Two vertebral bodies are linked by an inter- vertebral disc. Typically the intervertebral disc consists of an outer annulus fibrosus and inner nucleus pulposus, although it is difficult to distinguish the outermost aspect of the nucleus pulposus from the innermost aspect of the annulus fibrosus. From a histologic perspective, the cartilagi- nous end plate can also be considered part of the intervertebral disc because of the intimate relationship between the collagen framework of the vertebral end plate and the annulus fibrosus (figure 5.5).
The shape of the lumbar intervertebral discs is roughly elliptic. Generally, the anterior height of the last two lumbar discs is nearly twice their posterior height. This 2:1 relationship, resulting in a wedge shape for the disc, is nearly always present in the L5–S1 disc. Although the ante- rior inclination of the sacrum is a contributing factor, the wedge shape of the disc contributes greatly to the development of lumbar lordosis.
Approximately 12 to 20 concentric rings of fibrocartilage, referred to as lamel- lae, form the annulus fibrosus (Taylor 1990) (see figure 5.5). The term ring is a bit of a misnomer, however, because not every lamella completely encircles the complete disc (Tsuji et al. 1993). Each layer of fibers is obliquely offset from the next layer, with the fibers forming an approximately 30° angle with the horizontal (White and Panjabi 1978). In the anterior annulus, the lamellae are thick and fairly distinguishable. In contrast, the posterior annulus is thinner, and the lamellae tend to be compressed and merged together. The proportion of collagen increases from the inner to outer annulus, and there is also a variation of the type of col- lagen within the intervertebral disc. Type I collagen, the type that is structured to counter tensile forces, is found mainly in the annulus. Type II collagen, which counters compressive forces, is in the nucleus (Buckwalter, Einhorn, and Simon 2000; Buckwalter 1995). In degenerating intervertebral discs, type II collagen is
E3723/Ellenbecker/Fig5.05/328866/JasonMc/R2-alw Annulus
fibrosus
Nucleus pulposus
Figure 5.5 The components of the intervertebral disc.
replaced with the more-fibrous type I collagen. Nicotine leads not only to a reduc- tion in viable cell numbers in the disc but also to a change in the type of collagen synthesis from type II to type I in the nucleus pulposus (Akmal et al. 2004).
The nucleus pulposus is primarily a hydrated gel of proteoglycans consisting of sulfated glycosaminoglycans bound to a protein core, with a small number of cells of collagen present. Water typically accounts for more than 80% of the weight of the nucleus. Because of the presence of negatively charged sulfate groups, water is attracted to the proteoglycan macromolecule, hence the term hydrophilic. One of the most striking age-related changes of the nucleus pulposus is loss of water. This loss is generally attributed to changes in the glycosaminoglycan type and content of the nucleus pulposus.
The major components of the intervertebral disc are collagen fibers, proteoglycans, and water. The histochemistry changes from the periphery to the center. A more densely arranged collagen framework is evident in the peripheral annulus, whereas a greater concentration of proteoglycans and less collagen are found in the central nucleus pulposus. Because the water content depends on the water-binding proper- ties of the proteoglycans, there is a greater amount of water in the nucleus pulpo- sus. It is beyond the scope of this text to discuss in detail the types and behavioral characteristics of the specific types of collagen and proteoglycans, and the reader is encouraged to review sources providing this information (Buckwalter et al. 1985; Bushell et al. 1977; McDevitt 1988). As the intervertebral disc ages, it is increasingly difficult to distinguish the nucleus pulposus from the annulus fibrosus.
The cartilaginous end plates consist of hyaline cartilage over the subchondral bone plates of the vertebral body. The cartilage is approximately 0.6 mm thick and is generally thicker peripherally and thinner toward the center (Roberts, Menage, and Urban 1989). The vertebral end plates are connected directly to the lamellae that form the inner one third of the annulus fibrosus. Thus the end plates completely cover the nucleus pulposus but cover only the inner portion of the annulus fibrosus. From a sport and exercise perpective, it is important to note that the cartilaginous end plate, not the intervertebral disc, is the “weak link” if compression loads are too great for the vertebral segment.
