General Characteristics of the Spine

Development of the Spine

After the early development of the neural groove into the neural tube and neural crest (see Fig. 12-7), paraxial mesoderm condenses to form somites (see Figs. 12-7 and 12-9, A). The somites, in turn, develop into dermomyotomes and sclerotomes. Portions of the lateral aspects of the dermomyotomes develop into the dermis and subcutaneous tissue, whereas the majority of the dermomyotomes develop into the axial musculature. The sclerotomes migrate centrally to surround the neural tube and notochord (see Fig. 12-9, B). The sclerotomal cells then form the vertebral column and associated ligaments.

While the paraxial mesoderm is developing into somites, the more inferior portion of the neural tube differentiates into the ependymal, mantle, and marginal layers of the future spinal cord. The ependymal layer surrounds the future central canal region of the spinal cord. The mantle layer develops into the cells of the nervous system (neurons and glia), and the outer marginal layer of the tube consists of the axons of tract cells. The neural crest develops into the sensory neurons of the peripheral nervous system and the postganglionic neurons of the autonomic nervous system.

Vertebral Body

The vertebral body (Fig. 2-3) is the large anterior portion of a vertebra that acts to support the weight of the human frame. Each vertebral body is designed to provide the greatest amount of strength with the least amount of bone mass (Feltrin et al., 2001). The vertebral bodies are connected to one another by fibrocartilaginous intervertebral discs, and when the bodies are combined with their intervening discs, they create a flexible column or pillar that supports the weight of the trunk and head. The vertebral bodies also must be able to withstand additional forces from contraction of the axial and proximal limb muscles. The bodies are cylindric in shape and have unique characteristics in each named region of the spine. The transverse diameter of the vertebral bodies increases from C2 to L3. This probably results from the fact that each successive vertebral body carries a slightly greater load. There is variation in the width of the last two lumbar vertebrae, but the width steadily diminishes from the first sacral segment to the apex (inferior tip) of the coccyx.

Vertical trabeculae predominate in the vertebral bodies. The vertical trabeculae are supported by horizontal trabeculae that function much like the struts or support beams in the frame of a building. Animal studies have shown that both the vertical and the horizontal trabeculae of a vertebral body increase in number after prolonged (weeks) and increased loading by superior-inferior compression (Issever et al., 2003).

Osteoporosis is associated with a decrease in mass primarily of the horizontal trabeculae, leaving less support for the vertical trabeculae when loads are placed on an osteoporotic vertebral body. This lack of horizontal support results in a weakening of the vertebral body beyond that anticipated by the percent reduction in bone mineral content. In fact, a 25% reduction in bone mass is accompanied by a 50% reduction in the ability of a vertebral body to resist loads applied to the spine.

Bone mineral density can vary significantly from one vertebra to another (Curylo et al., 1996). Although determining the presence or absence of osteoporosis by means of x-ray bone densitometry to measure bone mineral density is reliable, fractal analysis of the trabecular pattern within vertebral bodies as imaged by computed tomography (CT) also shows promise (Kim and Nah, 2007).

The vertebral bodies have been found to change (remodel) after degeneration of the intervertebral discs, by adding bone to the region adjacent to the intervertebral disc. This addition of bone is known as subchondral sclerosis, and allows the vertebral bodies to more effectively absorb the additional compressive loads received by the vertebral bodies after intervertebral disc degeneration (Moore et al., 1996a,b).

Mosekilde and Mosekilde (1990) found that the cross-sectional area of vertebral bodies is larger in men than in women. They also found that the cross-sectional area of the vertebral body increases with age in men, but no similar finding was discovered in women.

The superior and inferior surfaces of vertebral bodies range from flat, but not parallel (Standring et al., 2008), to interlocking (see Chapter 5). A raised, smooth region around the edge of the vertebral body is formed by the anular apophysis. The superior and inferior surfaces of the vertebral body are rougher inside the anular apophyses.

Most vertebral bodies are concave posteriorly (in the transverse plane), where they help to form the vertebral foramina. Small foramina for arteries and veins appear on the front and sides of the vertebral bodies. Posteriorly there are small arterial foramina and one or two large, centrally placed foramina for the exiting basivertebral vein(s) (Standring et al., 2008).

A series of relatively large arteries pierce the center of the vertebral bodies along their entire circumference (Fig. 2-4). On entering a vertebral body, these large nutrient arteries form a dense plexus of arteries within the central horizontal plane of the vertebral body. From this central plexus, many small branches ascend and descend to reach the superior and inferior margins of the vertebral bodies; these margins are adjacent to the cartilaginous end plates of the intervertebral discs.

Large numbers of small veins drain the superior and inferior margins of the vertebral bodies. These very small veins enter into large tributaries that are oriented in the horizontal plane very close to each superior and inferior vertebral margin. These large tributaries have been called the horizontal subarticular collecting vein system (Crock & Yoshizawa, 1976). Branches of the horizontal subarticular collecting vein system, in turn, drain into large, vertically oriented channels that course toward the central horizontal plane of the vertebral body, where a dense venous network is formed. The dense network is drained by the basivertebral vein (occasionally there are two basivertebral veins in the same vertebral body). The subarticular collecting vein system also sends small tributaries laterally. These small tributaries leave the vertebral body and drain into veins of the external vertebral venous plexus (see later information).

Vertebral Arch

The vertebral (posterior) arch has several unique structures (see Fig. 2-3). These include the pedicles, laminae, and superior articular, inferior articular, transverse, and spinous processes. Each of these subdivisions of the vertebral arch is discussed separately in the following sections.

Functional Components of a Typical Vertebra

Each region of a typical vertebra is related to one or more of the functions of the vertebral column mentioned at the beginning of this chapter (support, protection of the spinal cord and spinal nerve roots, and movement) (Fig. 2-5). In general, the vertebral bodies help with support, whereas the pedicles and laminae protect the spinal cord. The superior and inferior articular processes help determine spinal movement by the facing of their facets. The transverse and spinous processes aid movement by acting as lever arms on which the muscles of the spine act.

The posterior arches also function to support and transfer weight (Pal et al., 1988), and the articular processes of the cervical region form two distinct pillars (left and right) that bear weight. In addition, the laminae of C2, C7, and the upper thoracic region (T1 and T2) help to support weight. Therefore a laminectomy at these levels results in marked cervical instability (Pal et al., 1988), whereas a laminectomy from C3 to C6 is relatively safe.

The pedicles also act to transfer weight from the posterior arch to the vertebral body, and vice versa in the cervical region (Pal et al., 1988), but only from the posterior arch to the vertebral bodies in the thoracic region. The role of the pedicles in the transfer of loads is yet to be completely determined in the upper lumbar region, but the trabecular pattern of the L4 and L5 pedicles seems to indicate that the majority of load may be transferred from the vertebral bodies to the region of the posterior arch in these two vertebrae. This is discussed in further detail in Chapter 7, which is devoted to the lumbar spine.

Zygapophysial Joints

The articulating surface of each superior and inferior articular process (zygapophysis) is covered with a 1- to 2-mm-thick layer of hyaline cartilage. This hyaline-lined portion of a superior and inferior articular process is known as the articular facet. The junction found between the superior and inferior articular facets on one side of two adjacent vertebrae is known as a zygapophysial joint. Therefore, a left Z joint and a right Z joint are between each pair of vertebrae. Figure 2-6, A, shows the Z joints of the cervical, thoracic, and lumbar regions. These joints also are called facet joints or interlaminar joints (Giles, 1992). The Z joints (Fig. 2-6, B to D) are classified as synovial (diarthrodial), planar joints. They are rather small joints, and although they allow motion to occur, they are perhaps more important in their ability to determine the direction and limitations of movement that can occur between vertebrae. In addition, the Z joints (more specifically, the articular processes) help to carry the loads placed on the spine, particularly during extension and rotation (Schultz et al., 1973). The Z joints are of added interest to those who treat spinal conditions because they have been found to be a source of back pain (Dreyer & Dreyfuss, 1996). Z joint pain may be generated as a result of direct injury to the joints, or as is the case with any synovial joint, loss of motion or aberrant motion of the Z joints may result in pain (Paris, 1983). In addition, degeneration of the hyaline cartilage that composes the articular facets can result in pain (Lewinnek & Warfield, 1986). In fact, the Z joints have been found to be a source of pain in 39% of chronic cervical pain patients, and in 34% and 27% of patients with chronic thoracic and lumbar pain, respectively (Manchukonda et al., 2007).

