Anatomic Approaches to the Spine

Anatomy of Vertebral Column

The vertebral column comprises 33 vertebrae divided into five sections (7 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 4 coccygeal) ( Fig. 37.1 ). The sacral and coccygeal vertebrae are fused, which typically allows for 24 mobile segments. Congenital anomalies and variations in segmentation are common. The cervical and lumbar segments develop lordosis as an erect posture is acquired. The thoracic and sacral segments maintain kyphotic postures, which are found in utero, and serve as attachment points for the rib cage and pelvic girdle. In general, each mobile vertebral body increases in size when moving from cranial to caudal. A typical vertebra comprises an anterior body and a posterior arch that enclose the vertebral canal. The neural arch is composed of two pedicles laterally and two laminae posteriorly that are united to form the spinous process. To either side of the arch of the vertebral body is a transverse process and superior and inferior articular processes. The articular processes articulate with adjacent vertebrae to form synovial joints. The relative orientation of the articular processes accounts for the degree of flexion, extension, or rotation possible in each segment of the vertebral column. The spinous and transverse processes serve as levers for the numerous muscles attached to them. The length of the vertebral column averages 72 cm in men and 7 to 10 cm less in women. The vertebral canal extends throughout the length of the column and provides protection for the spinal cord, conus medullaris, and cauda equina.

Anatomy of Spinal Joints

The individual vertebrae are connected by joints between the neural arches and between the bodies. The joints between the neural arches are the zygapophyseal joints or facet joints. They exist between the inferior articular process of one vertebra and the superior articular process of the vertebra immediately caudal. These are synovial joints with surfaces covered by articular cartilage, a synovial membrane bridging the margins of the articular cartilage, and a joint capsule enclosing them. The branches of the posterior primary rami innervate these joints.

The interbody joints contain specialized structures called intervertebral discs. These discs are found throughout the vertebral column except between the first and second cervical vertebrae. The discs are designed to accommodate movement, weight bearing, and shock by being strong but deformable. Each disc contains a pair of vertebral endplates with a central nucleus pulposus and a peripheral ring of annulus fibrosus sandwiched between them. They form a secondary cartilaginous joint or symphysis at each vertebral level.

The vertebral endplates are 1-mm thick sheets of cartilage-fibrocartilage and hyaline cartilage with an increased ratio of fibrocartilage with increasing age. The nucleus pulposus is a semifluid mass of mucoid material, 70% to 90% water, with proteoglycan constituting 65% and collagen constituting 15% to 20% of the dry weight. The annulus fibrosus consists of 12 concentric lamellae, with alternating orientation of collagen fibers in successive lamellae to withstand multidirectional strain. The annulus is 60% to 70% water, with collagen constituting 50% to 60% and proteoglycan about 20% of the dry weight. With age, the proportions of proteoglycan and water decrease. The annulus and nucleus merge in a junctional zone without a strict demarcation. The discs are the largest avascular structures in the body and depend on diffusion from a specialized network of endplate blood vessels for nutrition.

Anatomy of Spinal Cord and Nerves

The spinal cord is shorter than the vertebral column and terminates as the conus medullaris at the second lumbar vertebra in adults and the third lumbar vertebra in neonates. From the conus, a fibrous cord called the filum terminale extends to the dorsum of the first coccygeal segment. The spinal cord is enclosed in three protective membranes—the pia, arachnoid, and dura mater. The pia and arachnoid membranes are separated by the subarachnoid space, which contains the cerebrospinal fluid. The spinal cord has enlargements in the cervical and lumbar regions that correlate with the brachial plexus and lumbar plexus. Within the spinal cord are tracts of ascending (sensory) and descending (motor) nerve fibers. These pathways typically are arranged with cervical tracts located centrally and thoracic, lumbar, and sacral tracts located progressively peripheral. This accounts for the clinical findings of central cord syndrome and syrinx. Understanding the location of these tracts aids in understanding different spinal cord syndromes ( Figs. 37.2 and 37.3 ; Table 37.1 ).

Spinal nerves exit the canal at each level. Spinal nerves C2-7 exit above the pedicle for which they are named (the C6 nerve root exits the foramen between the C5 and C6 pedicles). The C8 nerve root exits the foramen between the C7 and T1 pedicles. All spinal nerves caudal to C8 exit the foramen below the pedicle for which they are named (the L4 nerve root exits the foramen between the L4 and L5 pedicles). The final dermatomal and sensory nerve distributions are shown in Figure 37.2 . Because the spinal cord is shorter than the vertebral column, the spinal nerves course more vertically as one moves caudally. Each level gives off a dorsal (sensory) root and a ventral (mostly motor) root, which combine to form the mixed spinal nerve. The dorsal root of each spinal nerve has a ganglion located near the exit zone of each foramen. This dorsal root ganglion is the synapse point for the ascending sensory cell bodies. This structure is sensitive to pressure and heat and can cause a dysesthetic pain response if manipulated.

