The spinal column is made up of four major vertebral regions: cervical (neck), thoracic (chest), lumbar (low back) and sacral (pelvic). Dogs have seven cervical, thirteen thoracic, seven lumbar and three sacral vertebrae. There are also variable numbers of coccygeal or tail vertebrae. Intervertebral disks are located between the vertebral bodies starting at the second and third cervical vertebrae (C2-3) and extending to the seventh lumbar and first sacral vertebrae (L7-S1). The three sacral vertebrae are fused and therefore do not have disks. Intervertebral disks are present between the coccygeal vertebra as well, but are of little clinical significance.
The vertebrae come together at three main points: the intervertebral disk between the end plates of the vertebral bodies, and two articular facets. The ends of the vertebral bodies are covered by thin cartilaginous plates. The fibrous portion of the disk connects the vertebral bodies by attaching to these cartilaginous plates as well as directly to the bone itself. In addition, two ligaments help connect the vertebral bodies, the dorsal and ventral longitudinal ligaments. The dorsal (top) longitudinal ligament runs on the floor of the spinal canal above the disk while the ventral (bottom) longitudinal ligament runs on the bottom of the vertebral bodies. Processes of bone (articular processes) extending from two adjoining vertebrae come together to make articular facets. These are located dorsolaterally (10 and 2 o'clock positions on a clock face) on each side of the vertebral column and are true diarthrodial joints which include a joint capsule, articular cartilage, and joint synovial fluid. In addition to these three main contact points, the vertebral column is also held together by numerous muscles and their tendinous attachments, and other specialized ligaments.
Intervertebral disks are composed of two major anatomic zones: the annulus fibrosus and the nucleus pulposus. The annulus is composed of laminated fibrous tissue wrapped around the gelatinous nucleus pulposus. Individual annular fibers radiate outwardly at varying angles to accommodate all the angles of force that can be applied to the disk. The outer or peripheral layers of the annulus are composed of type I collagen. In comparison, the inner annular layers lying next the nucleus pulposus are referred to as the transitional zone and are composed of fibrocartilaginous material. For the purpose of localization, the annulus is often subdivided into the dorsal (top or 12 o'clock position) annulus lying directly below the spinal cord and above the nucleus pulposus, the two lateral (side or 9 and 3 o'clock positions) annuli on the sides of the nucleus, and the ventral (bottom or 6 o'clock position) annulus below the nucleus. The dorsal annulus of the disk is approximately 1/3 the thickness of the ventral annulus; therefore the nucleus pulposus lies in the middle to the dorsal third of the disk. As a result, when disk herniation occurs, the tendency is for nuclear material to "escape" dorsally or upward into the spinal canal. The nucleus is composed of a lattice framework of collagen embedded in a highly hydrated gelatinous mass containing cells called chondrocytes and the biochemical glycosaminoglycans hyaluronic acid, chondroitin sulfate and keratin sulfate. The relative proportions for these biochemicals become important in determining those breeds of dogs predisposed to intervertebral disk herniation.
Nutrients reach the peripheral layer of the annulus fibrosus through small blood vessels adjacent to the annulus and small canals that perforate the vertebral body end plates providing direct access to the underlying vascular marrow. The transitional zone of the annulus and the nucleus pulposus receive nutrients by diffusion from the periphery and adjacent vertebral bodies. Normal body movements facilitate the removal of cellular wastes and diffusion of metabolites. Overall blood supply (vascularity) in the disk appears to decline from maturity to old age with the blood vessels remaining intact in only the outermost layers of the annulus fibrosus.
The outer layers of the annulus fibrosus and the dorsal longitudinal ligament contain sensory nerve fibers as opposed to the nucleus pulposus and transitional zone fibers which have none. So called diskogenic pain arises when there is stretching and tearing damage of the outer laminated layers of the annulus fibrosus.