Each Z joint is surrounded posterolaterally by a capsule. The outer capsule and inner layers of the capsule differ significantly in composition; this is possibly unique to Z joints (Yamashita et al., 1996). The capsule consists of an outer layer of dense fibroelastic connective tissue, a vascular central layer made up of areolar tissue and loose connective tissue, and an inner layer consisting of a synovial membrane (Giles & Taylor, 1987). Figure 2-6, B, shows the previously listed regions of the capsule. The anterior and medial aspects of the Z joint are covered by the ligamentum flavum. The synovial membrane lines the articular capsule, the ligamentum flavum (Xu et al., 1991), and the synovial joint folds (see the following), but not the hyaline articular cartilage that covers the joint surfaces of the articular processes (Giles, 1992).

The Z joint capsules throughout the vertebral column are thought to do little to limit motion (Onan, Heggeness, & Hipp, 1998), although the capsules probably help to stabilize the Z joints during motions (Boszczyk et al., 2001).

Generally, the Z joint capsules are relatively thin and loose and are attached to the margins of the opposed superior and inferior articular facets of the adjacent vertebrae (Standring et al., 2008). Superior and inferior external protrusions of the joint capsules, known as recesses, bulge from the joint and are filled with adipose tissue. The inferior recess is larger than the superior one (Jeffries, 1988). The capsules are longer and looser in the cervical region than in the lumbar and thoracic regions.

The Role of Spinal Ligaments

The ligaments of the spine are discussed from superior to inferior with the region in which they first occur (e.g., ligamentum nuchae and anterior longitudinal ligament with the cervical spine; supraspinous ligament with the thoracic spine). Thereafter, the ligaments are mentioned only when they have unique characteristics in a specific region. The intervertebral disc is covered later in this chapter. However, this section discusses some of the characteristics common to the majority of spinal ligaments. In addition, Tables 2-2 and 2-3 summarize the attachment sites and functions (motions restricted) of the most important ligaments of the spine.

The role of the spinal ligaments is to allow smooth motion, with the least amount of resistance and maximum conservation of energy, within the spine’s full normal range of motion. The ligaments also help to provide protection to the spinal cord, by limiting too much motion and absorbing a significant amount of the loads placed on the spine during trauma. The functional properties of a ligament are a combination of the physical properties of the ligament (as described by its load displacement curve; see the following) and also the orientation and location of the ligament with respect to the moving vertebrae.

The ligaments of the vertebral column are most effective in carrying loads along the direction in which their fibers run. They readily resist tensile forces but buckle when subjected to compression.

Although each ligament has a unique combination of stiffness, maximum deformation, and failure load, the similarities of the spinal ligaments with regard to these parameters are striking (i.e., biomechanically speaking, the load-deformation curves for all of the spinal ligaments are similar). For all ligaments of the spine, there is a sharp increase in stiffness once full physiologic motion has been attained (little stiffness up to the end of physiologic motion). Spinal degeneration generally leads to increased motion (in the initial stages). This increased motion can result in increased strain on the ligaments, which in turn can result in ligamentous sprain. Consequently, degenerated spines may carry higher risks for increased ligament sprains during flexion, extension, axial rotation, and lateral flexion.

Using nerve tracing techniques in an animal model, Jiang (1997) found that stretching of a spinal ligament resulted in “a barrage of sensory feedback from several spinal cord levels on both sides of the spinal cord.” The sensory information was found to ascend to many higher centers by way of the dorsal columns and spinocerebellar tracts (see Chapter 9). These higher centers included the thalamus and vestibular nuclei. Jiang’s (1997) findings provide provocative evidence that the spinal ligaments, along with the Z joint capsules and the small muscles of the spine (interspinales, intertransversarii, and transversospinales muscles), play an important role in mechanisms related to spinal proprioception (joint position sense). In addition, Solomonow and colleagues (1998) found that compression of the supraspinous ligament of cats or electrical stimulation of the same ligament in humans resulted in contraction of the multifidus muscle (see Chapter 4) at the same level as the stimulation and also at the segmental levels above and below the stimulation. These authors concluded that the ligaments of the vertebral column recruit the help of spinal muscles to achieve general spinal stability.

Structures That Limit Spinal Movement

Spinal motion is limited by a series of bony stops and ligamentous brakes (Louis, 1985). Table 2-4 shows some of the structures limiting spinal motion.

Other factors associated with each type of spinal motion include the following:

Rotation with Lateral Flexion (“Coupled Motion”)

The main motions of flexion-extension, left and right axial rotation, and left and right lateral bending are accompanied by subtle motions of the vertebrae in the other directions. These subtle motions that attend the main motions of the spine are called “coupled motions.” Haher and colleagues (1992) defined coupled motion more precisely as “consistent association of one motion about an axis with another motion about a second axis.” Coupled motions are complex and vary from one vertebral segment to the next (Ochia et al., 2006). There is little consensus among investigators for many of the coupled motions. These motions are most predictable, and consensus among investigators is highest in the cervical and lumbar regions when the main motion is lateral flexion (Harrison, Harrison, & Troyanovich, 1998).

As a result of the facing of the superior and inferior articular facets, lateral flexion of the cervical and lumbar regions is accompanied by axial rotation (Fig. 2-11). These are coupled motions. However, the direction of the rotation is opposite in these two regions, and more rotation occurs with lateral flexion in the cervical than the lumbar region (Moroney et al., 1988). Lateral flexion of the cervical spine is accompanied by rotation of the vertebral bodies into the concavity of the arch formed by the lateral flexion (vertebral body rotation to the same side as lateral flexion). For example, right lateral flexion of the cervical region is accompanied by right rotation of the vertebral bodies (see Fig. 2-11, A). Because the spinous processes move in the direction opposite that of the vertebral bodies during rotation, right lateral flexion of the cervical region is accompanied by left rotation of the spinous processes.

Lateral flexion of the lumbar spine, on the other hand, is accompanied by rotation of the vertebral bodies toward the convexity of the arch formed by lateral flexion (vertebral body rotation away from the side of lateral flexion). For example, left lateral flexion of the lumbar region is accompanied by right rotation of the vertebral bodies and left rotation of the spinous processes (see Fig. 2-11, B).

The upper four thoracic vertebrae move in a fashion similar to that of the cervical vertebrae during lateral flexion (i.e., vertebral body rotation into the side of concavity), whereas the lower four thoracic vertebrae mimic the motion of lumbar vertebrae (i.e., vertebral body rotation toward the side of convexity). The middle four thoracic vertebrae have little coupled motion (White & Panjabi, 1990). Other investigators feel that currently there is no strong consensus as to the amount or even the direction of coupled motion of axial rotation when the main motion is lateral bending in the thoracic region (Harrison, Harrison, & Troyanovich, 1998).

Composition of the Intervertebral Disc

Like cartilage elsewhere in the body, the disc consists of water, cells (primarily chondrocyte-like cells and fibroblasts), proteoglycan aggregates, and type I and II collagen fibers (see Chapter 14). The proteoglycan aggregates are composed of many proteoglycan monomers attached to a hyaluronic acid core. However, the proteoglycans of the IVD have a smaller size and different composition than the proteoglycans of cartilage found in other regions of the body (e.g., articular cartilage, nasal cartilage, and cartilage of growth plates) (Buckwalter et al., 1989). The cartilaginous IVD is a dynamic structure that has been shown to be able to repair itself and is capable of regeneration (Humzah & Soames, 1988; Nitobe et al., 1988; Mendel et al., 1992), although its ability to regenerate seems to diminish after injury or degeneration (Moore et al., 1996a,b).