Anatomy of Cervical, Thoracic, and Lumbar Pedicles

Numerous studies have documented the anatomic morphology of the cervical, thoracic, and lumbar vertebrae. Advanced internal fixation techniques, including pedicle screws, have been developed and used extensively in spine surgery, not only for traumatic injuries but also for degenerative conditions. As the role for anterior and posterior spinal instrumentation continues to evolve, understanding the morphologic characteristics of the human vertebrae is crucial in avoiding complications during fixation.

Placement of screws in the cervical pedicles is controversial and carries more risk than anterior plate or lateral mass fixation. Although cervical pedicles can be suitable for screw fixation, uniformly sized cervical pedicle screws cannot be used at every level. Screw placement in the pedicles at C3, C4, and C5 requires smaller screws (<4.5 mm) and more care in placement than those of the other cervical vertebrae. CT measurements of cervical pedicle morphology found that C2 and C7 pedicles had larger mean interdiameters than all other cervical vertebrae, and that C3 had the smallest mean interdiameter. The outer pedicle width-to-height ratio increased from C2 to C7, indicating that pedicles in the upper cervical spine (C2-4) are elongated, whereas pedicles in the lower cervical spine (C6-7) are rounded. It also is crucial to know that cervical pedicles angle medially at all levels, with the most medial angulation at C5 and the least at C2 and C7. The pedicles slope upward at C2 and C3, are parallel at C4 and C5, and are angled downward at C6 and C7.

The vertebral artery from C3 to C6 is at significant risk for iatrogenic injury during pedicle screw placement. The pedicle cortex is not uniformly thick. The thinnest portion of the cortex (the lateral cortex) protects the vertebral artery, and the medial cortex toward the spinal cord is almost twice as thick as the lateral cortex. Variations in the course of the vertebral artery also place it at risk during placement of pedicle screws. At the C2 and C7-T1 levels, the vertebral artery is less at risk during pedicle screw fixation. The vertebral artery follows a more posterior and lateral course at C2, whereas at C7-T1 it is outside the transverse foramen.

Pedicle dimensions and angles change progressively from the upper thoracic spine distally. A thorough knowledge of these relationships is important when considering the use of the pedicle as a screw purchase site. A study of 2905 pedicle measurements made from T1 to L5 found that pedicles were widest at L5 and narrowest at T5 in the horizontal plane ( Fig. 37.4 ). The widest pedicles in the sagittal plane were at T11, and the narrowest were at T1. Because of the oval shape of the pedicle, the sagittal plane width was generally larger than the horizontal plane width. The largest pedicle angle in the horizontal plane was at L5. In the sagittal plane, the pedicles angle caudal at L5 and cephalad at L3-T1. The depth to the anterior cortex was significantly longer along the pedicle axis than along a line parallel to the midline of the vertebral body at all levels except T12 and L1.

The thoracic pedicle is a convoluted, three-dimensional structure that is filled mostly with cancellous bone (62% to 79%). Panjabi et al. showed that the cortical shell is of variable density throughout its perimeter and that the lateral wall is significantly thinner than the medial wall. This seemed to be true for all levels of thoracic vertebrae.

The locations for screw insertion have been identified and described in several studies. The respective facet joint space and the middle of the transverse process are the most important reference points. An opening is made in the pedicle with a drill or handheld curet, after which a self-tapping screw is passed through the pedicle into the vertebral body. The pedicles of the thoracic and lumbar vertebrae are tube-like bony structures that connect the anterior and posterior columns of the spine. Medial to the medial wall of the pedicle lies the dural sac. Inferior to the medial wall of the pedicle is the nerve root in the neural foramen. The lumbar roots usually are situated in the upper third of the foramen; it is more dangerous to penetrate the pedicle medially or inferiorly as opposed to laterally or superiorly.

