Figure. The adult vertebral column and typical vertebrae in each region, lateral views.
There are at least 24 intervertebral discs interposed between the vertebral bodies: six in the cervical, twelve in the thoracic and five in the lumbar region, with one between the sacrum and coccyx. (Additional discs may be present between fused sacral segments.) The discs account for approximately one-quarter of the total length of the vertebral column, and are primarily responsible for the presence of the various curvatures. On descending the vertebral column, the discs increase in thickness, being thinnest in the upper cervical region and thickest in the lower lumbar. In the upper thoracic region, however, the discs appear to narrow slightly. In the cervical region the disc is about two-fifths the height of the vertebrae, being approx-imately 5 mm thick. In the thoracic region the discs average 7 mm in thickness, so that they are one-quarter of the height of the vertebral bodies. The discs in the lumbar regions are at least 10 mm thick, equivalent to one-third of the height of the lumbar vertebral bodies. The relative height of the disc to the vertebral bodies is an important factor in determining the mobility of the vertebral column in each of the regions. Individual discs are not of uniform thickness; they are slightly wedge-shaped in conformity with the curvature of the vertebral column in the region of the disc. The curvatures in the cervical and lumbar regions are primarily due to the greater anterior thickness of the discs in these regions.
The overall shape of the discs also varies from one region to another, being similar to the shapes of the adjacent vertebral bodies. Consequently, in the cervical region they tend to be oval, in the thoracic region almost heart-shaped and in the lumbar region kidney-shaped.
It is of considerable practical importance to remember that the intervertebral disc forms one of the anterior boundaries of the intervertebral foramen, and as the spinal nerves pass through the foramina they lie directly behind the corresponding discs. In addition the discs also form part of the anterior wall of the vertebral canal. Consequently, any posterior bulging of the disc may compress the spinal cord as well as the individual spinal nerves.
Figure. (A) Components of the intervertebral disc, (B) Fluid content of the nucleus pulposus and changes with trauma. (Adapted from Hendry N (1958) The hydration of the nucleus pulposus and its relation to the intervertebral disc derangement Journal of Bone and Joint Surgery, 40B, 132-144.)
Each disc is structurally characterized by three integrated tissues: the central nucleus pulposus, the surrounding annulus filvvsus and the limiting cartilage end plates (Fig. A). It is anchored to the vertebral body by the annulus fibrosus fibres and the cartilage end plate.
The soft, highly hydrophilic substance contained within the centre of the disc. There appears to be no clear division between the nucleus pulposus and the surrounding annulus fibrosus, the main difference being in tlie density of the fibres contained, with the nucleus having large extrafibrillar spaces containing glycos-aminoglycans enabling it to retain fluid. The classical idea, therefore, of a distinct division between the two regions is not true. Furthermore, the concept of the nucleus being round or oval has not been supported by discography, which has shown it to be more rectangular in infants and young children, and anything from oval to multilobed in adults. (Discography is a radiographic technique which allows visualization of the disc in the living subject; clinically it enables the health of the disc to be assessed.) The region between the nucleus and the annulus fibrosus is an area of maximum metabolic activity. It is also sensitive to physical forces as well as to chemical and hormonal regulation of growth processes. Consequently, it may be considered to represent the growth plate of the nucleus pulposus, similar to epiphyseal growth plates, since the nucleus can only increase in size and remodel itself at the expense of the inner part of the annulus fibrosus. The annulus on the other hand increases in horizontal diameter by the addition of new lamellae at the periphery.
Figure. The relative position of the nucleus pulposus within the intervertebral disc, and its relation to the axis about which movements occur.
The position of the nucleus pulposus within the disc varies regionally, being more centrally located in cervical and thoracic discs and posteriorly located in lumbar discs. Nucleus position is related to certain aspects of function.
The nucleus pulposus consists of a three-dimensional lattice of collagen fibres in which is enmeshed a proteoglycan gel, which is responsible for the hydrophilic nature of the nucleus. Patchy loss and disappearance of this gel occurs with ageing, which lowers the water content until in advanced degeneration the collagen may be devoid of proteoglycan material (Fig. 4.29B). This is the major change underlying dehydration of the nucleus in later life. In early life, a water content of 80-88% is usual. However, from about the fourth decade onwards this decreases to 70%. These changes in the proteoglycans of the nucleus, both in terms of their loss and composition, change the mechanical behaviour of the disc.