The IVD is an osmotic system that is sensitive to load, pressure, and the concentration of proteoglycans within its component parts. When the disc is contained (i.e., no extrusions), loading of the vertebral column leads to an outflow of fluid from the IVD and a loss of IVD height; unloading of the vertebral column results in an uptake of both fluid and nutrient substances, and a corresponding increase in IVD volume. Therefore fluid uptake takes place in lying positions and fluid loss occurs in upright and standard sitting positions. The response of the IVDs to compressive loads is complex and varies with the frequency, magnitude, and duration of the loads; and different anatomic parts of the IVD (e.g., nucleus pulposus versus anulus fibrosus) respond differently to the same loading forces (Hee, Zhang, & Wong, 2010). The IVDs thrive on reasonable motion within normal physiologic limits. That is, frequent changes of postures improve and maintain the internal environment of the IVD, whereas high-pressure postures, such as prolonged standing and sitting, inhibit disc nutrition. Under experimental conditions, prolonged static compression results in IVD degeneration (Guehring et al., 2006a,b) and loss of IVD height (O’Connell et al., 2007), whereas dynamic loading (changes in IVD pressure as would occur during normal daily activities) results in changes consistent with biologic remodeling (Wang, Jiang, & Dai, 2007; Korecki, MacLean, & Iatridis, 2008). A sedentary lifestyle has negative consequences for the IVD and the entire spine (Kraemer, 1995).

Calcification of the IVD during the aging process is much more common than previously believed. Cheng and colleagues (1996) found such calcification in 58.3% of subjects at autopsy. They concluded that calcification of the IVD is “significantly underestimated” by conventional radiography.

The IVD is composed of three regions (Fig. 2-12) known as the anulus fibrosus (including the attachment of the anulus fibrosus to the ring apophyses), nucleus pulposus, and vertebral (cartilaginous) end plate (Humzah & Soames, 1988; Fardon, 2001). Together the regions constitute the anterior interbody joint or intervertebral symphysis. Each region consists of different proportions of the primary materials that make up the disc (e.g., water, cells, proteoglycan, and collagen). Table 2-5 compares some of the characteristics of the anulus fibrosus with those of the nucleus pulposus.

Although each region of the disc has a distinct composition, the transition between the anulus fibrosus and the nucleus pulposus is rather indistinct. The main difference between the two regions is their fibrous structure (Humzah & Soames, 1988). Type I collagen (typical in tendons) predominates in the anulus fibrosus, and type II collagen (typical for articular cartilage) predominates in the nucleus pulposus. The histologic and biochemical composition of the IVD is currently an active field of research and has a great deal of potential clinical relevance. As mentioned previously, Chapter 14 discusses the histologic characteristics of the IVD in more detail, and Chapter 11 discusses the clinical relevance of pathology of the IVD in low back pain. The gross morphologic characteristics of the three regions of the disc are discussed in the following sections.

Nucleus Pulposus

The nucleus pulposus (NP) is a rounded region located within the center of the IVD (see Fig. 2-12). The NP is thickest from superior to inferior in the lumbar region, followed in thickness by the cervical region; it is the thinnest in the thoracic region. It is most centrally placed within the horizontal plane in the cervical region and is more posteriorly placed in the lumbar region (Humzah & Soames, 1988).

The NP develops from the embryologic notochord. It is gelatinous and relatively large just after birth, and several multinucleated notochordal cells can be found within its substance (Standring et al., 2008). The remnants of the notochord can be recognized in MRI scans as an irregular dark band, usually confined to the NP (Breger et al., 1988). The notochordal tissue has been found to be more apparent in fetal spines than in the spines of infants (Ho et al., 1988). The notochordal cells decrease in number over time and are almost completely replaced by fibrocartilage by approximately the eleventh year of life (Standring et al., 2008), and no notochordal cells normally remain after the mid-teens (Oda, Tamaka, & Tsukuki, 1988). As the notochordal cells are replaced, the outer aspect of the NP blends with the inner layer of the AF, making it more difficult to determine the border between the two regions. Notochordal cells may remain throughout the spine. These remnants are known as notochordal “rests” and may develop into neoplasms known as chordomas. Chordomas most commonly occur at the base of the skull and in the lumbosacral region (Humzah & Soames, 1988).

The adult disc is an avascular structure, except for the most peripheral region of the AF, and the NP is responsible for absorbing the majority of the fluid received by the disc. The process by which a disc absorbs fluid from the vertebral bodies above and below it has been termed imbibition. The size of the NP and its capacity to swell are greatest in the lumbar region, followed by the cervical region. The NP is 70% to 90% water and it reaches its peak hydration between the ages of 20 to 30 years, and the process of degeneration begins shortly thereafter (Coventry, 1969). The disc loses water when a load is applied but retains sodium and potassium. This increase in electrolyte concentration creates an osmotic gradient that results in rapid rehydration when the loading of the disc is stopped (Kraemer et al., 1985). This osmotic system of the IVD is sensitive to the forces applied to the disc, the pressure within the NP, and the composition and concentration of the proteoglycan molecules within the IVD (Kraemer, 1995). Frequent changes of posture are beneficial to this osmotic system, and the disc apparently benefits from both activity during the day (Holm & Nachemson, 1983; Kraemer, 1995) and the rest it receives during the hours of sleep. Because of the loss of fluid in the IVD during compression, the total body height is approximately 10 mm less at the end of an average day. This height is completely regained during 8 hours of sleep (Boos et al., 1996; McGill & Axler, 1996). Prolonged bed rest (up to 32 hours) does not significantly increase total body height, but the weightless environment of space can increase total body height by 40 to 60 mm (McGill & Axler, 1996). The diurnal variation in IVD height has been verified with both MRI (Botsford, Esses, & Olgilvie-Harris, 1994) and ultrasound (Boos et al., 1996; Ledsome et al., 1996). However, too much rest may not be beneficial. A decrease in the amount of fluid (hydration) of the IVDs has been noted on MRIs after 5 weeks of bed rest (LeBlanc et al., 1988).

The NP is a viscoelastic structure that can act as a fluid or solid depending on the rate and magnitude of loading. In addition, the NP can change its shape and position during different motions of the spine and when different loads are placed on the spine (Iatridis et al., 1996). For example, the NP of the lumbar region moves posteriorly during flexion and anteriorly during extension (Fennell, Jones, & Hukins, 1996).

In addition, the pressure within the NP changes as body position changes. When a person is positioned supine (lying on the back), the pressure within a lower lumbar NP is 20% of the pressure during standing. Leaning forward while sitting or standing substantially increases intradiscal pressure (IDP) (Nachemson, 1966) and causes the NP to migrate posteriorly (Alexander et al., 2007), and lifting a 20-kg load results in a 4.5-fold increase in IDP. This IDP is reduced by 25% if the knees are bent during lifting. In addition, if the load is held close to the body the IDP is reduced twofold. The IDP is lower in relaxed sitting than in relaxed standing, and slightly slouching while sitting actually decreases IDP, whereas straight sitting and straight standing both increase IDP. As might be expected, muscle activity increases IDP. During 7 hours of sleep the IDP increases approximately 240% compared with when one initially goes to bed (Wilke et al., 1999). This may support the hypothesis that herniation of the NP is more likely during the beginning of the day than at the end of the day.