Each vertebra has a central cavity which, when lined up sequentially, forms the vertebral or spinal canal in which the spinal cord lies. The size of this canal and the spinal cord vary in relation to each other at different levels of the spinal column. In the cervical and low lumbar areas the spinal cord that occupies it, thus allowing a great deal of space (extradural space) around the spinal cord. In comparison, the thoracolumbar spinal canal is almost entirely filled by the spinal cord; consequently there is very little extradural space at this level. These spinal canal to spinal cord relationships help explain why disk herniations in the thoracolumbar area are often far more debilitating than cervical disk herniations.
Where vertebrae come together, a "window' is formed on each side of the spinal column where the spinal nerves and the blood vessels exit and enter the spinal canal. These "windows" are called intervertebral foramina. One nerve exits through each intervertebral foramen. Consequently, the spinal cord is divided into numbered segments which correspond to the number of the vertebra where the paired nerves exit. Although there are only seven (7) cervical vertebrae, there are eight (8) pairs of cervical nerves because the first pair exits in front of the first cervical vertebra and the second pair exits at the junction between the first and second cervical vertebrae. For the remaining vertebrae, there is one pair of nerves per vertebra (i.e. 13 thoracic, 7 lumbar, 3 sacral and corresponding numbers of coccygeal). In the cervical and thoracic areas of dogs, the spinal cord segments and nerves lie approximately inside the correspondingly numbered vertebral segments. However, as the spinal cord enters the lumbar segments, the spinal cord segments start to lie ever-increasingly forward of their correspondingly numbered vertebrae so that nerves exit the spinal cord and run caudally (toward the tail) inside the spinal canal before exiting.
The spinal cord is covered by protective membranes collectively called the meninges. The innermost layer, the pia, contains the highly vascular network that delivers nutrients and removes wastes from the nervous system. Surrounding it is the arachnoid layer which, with the pia, forms the subarachnoid space where cerebrospinal fluid flows. The outermost and strongest layer is the dura mater to which the arachnoid is closely associated. The motor and sensory nerve fibers of each cord segment join inside the meninges to become the spinal nerves before exiting this protective sack as peripheral nerves. (See figure below) The meninges are innervated by numerous sensory nerve fibers called meningeal nerves. When a disk herniates into the spinal canal, the meningeal nerves become compressed and inflamed causing the animal a great deal of pain. In addition, the nerve roots themselves are often compressed, also resulting in a great deal of pain for the affected animal.
Purpose of the Spinal Column and Intervertebral Disks
The spinal column provides a rigid support for the attachment of muscles and bones. It also protects the delicate nervous system while still allowing for spinal column movement. The intervertebral disks form elastic cushions between the vertebrae which allow movement, minimize trauma and shock, and help connect the spinal column.
The intervertebral disk is designed well to dispose of compressive forces, but not as well to combat twisting or bending forces applied to the axis ("straight" line down the length) of the spine. When a disk is compressed, both the nucleus pulposus and the annulus fibrosus share in bearing the load, neither being able to accomplish the work effectively alone. The nucleus is a relatively incompressible mass which, when compressive forces are applied, tries to deform by spreading radially (away from the axis; approximately at right angles to the axis). The annulus fibrosus with its unique layers of fibers is present to prevent that expansion. The annulus effectively "braces" from within thus preventing buckling. Consequently the vertebral end plates are prevented from "contacting' and the compressive force is ultimately transmitted to the vertebral bodies. The disk's ability to combat compressive forces is so efficient that failure is usually by fracture of a vertebral endplate rather than nuclear herniation or annular tearing.
In contrast, twisting forces must be borne by the annular fibers alone with the nucleus serving only as a"ball bearing" in the activity. When the spinal column is rotated around its axis, only half of the annular fibers are appropriately oriented to resist the rotation. The other half are shortened or become slack and therefore can not help prevent the movement. Consequently, far more possibility for a tearing injury of the annular fibers, with associated inflammatory response, exists and usually occurs. With time, repetitive twisting forces may significantly weaken the annulus fibrosus and prevent its ability to resist compressive forces as efficiently.
