenerative process, disc bulging and the resultant traction on Sharpey fibers results 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 annulus fibrosis 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 spinal 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 it is less substantial than the ALL. It is located along the dorsal surface of the vertebral bodies, within the spinal canal ( Fig. 9-7 ). 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 annulus 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. Though the PLL’s contribution to spinal stability is modest, it serves to direct disc herniations posterolaterally, away from the central portion of the spinal canal.
Pedicle
With increasing popularity of the use 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 on whom he or she will operate.
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 caudally to meet the vertebral body, whereas 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 usually smallest in the region of T4-6, but increases only minimally until one arrives at the thoracolumbar junction. The pedicle-rib unit can provide abundant space for medially angled pedicle screw placement at every level from T1 to T12, with the surgeon aware that the screws may safely penetrate the lateral pedicle wall adjacent to the rib head and that a medial pedicle wall breach is less likely. A graphical depiction of transverse pedicle width and sagittal pedicle width across the spinal axis is shown in Figures 9-8 and 9-9 .
For the same reason that pedicle width constrains screw placement more than pedicle height, transverse pedicle angle is a more relevant anatomic variable than 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 transverse angle of 25 to 30 degrees ( Fig. 9-10 ).
Transpedicular instrumentation in the thoracic spine is more challenging than in the lumbar spine because of the presence of adjacent spinal cord, the smaller pedicle diameter, and the relative proximity of the bony elements to neural structures. In the lumbar spine, there is approximately 1.5 mm of epidural space between the medial pedicle wall and the thecal sac. In the thoracic spine, the medial pedicle border is contiguous with the edge of the thecal sac. 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 is approximately 1.5 mm. 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 from the lower edge of the thoracic pedicle to the nerve root below it is approximately 2 to 3 mm.
Facet and Pars Interarticularis
The facet joint, or zygapophyseal joint, is formed by an inferior articulating process emanating from the rostral level and a superior articulating process emanating from the caudal level. The facet joint is a synovial joint, possessing a true 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. Gliding articulation at the facet joint is limited by the capsular fibers, which are relatively lax in the cervical region, but which 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. 9-2 ). 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, which 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 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; as one dissects from medial to lateral (i.e., spinous process to lamina to transverse process), 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 and 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 is easily fractured during wide dorsal exposure for posterolateral 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 is shown in Figure 9-2 .
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. 9-1 ). The first rib articulates exclusively with T1, via a single, complete facet located at the lateral aspect of the vertebral body. Ribs 2 through 9 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, and 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 transthoracic access to the disc space or neural foramen inevitably requires removal of some or all of the adjacent rib head, regardless of level ( Fig. 9-11 ). 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 visible through the pleura is the sympathetic chain, coursing over the rib heads ( Fig. 9-12 ).