Hysteresis is a viscoelastic phenomenon that refers to deformation of a tissue because of short duration loading. Hysteresis helps to protect the spine and nervous system during rapid loadings. For example, the successive vertebrae, intervertebral discs, and other tissues from the feet to the brain absorb the shock of jumping by means of hysteresis. Hysteresis increases as loads increase, is greatest in young tissues, and decreases with age. This phenomenon is diminished in the lower thoracic and upper lumbar regions, compared with the lower lumbar region of the spine. Hysteresis also decreases when the same disc is loaded a second time, and continues to decrease with repetitive loading. Decreased hysteresis may be a factor in the increased incidence of extruded NP in those with driving occupations (i.e., repetitive loading from subtle and erratic bouncing).

As the NP ages, it becomes less gelatinous in consistency, its ability to absorb fluid diminishes, and the intradiscal pressure diminishes. The changes in composition and structure that are common to all sources of cartilage with aging occur earlier and to a greater extent in the IVD (Bayliss et al., 1988). In fact, the normal aging process of the IVD is difficult to differentiate from IVD degeneration (Boos et al., 2002), although there are differences between these two processes (Adams, 2005). Breakdown of the proteoglycan aggregates and monomers (see Fig. 14-6) is thought to contribute to this process of aging and degeneration. The breakdown of proteoglycans results in a decreased ability of the disc to absorb fluid, which leads to a decrease in the ability of the disc to resist loads placed on it. The degeneration associated with the decrease in ability to absorb fluid (water) has been identified through use of CT (Bahk & Lee, 1988) and MRI and has been correlated with histologic structure and fluid content. As the disc degenerates, it narrows in the superior to inferior dimensions and the adjacent vertebral bodies may become sclerotic (thickened and opaque on x-ray) (Moore et al., 1996a,b). Much of the disc thinning with age seen on x-ray may also be the result of the disc sinking into the adjacent vertebral bodies over the course of many years (Humzah & Soames, 1988).

Pathologic conditions of the IVD are frequently seen in clinical practice. As mentioned, the NP may cause bulging of the outer anular fibers or may protrude (herniate) through some or all of the lamellae of the AF. This was first described by Mixter and Barr (1934). Bulging or protrusion of the disc may be a primary source of pain, or pain may result because of pressure on the exiting nerve roots within the medial aspect of the intervertebral foramen. Such bulging is usually associated with heavy lifting or trauma, although such a history may be absent in as many as 28% of patients with confirmed disc protrusion (Martin, 1978). Vibration along with sudden loading has been shown to exacerbate disc herniations (Yates & McGill, 2011). Some investigators believe that proteoglycan and other molecules leaking from a tear in the anulus also may cause pain by creating a chemical irritation of the exiting nerve roots. The pain that results from pressure on or irritation of a nerve root radiates in a distribution along the nerves supplied by the compressed nerve root (see Chapter 11). Such pain is termed radicular pain because of its origin from the dorsal root (radix) or dorsal root ganglion. Treatment for protrusion of the NP ranges from excision of the disc (discectomy) to employment of conservative methods (Sanders & Stein, 1988; Brønfort, 1997).

Vertebral (Cartilaginous) End Plates

The vertebral end plates, or cartilaginous end plates (CEPs), limit all but the most peripheral rim of the disc superiorly and inferiorly. They are attached both to the disc and to the adjacent vertebral body (see Fig. 2-12). Although a few authors consider the vertebral end plate to be a part of the vertebral body, most authorities consider it to be an integral portion of the disc (Coventry, 1969; Bogduk, 2005a). The CEPs are approximately 1 mm thick peripherally and 3 mm thick centrally. They are composed of both hyaline cartilage and fibrocartilage. The hyaline cartilage is located against the vertebral body, and the fibrocartilage is found adjacent to the remainder of the IVD. The end plates help to prevent the vertebral bodies from undergoing pressure atrophy and, at the same time, contain the AF and NP within their normal anatomic borders.

The CEPs are very important for proper nutrition of the disc (Humzah & Soames, 1988). They are extremely porous and allow fluid to enter and leave the AF and NP by osmotic action (Humzah & Soames, 1988). Very early in postnatal life, small vascular channels enter the vertebral side of the vertebral end plate and a few channels enter the outermost lamella of the AF. These channels disappear with age and are almost completely gone by the age of 12, leaving the IVD to obtain all of its nutrition by means of imbibition through the CEP (Boos et al., 2002). The end plate is more permeable in the region adjacent to the NP and is relatively impermeable in the region associated with the AF. This may result from morphologic differences in the capillary beds of the bony end plates. These beds are more complex in the region surrounding the NP (Oki et al., 1996).

The first structures to fail with compressive loading of the vertebral column are the CEPs and the adjacent subchondral bone of the vertebral bodies (bony end plates) (Hickey & Hukins, 1980). Such fractures allow the NP to rupture through the CEP, causing a lesion known as a Schmorl’s node. These nodes cause the vertebrae surrounding the lesion to move closer together. This movement is thought to increase pressure on the posterior and anterior joints between the vertebrae, increasing the degenerative process of the anterior interbody joint (the remainder of the IVD). In addition, the disc thinning or narrowing that results from these end plate herniations causes more force to be borne by the Z joints and may result in more rapid degeneration of these structures as well.

The CEPs begin to calcify and thin with advancing years. This leaves them more brittle. The central region of the end plate in some vertebrae of certain individuals may be completely lost in the later years of life.

Innervation of the Intervertebral Discs

The entire outer third of the AF has been found to receive both sensory and vasomotor innervation (Bogduk, Tynan, & Wilson, 1981). The sensory fibers probably are both nociceptive (pain sensitive) and proprioceptive in nature, and an extensive distribution of small nerve fibers (both A-delta and C) has been found throughout the peripheral aspect of the AF (Cavanaugh et al., 1995). The vasomotor fibers of the AF are associated with the small vessels located along its most superficial aspect. The posterior aspect of the disc receives its innervation from the recurrent meningeal nerve (sinuvertebral nerve). The posterolateral aspect of the anulus receives both direct branches from the anterior primary division and also branches from the gray communicating rami of the sympathetic chain. The lateral and anterior aspects of the disc receive their innervation primarily from branches of the gray communicating rami and also branches from the sympathetic chain. The central CEP adjacent to the NP also receives a sensory innervation similar to that of the outer AF.

The very strong contribution of afferents associated with the sympathetic nervous system to the innervation of the IVD has led some to observe that the innervation to the disc is different from the innervation to most musculoskeletal tissues, and that the innervation to the IVD is actually similar to that of visceral, specifically enteric, organs. These findings indicate that discogenic pain may be a type of visceral pain (Edgar, 2007).

Degenerated discs receive increased innervation by sensory fibers conducting nociception. Both the number of sensory nerve endings increases and the nerve endings extend deeper into the AF. The nerve endings are both nociceptive and mechanoreceptive and inflammation may sensitize (peripheral sensitization) the mechanoreceptive endings. The increased innervation by nociceptive and mechanoreceptive sensitive endings indicates that injured or degenerated discs are more sensitive to inflammation and pressure and that stimulation of these endings may be interpreted as pain (Coppes et al., 1997; Edgar, 2007). Schwann cells of the nerves innervating the outer layers of the AF appear to play a role in this ingrowth of sensory nerves (Johnson et al., 2001).

These findings just described indicate that the IVD itself is most likely able to generate pain. Therefore disorders affecting the IVDs alone (e.g., internal disc disruption, tears of the outer third of the AF, and possibly even marked disc degeneration) can be the sole cause of back pain. The disc can also generate pain by compressing (entrapping) an exiting dorsal root. As mentioned, leakage of nerve-irritating (histamine-like) molecules from disrupted IVDs also has been found to be a cause of irritation to the exiting dorsal root. These latter conditions cause a sharp, stabbing pain that radiates along the distribution of the nerves receiving fibers from the dorsal root. This type of pain is known as radicular pain because it results from irritation of a nerve root (radix). Chapter 11 describes the differentiation of radicular pain from somatic referred pain. The unique characteristics of the innervation to the IVDs of the specific spinal regions are discussed in Chapters 5 through 7.

