The intervertebral disc is a cartilaginous structure that resembles articular cartilage in its biochemistry, but morphologically it is clearly different. It shows degenerative and ageing changes earlier than does any other connective tissue in the body. It is believed to be important clinically because there is an association of disc degeneration with back pain. Current treatments are predominantly conservative or, less commonly, surgical; in many cases there is no clear diagnosis and therapy is considered inadequate. New developments, such as genetic and biological approaches, may allow better diagnosis and treatments in the future.

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Degeneration of the intervertebral disc

This review article describes anatomy, physiology, pathophysiology and treatment of intervertebral disc. The intervertebral discs lie between the vertebral bodies, linking them together. The components of the disc are nucleus pulposus, annulus fibrosus and cartilagenous end-plates. The blood supply to the disc is only to the cartilagenous end-plates. The nerve supply is basically through the sinovertebral nerve. Biochemically, the important constituents of the disc are collagen fibers, elastin fibers and aggrecan. As the disc ages, degeneration occurs, osmotic pressure is lost in the nucleus, dehydration occurs, and the disc loses its height. During these changes, nociceptive nuclear material tracks and leaks through the outer rim of the annulus. This is the main source of discogenic pain. While this is occurring, the degenerative disc, having lost its height, effects the structures close by, such as ligamentum flavum, facet joints, and the shape of the neural foramina. This is the main cause of spinal stenosis and radicular pain due to the disc degeneration in the aged populations. Diagnosis is done by a strict protocol and treatment options are described in this review. The rationale for new therapies are to substitute the biochemical constituents, or augment nucleus pulposus or regenerate cartilaginous end-plate or finally artificial disc implantation..

Intervertebral disc: anatomy-physiology-pathophysiology-treatment.

https://www.oarsijournal.com/article/S1063-4584(15)00877-8/pdf

Mechanics and biology in intervertebral disc degeneration: a vicious circle

The mechanical behavior of the entire anulus fibrosus is determined essentially by the tensile properties of its lamellae, their fiber orientations, and the regional variation of these quantities. Corresponding data are rare in the literature. The paper deals with an in vitro study of single lamellar anulus lamellae and aims to determine (i) their tensile response and regional variation, and (ii) the orientation of lamellar collagen fibers and their regional variation. Fresh human body-disc-body units (L1–L2, n=11) from cadavers were cut midsagittally producing two hemidisc units. One hemidisc was used for the preparation of single lamellar anulus specimens for tensile testing, while the other one was used for the investigation of the lamellar fiber orientation. Single lamellar anulus specimens with adjacent bone fragments were isolated from four anatomical regions: superficial and deep lamellae (3.9±0.21 mm, mean ± SD, apart from the outer boundary surface of the anulus fibrosus) at ventro-lateral and dorsal positions. The specimens underwent cyclic uniaxial tensile tests at three different strain rates in 0.15 mol/l NaCl solution at 37°C, whereby the lamellar fiber direction was aligned with the load axis. For the characterization of the tensile behavior three moduli were calculated: Elow (0–0.1 MPa), Emedium (0.1–0.5 MPa) and Ehigh (0.5–1 MPa). Additionally, specimens were tested with the load axis transverse to the fiber direction. From the second hemidisc fiber angles with respect to the horizontal plane were determined photogrammetrically from images taken at six circumferential positions from ventral to dorsal and at three depth levels. Tensile moduli along the fiber direction were in the range of 28–78 MPa (regional mean values). Superficial lamellae have larger Emedium (p=0.017) and Ehigh (p=0.012) than internal lamellae, and the mean value of superficial lamellae is about three times higher than that of deep lamellae. Tensile moduli of ventro-lateral lamellae do not differ significantly from the tensile moduli of dorsal lamellae, and Elow is generally indifferent with respect to the anatomical region. Tensile moduli transverse to the fiber direction were about two orders of magnitude smaller (0.22±0.2 MPa, mean ± SD, n=5). Tensile properties are not correlated significantly with donor age. Only small viscoelastic effects were observed. The regional variation of lamellar fiber angle ϕ is described appropriately by a regression line |ϕ|=23.2+0.130×α (r2=0.55, p<0.001), where α is the polar angle associated with the circumferential position. The single anulus lamella may be seen as the elementary structural unit of the anulus fibrosus, and exhibits marked anisotropy and distinct regional variation of tensile properties and fiber angles. These features must be considered for appropriate physical and numerical modeling of the anulus fibrosus.

Single lamellar mechanics of the human lumbar anulus fibrosus

Study Design. The transport properties of solutes of different sizes and conformations were studied in cartilage endplates. Objectives. The results were correlated with the composition of the cartilage matrix to determine if a relationship existed between this and the movement of molecules within it. Summary of Background Data. Solute transport through the hyaline cartilage endplate is important not only for the physiologic and metabolic processes of that tissue, but also for those of the adjacent intervertebral disc. Movement of solutes depends on solute size, shape or charges, and the composition of the matrix itself. Changes in composition of the cartilage endplate, such as those that occur in degeneration or scoliosis, may affect transport. Methods. Partition and diffusion coefficients of solutes ranging in molecular weight from 115 to 70,000 d have been measured on cores of cartilage endplate. Transport properties were assessed in relation to core composition. Results. The shape and size of the solutes were found to affect their transport through cartilage matrix, with larger molecules being more highly excluded and diffusing more slowly. Long-chain polymers were able to penetrate the matrix less readily than the more globular molecules. The more hydrated the matrix, the higher the degree of penetration and the more easily solutes could move, in contrast to the inverse relationship between the other components of the matrix and solute transport. With increased proteoglycan, collagen, or calcification in the tissue, there was greater restriction of solute movement. Conclusions. The proteoglycans normally found in the endplate regulate movement of solutes into and out of the disc. It has been shown previously that removal of proteoglycans from the endplate accelerates the loss of proteoglycans from the nucleus. Hence, a major function of the cartilage endplate may be to prevent fragments of osmotically active proteoglycans from leaving the disc.

Transport properties of the human cartilage endplate in relation to its composition and calcification

The fluid content of the intervertebral disc is important in determining its mechanical response and also its transport and biologic properties. Fluid content depends on the proteoglycan content of the tissue and on the relationship of the external load to the disc's swelling pressure. The influence of proteoglycan content and external load on the hydration of nuclei from 32 human lumbar discs was measured. Swelling pressure of the same specimens was measured by equilibrium dialysis. The influence of age (14-91 years) and spinal level was noted. Proteoglycan content of the discs fell with age, and for all spines tested, proteoglycan content was lowest in the L5-S1 disc; no systematic change in collagen content was found. The hydration of the discs, as received, also fell with increase in age; in each complete lumbar spine tested, the L1-L2 and the L5-S1 discs had the lowest hydration at postmortem examination. As the stress applied to the discs was increased, hydration decreased. Although a stress of 0.10-0.23 MPa maintained the disc slices at their postmortem hydration, under a stress of about 0.6-0.8 MPa, most discs lost 40-60% of their initial fluid. The relationship between change in hydration and swelling pressure was found to depend on the composition of the disc rather than on age or degree of degeneration; the relationship between equilibrium hydration and swelling pressure could be predicted satisfactorily for a disc of known collagen and proteoglycan content.

J. F. Mcmullin

Papers

Swelling pressure of the lumbar intervertebral discs: influence of age, spinal level, composition, and degeneration.