Studies suggest that the nucleus pulposus represents the functional centre of the disc, and that systemic changes within it may be important as a primary cause of pathological change within the disc, and consequently of all pathological change within the intervertebral space. There is, however, the view that in disc degeneration the first morphological change to be observed is the separation of part of the cartilage end plate from the adjacent vertebral body.
A series of annular bands whose geometry varies as a function of vertebral level and intradiscal region. Each annular band has a roughly parallel course, with the directional arrangement of fibres alternating in adjacent bands (Fig. A), with the obliquity of these bands being greatest in the innermost layer of any given disc. The number of lamellae, as well as their size, thickness and obliquity of arrangement, shows large variations for any given band within different parts of the same disc, for any particular ver-tebral level, and from individual to individual. Nevertheless, the average number of lamellae is 20 and in general their thickness varies from 200 to 400 |im, increasing from the inside out. Within each lamella the fibrils (0.1-0.2 mm) are uniformly arranged, but their orientation varies considerably from one lamella to another.
Each lamella is composed of obliquely arranged bundles of fibrils, varying in size between 10 and 50 mm. Except for thin fibrils, there is little interconnection between adjacent lamellar sheets; consequently there will be only limited restriction to movement during compression and tension. The question arises as to whether orientation of the collagen bundles is predetermined or mechanically induced when movement occurs. There are considerable differences in fibril thickness and lamellar organization in the fetus. It is therefore likely that mechanical phenomena, particularly torsion, are responsible for the arrangement seen in adults.
The density of the fibrocartilaginous lamellae is a function of the annular region, being more closely packed anteriorly and posteriorly than laterally. The lamellar bands do not form complete rings but split intricately or merge to interlock with other bands. The posterolateral regions of the annulus appear to have marked irregularities and are much less orderly. With ageing, the annulus becomes weakest in these posterolateral regions, thereby predisposing to nucleus herniations.
Elastic fibres are present within both the annulus fibrosus and nucleus pulposus. In the annulus they are circularly, obliquely and vertically arranged, although they are not distributed throughout, but are restricted to the lamellae at the vertebral epiphysis and disc interface. Interlamel- lar elastic fibres branch and join, freely imparting a dynamic flexibility to the tissue, with obvious implications for function. The intralamellar elastic fibres penetrate the bony vertebrae as perforating fibres.
Figure. Type and ratio of collagen found within the intervertebral disc. (Adapted from Taylor TKF, Ghosh Pand Bushel GR (1981) The contribution of the intervertebral disc to scoliotic deformity. Clinical Orthopaedics and Related Research, 156, 79-90.)
Within the annulus the total collagen content is not constant, decreasing from the outer layers towards the nucleus. However, the proportion of type I to type II collagen (the principal collagen types within the disc) decreases from the outer layer of the annulus to the nucleus, and also varies from region to region. In other words, type I collagen predominates in the outermost regions of the annulus and type II the innermost; the nucleus pulposus contains type II only. Since type I collagen is typical of tendons and type II of articular cartilage, where large transient compressive forces are generated, the tensile strength of the annulus is probably provided by type I collagen, while the compressive component involves type II. With increasing age, the collagen content of the annulus increases from the inside outwards in the disc, and also downwards from cervical to lumbar regions. However, the proportion of type II collagen does not appear to change with age.
The attachment of the annulus fibrosus to the vertebrae is fairly complicated. The annulus fibres pass over the edges of the cartilage end plate and anchor themselves to and beyond the compact bony zone that forms the outside of the vertebral rim, as well as to the margins of the adjacent vertebral body and its periosteum, thereby forming stable connections between adjacent vertebral bodies. These perforating fibres become interwoven with fibrillar lamellae of the bony trabeculae. This fibrillar anchorage is already present at birth even though the vertebral rim is not ossified.