• Axial-occipital syndesmosis (between odontoid and clivus; ligaments include cruciform, apical-odontoid, and alar)

• Ligamentum nuchae (syndesmosis between occiput and C1-7)

• Laminar syndesmosis (ligamentum flavum)

• Intertransverse syndesmosis (intertransverse ligament)

• Supraspinous syndesmosis (supraspinous ligament)

• Interspinous syndesmosis (interspinous ligament)

• See Appendix I for additional syndemoses

• Vertebral body

• Left and right pedicles

• Left and right laminae

• Spinous process

External Vertebral Venous Plexus

Before investigating the contents of the vertebral canal, it is necessary to discuss a plexus of veins that surrounds the outside of the vertebrae and the vertebral canal. This network of veins surrounding the external aspect of the vertebral column is known as the external vertebral venous plexus. The external vertebral venous plexus is associated with both the posterior and the anterior elements of the vertebral column and can be divided into an anterior external vertebral venous plexus surrounding the vertebral bodies and a posterior external vertebral venous plexus associated with the posterior arches of adjacent vertebrae. These plexuses communicate with segmental veins throughout the spine (e.g., deep cervical veins, intercostal veins, lumbar veins, and ascending lumbar veins) and also with the internal vertebral venous plexus, which lies within the vertebral canal. The external and internal vertebral plexuses communicate through the IVFs and also directly through the vertebral bodies (anterior plexus) and through the ligamenta flava (posterior plexus). The posterior external vertebral plexus has valves that direct flow into the posterior internal vertebral venous plexus (Stringer et al., 2012). Intervertebral veins, located at the superior and inferior aspects of the IVFs, connect the internal and external venous plexuses (Parke, 2005). The superior intervertebral veins surround the dorsal root ganglion, exiting nerve roots (Parke, 2005), and form a vascular cuff around the (more distal) spinal nerve (Humzah & Soames, 1988).

Epidural Space

The region immediately beneath the bony and ligamentous elements forming the vertebral canal is known as the epidural space (see dura mater in Fig. 2-15). Generally throughout the length of the vertebral canal, the epidural space is approximately 4 to 6 mm deep to the osseous and ligamentous anterior and posterior canal borders (Chen et al., 1989; Hackney, 1992). The epidural space is sometimes entered at the L3-4 interspinous space for the purpose of administering anesthetics. The depth to the epidural space at this level is 4.77 ± 0.55 cm in males and 4.25 ± 0.55 cm in females. The range of depth is 3.0 to 7.0 cm (1.2 to 2.8 inches), and there is a positive correlation between both body weight and body height with the depth to the epidural space (Chen et al., 1989).

The epidural space contains a venous plexus embedded in a thin layer of adipose tissue. The adipose tissue is known as the epidural adipose tissue, or epidural fat, and the venous plexus is known as the internal vertebral venous plexus.

Internal Vertebral Venous Plexus

The internal vertebral venous plexus is located beneath the bony elements of the vertebral foramina (e.g., laminae, spinous processes, pedicles, and vertebral body). As mentioned, it is embedded in a layer of loose areolar tissue known as the epidural (extradural) adipose tissue, although less adipose tissue surrounds the veins located in the anterior aspect of the epidural space. The internal vertebral venous plexus provides an alternate route of venous return when the jugular veins of the neck are compressed, when the flow through the inferior vena cava is obstructed, and when intrathoracic or intraabdominal pressures are increased. They also provide a protective cushion for the important contents of the vertebral canal (Stringer et al., 2012). The internal vertebral venous plexus is a clinically important plexus, and perhaps for this reason it has been given many names. It is known as the internal vertebral venous plexus, the epidural venous plexus, the extradural venous plexus, and also as Batson’s channels.

The internal vertebral venous plexus consists of approximately four interconnected longitudinal channels. Two course along the posterior aspect of the vertebral canal, and two channels of larger diameter course along the anterior aspect of the canal (Stringer et al., 2012). The posterior channels are rudimentary in the cervical region, but well developed in the thoracic and lumbar regions (Chaynes et al., 1998). The posterior channels are located along the posterolateral aspect of the vertebral canal in the thoracic region and are more laterally placed in the cervical (rudimentary channels) and lumbar regions (Stringer et al., 2012).

The anterior channels are located on either side of the posterior longitudinal ligament and drain the vertebral bodies via large basivertebral veins. The basivertebral veins pierce the center of each vertebral body and communicate posteriorly with the internal plexus and anteriorly with the external vertebral venous plexus. Posteriorly the basivertebral veins connect with horizontally oriented transverse veins that course anterior to the posterior longitudinal ligament. These transverse veins connect the left and right longitudinal channels of the anterior internal vertebral venous plexus (Stringer et al, 2012).

The veins of the internal vertebral venous plexus contain no valves; therefore the direction of drainage is posture and respiration dependent. Inferiorly this plexus is continuous with the prostatic venous plexus of the male, and superiorly (in both sexes) it is continuous with many veins and also to dura mater venous sinuses of the posterior cranial fossa. These superior connections include the following: vertebral veins, occipital veins, occipital sinus, sigmoid sinus, and inferior petrosal sinus (Stringer et al., 2012). Therefore prostatic carcinoma may metastasize via the intervertebral venous plexus to all regions of the spine and to the meninges and brain. Because of venous communications in the thoracic region, lung and breast cancers can metastasize to these veins as well. However, the veins of the internal vertebral venous plexus eventually (by means of intervertebral and ascending lumbar veins) drain into large veins. These large veins include the vertebral, for the cervical region; and the azygos, hemiazygos, and right highest intercostal veins, for blood draining the thoracic and lumbar regions. These large veins each have one or two valves at their entrance to the brachiocephalic veins (for the vertebral veins) or at their entrance to the azygos vein (for the right highest intercostal and hemiazygos veins). The azygos vein then drains directly into the superior vena cava, and there is also a valve at this entrance. These valves act as a protective mechanism, preventing reflux of blood (and the accompanying increase in pressure) into the internal vertebral venous plexus and the important neural tissues they serve (Scapinelli, 2000). However, diminished right-sided heart function can lead to congestion and engorgement of the intervertebral veins, coursing through the IVFs, and veins of the internal vertebral venous plexus. Such venous congestion, usually when coupled with narrowing (stenosis) of the vertebral canal, can exacerbate inflammation of dorsal roots and/or dorsal root ganglia and lead to pain typically associated with prolonged recumbency (e.g., during sleep) (Parke, 2005).

The walls of the longitudinal veins of the internal vertebral venous plexus are very distinct. These walls have many trabeculae composed of collagen and smooth muscle that create a series of interconnected, parallel channels for blood flow within the individual longitudinal veins. The trabeculae are supplied by small nerve endings and arteries. The trabeculae are thought to prevent overdistension or collapse of the vessels and may help to regulate the direction and velocity of blood flow within the vessels (Stringer et al., 2012). However, segments of the longitudinal veins, and all of the horizontal connecting and intervertebral veins (the latter coursing through the intervertebral foramina), are devoid of these reinforcements. These trabeculae-free regions appear to be more vulnerable to distension and collapse. Isolated regions of the veins have been found to become dilated (varices of epidural veins; that is, epidural venous engorgement) (Parke, 2005). These regions are likely related to the trabeculae-free areas of the veins. The varices that can develop in these regions, and in the intervertebral veins, may compress the exiting spinal nerves and cause radiculopathy (i.e., pain coursing along the distribution of the dorsal root that contributes to the formation of the spinal nerve). Radiculopathy from this cause can mimic that more commonly caused by an IVD protrusion (Wong et al., 2003). In addition, the veins can collapse from the pressure of an IVD protrusion. This fact has been used in a procedure known as epidural venography (Fig. 2-17) to aid in the diagnosis of IVD disease. In epidural venography, radiopaque dye is injected into the epidural veins and x-ray films are taken. This allows the veins filled with dye to be visualized (Jayson, 1980). Pressure from a disc protrusion prevents the veins from filling and is seen as an area devoid of dye on the x-ray film.