We use three techniques for open localization of the pedicle: (1) the intersection technique, (2) the pars interarticularis technique, and (3) the mammillary process technique. It is important in preoperative planning to assess individual spinal anatomy with the use of high-quality anteroposterior and lateral radiographs of the lumbar and thoracic spine and axial CT or MRI at the level of the pedicle. In the lumbar spine, coaxial fluoroscopy images are a reliable guide to the true bony cortex of the pedicle. The intersection technique is perhaps the most commonly used method of localizing the pedicle. It involves dropping a line from the lateral aspect of the facet joint, which intersects a line that bisects the transverse process at a spot overlying the pedicle ( Fig. 37.5 ). The pars interarticularis is the area of bone where the pedicle connects to the lamina. Because the laminae and the pars interarticularis can be identified easily at surgery, they provide landmarks by which a pedicular drill starting point can be made. The mammillary process technique is based on a small prominence of bone at the base of the transverse process. This mammillary process can be used as a starting point for transpedicular drilling. Usually, the mammillary process is more lateral than the intersection technique starting point, which also is more lateral than the pars interarticularis starting point. Thus, different angles must be used when drilling from these sites. With the help of preoperative CT scanning or MRI at the level of the pedicle and intraoperative fluoroscopy, the angle of the pedicle to the sagittal and horizontal planes can be determined.

For percutaneous pedicle screw placement, we use fluoroscopy that is orthogonal to the target vertebral body in the anteroposterior and lateral planes and allows clear visualization of the medial wall of the pedicle and pedicle/vertebral body junction. A Jamshidi needle typically is docked on the pedicle of interest at the lamina/pedicle junction on the anteroposterior view (9 o’clock position for left pedicles and 3 o’clock position for right pedicles). For the most cephalad screw of a construct in the lumbar spine, we prefer to place the starting point slightly below midline on the anteroposterior view (8 o’clock for left pedicles and 4 o’clock for right pedicles) to limit encroachment of the next cephalad facet joint. Under anteroposterior imaging, the needle is then advanced down the pedicle 20 to 25 mm at a trajectory that will allow the tip of the needle to be placed at the pedicle/vertebral body junction without violating the medial wall of the pedicle. When seated, the needle should pass obliquely across the pedicle with the tip just lateral to the medial wall on the anteroposterior view and just deep to the base of the pedicle on the lateral view. This will allow passage of a guidewire into the vertebral body and placement of a cannulated percutaneous pedicle screw using a Seldinger technique. The technique of connecting rod passage depends on the implant manufacturer.

Midline cortical screw placement is another technique that can be used as a slightly less invasive means of fixation in conjunction with a spinous process splitting or limited midline approach and with a posterior lumbar interbody fusion when a posterolateral fusion is not performed. The initial screw starting point is at the medial caudal border of the target pedicle on true anteroposterior view of the target vertebrae (Fig. 37.6) . A burr is used to dimple the cortical bone at the entry site. A pedicle awl is then used to traverse the pedicle in a medial-to-lateral and caudal-to-cranial trajectory, taking care not to violate the cortical wall of the pedicle as seen on anteroposterior and lateral fluoroscopy. Once the track is formed, it should be tapped to the size of the screw that will be inserted to prevent fracture of the thick cortical bone of the pars. A sound is used to confirm bony continuity along the screw path, and markers can be inserted to confirm position with imaging. The desired decompression and interbody fusion is then performed and cortical bone screws placed at the end of the case.

Circulation of Spinal Cord

The arterial supply to the spinal cord has been determined from gross anatomic dissection, latex arterial injections, and intercostal arteriography. Dommisse contributed significantly to knowledge of the blood supply, stating that the principles that govern the blood supply of the cord are constant, whereas the patterns vary with the individual. He emphasized the following factors:

• 1.

  Dependence on three vessels. These are the anterior median longitudinal arterial trunk and a pair of posterolateral trunks near the posterior nerve rootlets.

• 2.

  Relative demands of gray matter and white matter. The longitudinal arterial trunks are largest in the cervical and lumbar regions near the ganglionic enlargements and are much smaller in the thoracic region. This is because the metabolic demands of the gray matter are greater than those of the white matter, which contains fewer capillary networks.

• 3.

  Medullary feeder (radicular) arteries of the cord. These arteries reinforce the longitudinal arterial channels. There are 2 to 17 anteriorly and 6 to 25 posteriorly. The vertebral arteries supply 80% of the radicular arteries in the neck; arteries in the thoracic and lumbar areas arise from the aorta. The lateral sacral, the fifth lumbar, the iliolumbar, and the middle sacral arteries are important in the sacral region.

• 4.

  Supplementary source of blood supply to the spinal cord. The vertebral and posterior inferior cerebellar arteries are important sources of arterial supply. Sacral medullary feeders arise from the lateral sacral arteries and accompany the distal roots of the cauda equina. The flow in these vessels seems reversible and the volume adjustable in response to the metabolic demands.