The cartilage resembles the intervertebral disc in that it has a higher concentra- tion of proteoglycans and water than collagen toward the center and more collagen than proteoglycans peripherally. Multiple perforations in the cartilage end plate permit contact with the vascular buds from the marrow of the cancellous bone of the vertebral body. This vascular communication plays an important role in the nutrition of the intervertebral disc. Because of these vascular contacts and the thinness of the cartilage, diffusion of nutrients such as oxygen and glucose is facilitated. Since the intervertebral discs are essentially avascular structures, this source of nutrition and the blood vessels that enter the periphery of the annulus fibrosus are the primary sources of nutrition for the disc. The central aspect is the most metabolically active region of the cartilaginous end plate, and the vascular channels in this region are the most numerous and complex (Fagan, Moore, and Vernon Roberts 2003).
Because collagen fibers reinforce a tissue, provided that the applied stress is in the axial direction of the fiber, it should be apparent that the orientation of the collagen fibers of the annulus fibrosus allows restraint to nearly all motions of the spine. For example, twisting (torsion), anterior shear forces that accompany weight bearing, or forward bending of the spine results in tension being applied to specific regions of the annulus fibrosus. The increase in tension to the annular
170 Effective Functional Progressions in Sport Rehabilitation
fibers minimizes motions between the two adjacent vertebrae. Any motion of the lumbar spine places at least a portion of the annulus fibrosus under tension, and thus the annulus serves as the key ligamentous restraint of the vertebral column.
The annulus fibrosus and the nucleus pulposus work in concert to resist com- pressive loads. The proteoglycan–water component of the nucleus provides a hydrostatic mechanism that helps dis- tribute compressive forces tangentially toward the annulus. This increases the tension of the annular rings, further stabilizing the intervertebral disc (figure 5.6). Increased compressive loads to the disc raise the pressure within the nucleus, which further increases tension of the col- lagen fibers of the inner annular lamellae. Attraction and binding of water by the proteoglycans result in a swelling pressure that ultimately allows the intervertebral discs to support compressive loads.
It is not simply trunk forces and ground forces that increase compression between adjacent vertebrae. Note also that contrac- tion of the muscles of the spine, in particu- lar the erector spinae group, multifidus, and psoas major, also increases vertebral compression as a result of the muscle fiber direction. In fact, intervertebral com- pressive forces that ultimately increase intradiscal pressure are dependent on, in probable decreasing order of influence, muscle contraction, preload by the liga- ments spanning the vertebral segment, and superincumbent weight. The synergistic action of the annulus and nucleus stabilizes the disc complex and distributes forces properly to restrict movement, maintain the size and shape of the neuroforamen, and ensure that the apophyseal joint maintains an invulnerable loading position during weight bearing.
The lumbar intervertebral disc has meager innervation. This is concentrated in the periannular connective tissue and the central end plate. Although receptor threshold appears closely related to nociceptive function, innervation density sug- gests consideration of mechanoreceptor contributions via this nerve supply as well (Fagan, Moore, and Vernon Roberts 2003). Noxious stimulation of the intervertebral disc appears to result in low back pain and distally referred symptoms, including symptoms below the knee, with the distal extent of the referral being dependent on the intensity of the stimulus (O’Neil et al. 2002). The innervation density of the periannular connective tissue and the central end plate is suggestive of propriocep- tive as well as nociceptive functions (Fagan, Moore, and Vernon Roberts 2003).
Joints
The joints that are especially important to detail are the zygapophyseal joints of the lumbar spine, the sacroiliac joints of the pelvis, and the pubic symphysis. These joints guide motion of the lumbopelvic region while simultaneously attenuating
E3723/Ellenbecker/Fig5.06/328869JasonMc/R1
Figure 5.6 An increase in pressure of the nucleus pulposus results in the force being applied tangentially, thus increasing the tension of the annular rings. The increase in tension of the annulus results in enhanced stability between the adjacent bony segments.
ground forces. Disruption of the delicate interplay of force attenuation, load bear- ing, and motion direction is one of the fundamental reasons for joint injuries of the low back in sport and functional activities. There is a very precise amount and specific direction of movement available at these joints, and this precision influences the exercise prescription. Motions or loading conditions that exceed the limits or that do not precisely follow the joint planes result in injury or accelerate the breakdown of the articulating elements. Low back injuries of this sort result in major limitations in activity and can often preclude a return to activity.