Spinal epidural hematoma is a condition in which bleeding occurs into the space surrounding the dura mater. It is usually the result of a ruptured epidural vein and is rather rare, with only 250 cases reported in the literature. Of these cases, approximately 50% are spontaneous and of unknown cause. The causes of the remainder of the cases include trauma, anticoagulant therapy, and arteriovenous malformation. Spinal epidural hematoma may simulate IVD protrusion but can usually be identified through MRI (Mirkovic & Melany, 1992). The treatment protocol usually involves the release of pressure (decompression) by the removal of a lamina (laminectomy), although several cases with spontaneous recovery have been reported (Sei et al., 1991).

Meningeal and Neural Elements within the Vertebral Canal

The meningeal and neural elements of the vertebral canal are thoroughly discussed in Chapter 3. This section focuses on the neural elements that enter and leave the vertebral canal.

Beneath the epidural venous plexus and epidural adipose tissue lie the meninges, which surround the spinal cord (Fig. 2-18). These layers of tissue are known as the dura mater, arachnoid mater, and pia mater. The space between the meninges and the borders of the vertebral canal is known as the epidural space. Recall that this space is approximately 4 to 6 mm anteriorly and posteriorly throughout the length of the vertebral canal (Chen et al., 1989; Hackney, 1992).

The spinal cord lies deep to the dura, arachnoid, and pia mater (see Fig. 2-18). Beneath the transparent pia mater, dorsal and ventral rootlets can be seen attaching to the spinal cord. These rootlets divide the spinal cord into spinal cord segments (see Chapter 3). A spinal cord segment is the region of the spinal cord delineated by those exiting dorsal and ventral rootlets that eventually unite to form a single spinal nerve. Spinal cord segments can be identified easily on a gross specimen of the spinal cord (see Figs. 3-6 and 3-11, C). The rootlets combine to form dorsal roots (from dorsal rootlets) and ventral roots (from ventral rootlets). The dorsal and ventral roots then unite to form a spinal nerve. The rootlets are “exceedingly delicate and vulnerable and when implicated in fibrous adhesions from whatever cause, undergo irreversible changes” (Domisse & Louw, 1990).

Arterial Supply to the Spine

The external aspect of the vertebral column receives its arterial supply from branches of deep arteries “in the neighborhood” (see Fig. 2-4). The cervical region is supplied by the left and right deep cervical arteries (from the costocervical trunks) and also the right and left ascending cervical arteries (from the right and left inferior thyroid arteries). The thoracic region of the spine is supplied by posterior intercostal arteries, and the lumbar region is supplied by lumbar segmental arteries.

The internal aspect of the vertebral canal receives its arterial supply from segmental arteries that send spinal branches into the IVFs (see Fig. 2-4). The segmental arteries are branches of the vertebral artery in the cervical region, the posterior intercostal arteries in the thoracic region, and the lumbar segmental arteries in the lumbar region.

On entering the IVF, each spinal branch (ramus) of a segmental artery further divides into three branches. One branch courses posteriorly to supply the laminae and ligamenta flava, and a large branch usually courses to the spinous process, where it enters the base of this process and continues posteriorly to reach its posterior tip. Other arterial twigs from the posterior branch supply the extradural adipose tissue and other small branches course to the posterior spinal dura mater. A large branch enters the lamina close to its junction with the pedicle. On entering the lamina, an ascending branch and a descending (the longer of the two) branch course through the superior and inferior articular processes, respectively, to reach the subchondral bone of the articular facets (Crock & Yoshizawa, 1976).

Another branch of the spinal ramus of a segmental artery courses anteriorly. This anterior branch bifurcates into large offshoots to the center of the posterolateral aspect of the vertebral body and then divides into an ascending and a descending branch. The ascending branch crosses the IVD above and joins the descending branch of the level above. The descending branch courses near the pedicle, sending branches to it. The ascending and descending branches both supply the posterior aspect of the vertebral bodies and also send twigs to the posterior longitudinal ligament (Crock & Yoshizawa, 1976).

The third branch of each spinal ramus of a segmental artery, known as the neural branch, courses to the spinal nerve. The neural branch then divides into an anterior and a posterior radicular artery to supply the ventral and dorsal nerve roots and rootlets, respectively. The posterior radicular artery also supplies a branch to the distal pole and another to the proximal pole of the dorsal root ganglion (polar dorsal root ganglion arteries) (Parke, 2005). The spinal cord, the vasculature of the cord (including radiculomedullary arteries), and its meningeal coverings are discussed in detail in Chapter 3, and the unique characteristics of the blood supply to each region of the spine are discussed in further detail in the chapters on specific regions of the spine (Chapters 5 through 8).

1. The pedicle of the vertebra above (more specifically, its periosteum)

2. The vertebral body of the vertebra above (again, its periosteum)

3. The IVD (posterolateral aspect of the AF)

4. The vertebral body of the vertebra below, and in the cervical region, the uncinate process (periosteum)

5. The pedicle (periosteum) of the vertebra below forms the floor of the IVF. A small part of the sacral base (between the superior articular process and the body of the S1 segment) forms the floor of the L5-S1 IVF.

6. The Z joint forms the “posterior wall.” Recall that the Z joint is composed of the following: (a) the inferior articular process (and facet) of the vertebra above, (b) the superior articular process (and facet) of the vertebra below, and (c) the anterior articular capsule, which is composed of the ligamentum flavum (Xu et al., 1991; Giles, 1992) (the posterior articular capsule is not directly related to the IVF).

• The spinal nerve (union of dorsal and ventral roots).

• The dural root sleeve.

• Lymphatic channel(s).

• The spinal branch (ramus) of a segmental artery. Recall that this artery divides into three branches: one to the posterior aspect of the vertebral body, one to the posterior arch, and one to the spinal nerve (neural branch).

• Communicating (intervertebral) veins between the internal and external vertebral venous plexuses.

• Two to four recurrent meningeal (sinuvertebral) nerves.

• Fibrocartilage of the AF

• NP (especially in earlier decades)

• Red bone marrow of the vertebral bodies

• Compact bone of the pedicles

• Z joints

• Capsules

• Synovial membranes

• Articular cartilage

• Fibroadipose meniscoids

• Fat pads

• Connective tissue rim (fibrous labra)

• Costocorporeal joints (in the thoracic region)

Accessory Ligaments of the Intervertebral Foramen

Accessory ligaments of the IVF were first studied in the early nineteenth century (Bourgery, 1832). However, these structures received very little attention until the mid- to late-twentieth century, when Golub and Silverman (1969) first used the term transforaminal ligaments (TFLs) in the description of ligamentous bands crossing the IVFs. Since then, several investigators have studied these structures in the cervical (Bakkum & Berthiaume, 1994), thoracic (Bakkum & Mestan, 1994), and lumbar (Amonoo-Kuofi et al., 1988a,b; Nowicki & Haughton, 1992a,b; Bakkum & Mestan, 1994; Cramer et al., 2002) regions of the spine. These ligaments are much more common in the lower thoracic and lumbar regions (Bakkum & Mestan, 1994) than in the cervical region (Bakkum & Berthiaume, 1994), and are now considered to be normal structures within the lumbar IVFs. However, the reported incidence of TFLs in the lumbar region varies considerably (Table 2-8). One reason for the difference in reported findings is that there is considerable variation in size, shape, and location of TFLs from one IVF to another. Also some investigators evaluated different regions of the IVFs (i.e., medial, lateral, or both), and some authors only identified very thick and substantial structures as TFLs, whereas others were less restrictive in their definition of TFLs.

Amonoo-Kuofi and colleagues (1988a) also studied TFLs of the IVFs, and mapped out the relationship of the spinal nerve, segmental veins and arteries, and the recurrent meningeal nerve through the openings between the TFLs. They concluded that these accessory ligaments tend to hold the previously mentioned structures in their proper position within the IVF.