Bending forces do not cause as much damage to the annulus fibrosus as twisting forces because the joint capsules of the articular facets, epaxial muscles and spinal ligaments help limit the degree of bend that any one vertebral interspace allows. However, if these supportive structures are compromised by injury or disease, the annular fibers may become separated from the sites of attachment leading to a weakened intervertebral disk space.
Types of Disks
Disks can be divided into two histochemical types: 1) chondrodystrophoid and 2) nonchondrodystrophoid or fibroid. The word "chondrodystrophoid" literally means faulty development or nutrition of cartilage. In humans, chondrodystrophoism is recognized physically (phenotypically) as dwarfism, where individuals are smaller than normal and whose parts (especially limbs) are disproportionate. Certain breeds of dogs, such as dachshunds, show their chondrodystrophism by having disproportionately short and angulated limbs. However, phenotypic characteristics alone can not be used to identify chondrodystrophoid dogs. Other breeds, such as miniature poodles and beagles, have been histochemically identified to have chondrodystrophoid disks and yet do not appear outwardly to be chondrodystrophoid.
When comparing the disks of nine month old dogs, chondrodystrophoid disks characteristically have a larger ratio of transitional versus peripheral zone in the annulus fibrosus. Also the cells of the transitional zone lack clear orientation as is typical in nonchondrodystrophoid disks. The nucleus pulposus in chondrodystrophoid is almost completely composed of dense fibrocartilage which appears to have completed the chondrofication process. There are only isolated "islands" of notochordal cell remnants seen. In contrast the intracellular matrix of the nonchondrodystrophoid disk is loose and fibrillar and contains notochordal cells only.
The amount of pressure that builds up inside the disk when forces are applied depends on two factors: 1) the water binding properties of the nucleus (more water equals more elasticity) and 2) the degree of resistance and elasticity of the annulus and surrounding structures. These factors are highly dependent on the histochemical makeup of the disk and the changes it undergoes during aging.
Changes In The Disk That Predispose It To Herniation
Biochemical differences between chondrodystrophoid and nonchondrodystrophoid disks are apparent shortly after birth and explain the differences in the types of degeneration that occur. The degeneration that occurs in chondrodystrophoid disks is called chondroid metaplasia because the nucleus pulposus is gradually replaced with cartilage. Degeneration takes place rapidly and begins as early as 6 months of age starting at the periphery of the nucleus pulposus and progressing centrally. A dramatic and rapid increase in collagen content, as much as 30-40% by dry weight, is seen between 6 and 12 months of age. In addition, total glucosaminoglycan content will be 30 to 50% lower than age matched nonchondrodystrophoid dogs within the first 3 years resulting in a great loss of water content in the nucleus. When this happens, the nucleus loses its elasticity and no longer acts as an efficient shock absorber. Eventually the hyaline cartilage which forms calcifies, leading to almost complete lose of elasticity intervertebral the nucleus pulposus. The overall result is that of placing more of the "workload" on the annulus fibrosus while it is simultaneously undergoing degeneration. Disruption of the annulus fibrosus eventually occurs, especially at its weakest point, the thinner dorsal area lying just below the spinal canal. This allows nuclear material to escape, usually dorsally into the spinal canal or dorsolaterally to impinge on the nerve roots exiting the intervertebral foramina.
In comparison, nonchondrodystrophoid disks degenerate by fibroid metaplasia with the process becoming clinically significant at 8 to 10 years of age. Fibroid degeneration involves a gradual process of dehydration, and therefore loss of elasticity, of the nucleus pulposus with the incorporation of increasing amounts of collagen and polysaccharides (chondroitin sulfate and keratin sulfate). This causes a gradual diminishing of the border between the annulus fibrosus and the nucleus pulposus, and thus a weakening of the disk's overall biomechanical abilities. Partial rupture of the annulus fibrosus may result allowing the nucleus pulposus to bulge into the annulus and possibly the spinal canal.