Cartilage end plate
Found on each surface of the vertebral body it represents the anatomical limit of the disc (Fig. A). It is approximately 1 mm thick at the periphery and decreases towards the centre. It can be considered to have three main functions: (i) it appears to protect the vertebral body from pressure atrophy; (ii) it confines the annulus fibrosus and nucleus pulposus within their anatomical boundaries, and (iii) it acts as a semipermeable membrane to facilitate fluid exchanges between the annulus, the nucleus and the vertebral body via osmotic action. Regarding this third function, however, studies suggest that only the central part of the end plate is permeable.
In the first few years of life, the end plates are loosely attached by a thin layer of calcified material to irregular, radiating, fanshaped ridges and furrows on the vertebral bodies. Later, a thin layer of calcified material on the end plate firmly adheres to the trabeculae of the porous surface of the vertebral body. It is thought that the end plate is in contact with the bone marrow, through which it receives its nutrients.
In the early part of life, numerous minute vascular channels (cartilage canals) penetrate deeply into the end plate from tlie vertebral side. However, these channels disappear with increasing age so that by the third decade they are largely obliterated. Following the third decade, retrogressive changes occur in the end plate: it begins to show signs of ossification and there is an increase in calcification. It becomes more brittle, with fimbriation becoming more evident, ranging from thinning to complete destruction of the central end plate zone.
Development of the intervertebral disc
Figure. Development of the intervertebral disc, frontal section: (A) at 4 weeks showing sclerotome cells around the notochord and (B) in the adult.
The vertebral column begins to develop in the embryonic mesoderm at about 4 weeks, with individual vertebrae developing under the combined inductive influence of the notochord and neural tube. Ablation of either the notochord or neural tube at an early stage results in failure of sclerotomal and myotomal segmentation. The segmental vessels of aortic origin pass between two sclerotomal zones, which then fuse to form the mesenchymal body of the vertebra.
The intervertebral disc therefore develops initially in an environment which contains few blood vessels and is surrounded by a perichondral layer, whose continuity foreshadows the longitudinal vertebral ligaments. The nerves come to lie close to the discs while the intersegmental arteries come to lie either side of the vertebral bodies.
In those regions where the notochord is surrounded by the developing vertebral body it degenerates and disappears. Between the vertebrae, however, the notochord expands as local aggregations of cells within a proteoglycan matrix, forming the gelatinous centre of the disc, the nucleus pulposus. The nucleus is later surrounded by the circularly arranged fibres of the annulus fibrosus, which are derived from the perichordal mesenchyme. The nucleus pulposus and annulus fibrosus constitute the embryonic intervertebral disc. Remnants of notochord may persist in any part of the axial skeleton and give rise to a chordoma. This slow-growing neoplasm occurs most frequently at the base of the skull and in the lumbosacral region.
Following the proliferation and later degeneration of the notochordal cells, there is a fibrocartilaginous invasion of the nucleus pulposus by the orginal mesenchymal intervertebral cells. This invasion occurs at about 6 months in utero.
Intervertebral discs lose their embryonic integrity with time, with structural changes occurring in the nucleus throughout adulthood. These normal processes are often considered to be signs of degeneration; they are merely stages in the natural evolution of connective tissue which is subjected to mechanical stress in the form of combined shear and compression forces. The growth of the intervertebral disc, together with the microscopic changes within it, has been correlated with changes associated with weight-bearing in the erect posture. This may be a similar mechanism to that associated with the formation of subcutaneous connective tissue bursae, e.g. housemaid’s knee, in which the alternate action of compression forces at right angles to the skin surface and tangential shear stresses induce thickening and delamination of the connective tissue.
Intervertebral discs are subjected to compression, torsion and shear. However, the shear stresses are constantly changing, being dependent on the instantaneous centre of rotation between adjacent vertebrae. This could explain the mechanical delamination of the central region of the disc at different vertebral levels. In other words, the appearance of an irregular cavity in the central region is mechanically induced.
The cartilage end plate also appears to follow this mechanical induction. It is thought to be derived not from the vertebra but from the undifferentiated cells which accumulate in early embryonic life, and develops as an organized structure under mechanical influences. The annular epiphysis of the vertebral body develops in the marginal part of this thin plate of hyaline cartilage, and could therefore be considered to be either part of the disc, or part of the vertebral body.