Transforaminal ligaments have been identified on both CT (Nowicki & Haughton, 1992b) and MRI scans (Nowicki & Haughton, 1992b; Cramer et al., 2002). Cramer and colleagues (2002) conducted a reliability study to evaluate the ability of trained radiologists to identify TFLs on MRI. The study was performed on cadaveric spines that were carefully dissected to identify all TFLs. The spines were then embedded in gelatin and MRI-scanned. Three radiologists were trained to identify TFLs on MRI scans and then evaluated the MRI scans of the cadaveric spines to determine the presence or absence of TFLs. The radiologists were blinded to the results of one another and to the anatomic specimens. The results showed that if a radiologist, trained to identify TFLs on MRI, determined that a TFL was present at a given IVF, there was approximately an 87% chance that one was actually present (i.e., a positive predictive value of 86.7%). However, if a trained radiologist stated a TFL was not present in an IVF, there remained approximately a 50% chance that one was present (i.e., a negative predictive value of 50.8%).

TFLs have been implicated as a cause of both low back pain and nerve root entrapment (Bachop & Janse, 1983; Giles, 1988; Macnab & McCulloh, 1990; Olsewski et al., 1991; Transfeldt, Robertson, & Bradford, 1993; Qian, Qin, & Zheng, 2011). Bakkum and Mestan (1994) found that when TFLs were present, the superior to inferior dimension of the compartment transmitting the anterior primary division of the spinal nerve was significantly decreased as compared with that of the osseous IVF (the mean decrease in size was 31.5%). They concluded that there is often less space at the exit zone of the IVF for the emerging anterior primary division than was traditionally considered to be the case. Furthermore, they felt that the decreased space may contribute to the incidence of neurologic symptoms in the region at times, especially after trauma or secondary to degenerative arthritic changes in the region of the IVF. Bachop and Janse (1983) reported that the higher a TFL is located within the IVF, the less space remains for the intervertebral vessels, which conceivably could lead to ischemia or venous congestion. They also postulated that lower placement of the ligament would increase the possibility of sensory and motor deficits. Qian and colleagues (2011) hypothesized that TFLs may be “the leading cause of lumbar nerve root compression in foraminal stenosis,” particularly in cases of IVD degeneration, where the narrowing IVD causes the IVF to narrow (become stenotic), positioning TFLs in the IVF closer to the exiting nerve roots and spinal nerve. Figure 2-23, A and B, shows examples of TFLs, and Figure 2-23, C, illustrates a composite drawing demonstrating the TFLs that have been reported in the literature.

The term corporotransverse ligament is used when referring to a TFL that courses between the vertebral body and the transverse process at the L5-S1 junction (Bachop & Janse, 1983). The lumbar spinal ramus of the segmental artery, intervertebral veins, and gray sympathetic ramus course above this structure, and the anterior primary division courses underneath it (Golub & Silverman, 1969; Bachop & Ro, 1984).

Corporotransverse ligaments can be either broad and flat or rodlike (McNab, 1971; Bachop & Hilgendorf, 1981). The rodlike ligaments are usually tougher (firmer) than the flat type. Wang and colleagues (1999) found that corporotransverse ligaments have the histologic composition of other ligaments of the spine. In addition, calcification often is found in corporotransverse ligaments. Therefore the corporotransverse ligaments are sturdy ligamentous bands that can calcify (Wang et al., 1999).

Like TFLs, the corporotransverse ligaments can be seen on CT (Church & Buehler, 1991) and MRI (Nowicki & Haughton, 1992b; Cramer et al., 2002), but not on standard x-rays (Winterstein & Bachop, 1990). Figure 2-24 shows a corporotransverse ligament at the L5-S1 level of a cadaveric spine. Figure 2-25 shows two MRIs of the same cadaveric spine. The TFL is shown on the MRI of Figure 2-25, panel B.

As with TFLs, the corporotransverse ligaments are also thought to be clinically significant. They may have a constricting effect on the anterior primary division (ventral ramus) because that nerve courses under the ligamentous band (McNab, 1971; Bachop & Janse, 1983). That is, in patients with sciatica, when the leg is raised, the anterior primary division could be stretched across the ligament, possibly mimicking the thigh and leg pain of a disc protrusion. In addition, Breig and Troup (1979) and Rydevik and colleagues (1984) have published information about the increased sensitivity of inflamed nerve roots. Factors such as facet arthrosis, disc protrusion, and ligamentum flavum hypertrophy could conceivably increase intraforaminal pressure. The presence of a corporotransverse ligament could further increase this pressure and possibly cause a subclinical problem to become clinical.

Other accessory ligaments of the spine that might impinge on nerves and blood vessels have been described by Bogduk (1981) and Nathan and colleagues (1982).

Magnetic Resonance Imaging

MRI is an important component of spinal imaging. MRI shows soft tissue especially well and represents a quantum leap in the evaluation of patients with disc disease (Woodruff, 1988). It has been found to be more sensitive than contrast-enhanced CT in demonstrating disc degeneration (Schnebel et al., 1989), and is currently the imaging modality of choice in the evaluation of lumbar disc protrusion and extrusion (Forristall, Marsh, & Pay, 1988; Jackson et al., 1989). MRI can also detect some tears of the AF (Herzog, 1996). However, MRI alone is not enough to determine the cause of back pain (Borenstein et al., 2001), and correlations with patient history, physical examination findings, and (if necessary) findings of laboratory and other diagnostic procedures (e.g., electromyography) are essential to establish the most likely cause of back pain.

MRI also can detect disruption of the posterior longitudinal ligament secondary to extrusion of the NP. MRI not only allows for visualization of the discs, cerebrospinal fluid, spinal cord, and the perimeter of the spinal canal, but also allows visualization of these structures in several planes without the use of intravenous contrast media. For these reasons MRI is currently the method of choice for detecting disorders of the spinal canal and spinal cord (Woodruff, 1988) and can detect spinal cord injury without radiographic abnormalities (SCIWORA syndrome) (Kasimatis et al., 2008). Edema of bone marrow, spinal cord tumors, syringomyelia, extramedullary tumors (e.g., meningiomas), early detection of metastatic disease and primary malignancies of the vertebrae, and spina bifida (dysraphism) are all evaluated exceptionally well with this technology (Alexander, 1988; Woodruff, 1988; An et al., 1995; Buckwalter & Brandser, 1997). MRI also has been found to be effective in the evaluation of failed back surgery syndrome by differentiating fibrotic scar formation secondary to spinal surgery from disc herniation (Frocrain et al., 1989; Kricun, Kricun, & Danlinka, 1990) and is becoming the most important modality for all imaging of the postoperative spine (Djukic et al., 1990). Discitis also can be evaluated with MRI (Woodruff, 1988). MRI and conventional films are considered adequate for the preneurosurgical evaluation of cervical radiculopathy and myelopathy, with CT myelography being the follow-up procedure of choice (Brown et al., 1988).

A discrete area of high signal, known as a “high-intensity zone,” has been identified as a marker for painful tears of the outer AF of the IVD, especially tears related to internal disc disruption (Aprill & Bogduk, 1992; Schellhas et al., 1996a,b). (See Chapter 11 for a discussion of internal disc disruption.) Perhaps a related finding is that the high-intensity zone also has been associated with decreased stiffness of the IVD, especially in axial rotation (Schmidt et al., 1998). Saifuddin, McSweeney, and Lehovsky (2003) found that the high-intensity zone became more apparent when MRI scans were taken during axial loading of the spine (loads causing compression of the IVDs; e.g., standing). However, other research questions the clinical relevance of the high-intensity zone (Narvani, Tsiridis, & Wilson, 2003).