• 5.

  Segmental arteries of the spine. At every vertebral level, a pair of segmental arteries supplies the extraspinal and intraspinal structures. The thoracic and lumbar segmental arteries arise from the aorta; the cervical segmental arteries arise from the vertebral arteries and the costocervical and thyrocervical trunks. In 60% of individuals, an additional source arises from the ascending pharyngeal branch of the external carotid artery. The lateral sacral arteries and, to a lesser extent, the fifth lumbar, iliolumbar, and middle sacral arteries supply segmental vessels in the sacral region.

• 6.

  “Distribution point” of the segmental arteries. The segmental arteries divide into numerous branches at the intervertebral foramen, which has been termed the distribution point ( Fig. 37.7 ). A second anastomotic network lies within the spinal canal in the loose connective tissue of the extradural space. This occurs at all levels, with the greatest concentration in the cervical and lumbar regions. The presence of the rich anastomotic channels offers alternative pathways for arterial flow, preserving spinal cord circulation after the ligation of segmental arteries.

  

• 7.

  Artery of Adamkiewicz. The artery of Adamkiewicz is the largest of the feeders of the lumbar cord; it is located on the left side, usually at the level of T9-11 (in 80% of individuals). The anterior longitudinal arterial channel of the cord rather than any single medullary feeder is crucial. The preservation of this large feeder does not ensure continued satisfactory circulation for the spinal cord. In principle, it would seem of practical value to protect and preserve each contributing artery as far as is surgically possible.

• 8.

  Variability of patterns of supply of the spinal cord. The variability of blood supply is a striking feature, yet there is absolute conformity with a principle of a rich supply for the cervical and lumbar cord enlargements. The supply for the thoracic cord from approximately T4 to T9 is much poorer.

• 9.

  Direction of flow in the blood vessels of the spinal cord. The three longitudinal arterial channels of the spinal cord can be compared with the circle of Willis at the base of the brain, but it is more extensive and more complicated, although it functions with identical principles. These channels permit reversal of flow and alterations in the volume of blood flow in response to metabolic demands. This internal arterial circle of the cord is surrounded by at least two outer arterial circles, the first of which is situated in the extradural space and the second in the extravertebral tissue planes. By virtue of the latter, the spinal cord enjoys reserve sources of blood supply through a degree of anastomosis lacking in the inner circle. The “outlet points” are limited, however, to the perforating sulcal arteries and the pial arteries of the cord.

The blood supply to the spinal cord is rich, but the spinal canal is narrowest and the blood supply is poorest at T4-9. T4-9 should be considered the critical vascular zone of the spinal cord, a zone in which interference with the circulation is most likely to result in paraplegia.

The dominance of the anterior spinal artery system has been challenged by the fact that many anterior spinal surgeries have been performed in recent years with no increase in the incidence of paralysis. This would seem to indicate that a rich anastomotic supply does exist and that it protects the spinal cord. The evidence suggests that the posterior spinal arteries may be as important as the anterior system but are as yet poorly understood. Venous drainage of the spinal cord is more difficult to define clearly than is the arterial supply ( Fig. 37.8 ). It is well known that the venous system is highly variable. Dommisse pointed out that there are two sets of veins: veins of the spinal cord and veins that fall within the plexiform network of Batson. The veins of the spinal cord are a small component of the entire system and drain into the plexus of Batson. The Batson plexus is a large and complex venous channel extending from the base of the skull to the coccyx. It communicates directly with the superior and inferior vena cava system and the azygos system. The longitudinal venous trunks of the spinal cord are the anterior and posterior venous channels, which are the counterparts of the arterial trunks. The three components of the Batson plexus are the extradural vertebral venous plexus; the extravertebral venous plexus, which includes the segmental veins of the neck, the intercostal veins, the azygos communications in the thorax and pelvis, the lumbar veins, and the communications with the inferior vena caval system; and the veins of the bony structures of the spinal column. The venous system plays no specific role in the metabolism of the spinal cord; it communicates directly with the venous system draining the head, chest, and abdomen. This interconnection allows metastatic spread of neoplastic or infectious disease from the pelvis to the vertebral column.

During anterior spinal surgery, we empirically follow these principles: (1) ligate segmental spinal arteries only as necessary to gain exposure; (2) ligate segmental spinal arteries near the aorta rather than near the vertebral foramina; (3) ligate segmental spinal arteries on one side only when possible, leaving the circulation intact on the opposite side; and (4) limit dissection in the vertebral foramina to a single level when possible so that collateral circulation is disturbed as little as possible.

Surgical Approaches