MRI continues to be a rapidly developing field, and the many technical advances should continue to improve its clinical utility. One such advance is the ability to decrease cerebrospinal fluid (CSF) flow artifact. This development results in better visualization of the spinal cord and the cord–CSF interface. Other advances are related to an increased variety of new imaging protocols used by radiologists. The imaging protocols of gradient-echo imaging (e.g., GRASS, FLASH, FISP, MPGR) allow for greater contrast between anatomic structures while decreasing scan time. Such gradient-echo techniques are the procedures of choice in patients with suspected cervical radiculopathy (Kricun, Kricun, & Danlinka, 1990), giving information of greater or equal value to that obtained from myelography or CT myelography (Hedberg, Dayer, & Flom, 1988). In addition, diffusion-weighted MR imaging allows for visualization of the nerve roots, spinal nerve, formation of the plexuses, and proximal aspects of peripheral nerves (Tsuchiya et al., 2007).

Two of the primary properties of MR images are related to the various responses of different tissues to the radiofrequency applied during the MRI evaluation. These two characteristics are known as T1 and T2. Various MRI protocols can highlight either of these characteristics and thereby selectively enhance different tissues. T1-weighted images are better for depicting anatomic detail. Adipose tissue has a high signal (is bright) on T1-weighted images, and these images are particularly useful in the evaluation of the spinal cord, bone marrow of vertebrae, IVDs, osteophytes, and ligaments (Woodruff, 1988; Kricun, Kricun, & Danlinka, 1990).

As a result of the increased acquisition time of the second echo, the resolution of T2-weighted images is not as good as that of T1-weighted images. However, T2-weighted images allow for better visualization of fluid and edema, and often reveal subtle, significant spinal cord pathology. T2-weighted images are also the most sensitive at showing a decreased signal intensity resulting from degeneration and desiccation of the disc (Woodruff, 1988; Herzog, 1996), and a decreased signal of the IVD on T2-weighted images may either precede or follow histologic evidence of IVD degeneration (Herzog, 1996). Because CSF has a very high signal on T2-weighted images, these images also are valuable in evaluating the amount of narrowing of the subarachnoid space in cases of spinal stenosis. T2-weighted images also create a “myelographic effect” related to the bright appearance of CSF on these films. This myelographic effect can help with the detection and characterization of subtle disc bulges and protrusions. T2-weighted images also are useful in the detection of multiple myeloma of the vertebral column (Avrahami, Tadmor, & Kaplinsky, 1993). Even with all of these advantages, in general, T1-weighted images are more valuable than T2-weighted images in the evaluation of the majority of spinal disorders (Moffit et al., 1988).

The contrast medium of gadolinium-diethylenetriaminepentaacetic acid (Gd-DTPA) can be used in conjunction with MRI and has been found to be safe and effective in increasing the contrast of certain pathologic conditions. Differentiation of scar formation (epidural fibrosis) from disc herniation in failed back surgery syndrome (recurrent postoperative sciatica) is improved with the use of Gd-DTPA (Hueftle et al., 1988). Gd-DTPA also may be useful in depicting disc protrusions and extrusions surrounded by scar tissue and free disc fragments. It also can be useful in identifying acute healing compression fractures and differentiating whether a compression fracture of the vertebral body is the result of a benign or malignant process. Gd-DTPA also is useful in evaluating patients with intradural tumors, but it is less useful in evaluating tumors external to the dura mater.

Magnetic resonance angiography (MRA) allows for imaging of arterial vasculature and is being used extensively in spinal imaging to assess the vertebrobasilar arterial system. In addition, Mordasini and colleagues (2012) used a high-field strength magnet (3 tesla [T]) to image the great radiculomedullary artery (of Adamkiewicz) in preparation for thoracoabdominal aortic aneurysm repair surgery. Advances in high-field strength MRA may allow for even more detailed assessment of spinal vasculature in the future.

Advances in MRI technology include developments in the hardware of the MRI unit, such as coil configurations (Woodruff, 1988). These changes allow large areas of the spine to be viewed at once, which is particularly useful in the evaluation of metastatic disease and syringomyelia. Other advances include three-dimensional reconstruction of spinal images with a video display that allows images to be rotated 360 degrees for viewing. Axial loaded (standing) MRI of the lumbar spine, which images the spine in a more physiologic state, may help to more accurately assess the dimensions of the spinal canal, when compared with routinely performed supine examinations with the hips and knees flexed (Saifuddin, Blease, & McSweeney, 2003). In addition, functional MRI (fMRI) evaluation of the spinal cord, which maps MRI signal changes after a specific stimulus designed to change neural activity, may become fundamental in the future work-up of spinal cord injury (Stroman et al., 2002).

Research is also being done involving morphometry of the spine by means of MRI (Cramer et al., 2003). Morphometry means the measurement of an organism or its parts. The digital images available from MRI (and CT) scans may be used to accurately quantify certain anatomic structures of the spine. This is the first time many such measurements can be made in living patients. Such measurements allow for an increased ability to study the structures influenced by a variety of therapeutic procedures (Cramer et al., 2002, 2003, 2012).

Diffusion and perfusion MRI allows information to be gained about the structure and function of tissue at a microscopic level. These procedures have an increasingly prominent clinical role, especially in neurovascular imaging (Valentini et al., 2003). Finally, extraordinary opportunities for advances in the clinical applications of MRI exist as imaging at the molecular level becomes possible (Rollo, 2003). Advances in spinal research will undoubtedly make use of such important applications as the imaging of the chemical mediators of back pain. (See Chapter 11 for a discussion of these chemical mediators.)

Computed Tomography

Conventional CT remains effective in the evaluation of many conditions. It is especially valuable when accurate depiction of osseous tissues is important. Pathologic conditions including spinal stenosis, bone tumors, congenital anomalies, degenerative changes, trauma, spondylolysis, and spondylolisthesis can all be accurately evaluated by CT (Wang, Wesolowski, & Farah, 1988). Images reformatted to the sagittal or coronal plane may help with the evaluation of complicated bone anatomy. Arachnoiditis ossificans, a rare ossification of the arachnoid mater as a consequence of trauma, hemorrhage, previous myelogram, or spinal anesthesia, can be better visualized on CT than MRI (Wang, Wesolowski, & Farah, 1988). Criteria for the diagnosis of intraspinal hemangiomas by means of CT also have been established (Salamon & Freilich, 1988). Although lumbar disc disease can be evaluated adequately by means of CT, “beam hardening” artifacts lead to inadequate evaluation of disc disease in the thoracic and, to a lesser extent, the lower cervical canal (Woodruff, 1988).

CT is especially valuable in the evaluation of lumbar spinal stenosis, although artifacts sometimes make the evaluation of cervical and thoracic spinal stenosis difficult (Wang, Wesolowski, & Farah, 1988). The evaluation of facet joint disease and calcification of the ligamentum flavum is currently more efficient with CT than with MRI (Wang, Wesolowski, & Farah, 1988).

CT is also quite effective in the evaluation of bone destruction and new bone formation secondary to neoplasia. It demonstrates the vertebral bony cortex and vertebral bodies well (Buckwalter & Brandser, 1997). In addition, CT is excellent in allowing the identification of osseous changes subsequent to spinal trauma. For example, CT is particularly good at identifying the presence of bony fragments in the spinal canal after posterior arch fracture (Wang, Wesolowski, & Farah, 1988).

Intrathecal contrast-enhanced CT (CT myelography) results in a more complete depiction of the spinal canal, the IVD relative to the spinal canal, and the perimeter of the spinal cord (Woodruff, 1988). Contrast-enhanced CT and MRI are comparable in their abilities to demonstrate spinal stenosis (Schnebel et al., 1989). CT and MRI have a complementary role in the evaluation of such disorders as spinal canal stenosis, congenital disorders, facet disorders, and acute spinal injury (Wang, Wesolowski, & Farah, 1988; Tracy, Wright, & Hanigan, 1989). Extraforaminal (far lateral and anterior) disc herniations can also be readily identified on both CT and MRI if scans include L2 through S1, and if the IVF and paravertebral spaces are closely examined (Osborn et al., 1988).

Other Imaging Modalities

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