The intervertebral disc

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The intervertebral disc provides mobility to the spine. Positioned between the bony vertebrae, the disc allows complex motion without the mechanical disadvantages of the opposing articular surfaces of a diarthrodial joint. The disc derives its function from its unique structure, whereby the amorphous, gel-like nucleus pulposus is surrounded by the highly oriented annulus fibrosus. In the healthy disc, a hydrostatic pressure is developed within the nucleus, which is contained by the strong lamellae of the annulus, and loads are thereby evenly distributed across the underlying vertebrae.

The degenerative changes to the vertebral disc which are often observed with aging have been well described in the review by Vernon-Roberts [38]. Macroscopic changes to the disc include the appearance of horizontal splits and clefts midway between the center of the disc and the cartilage endplates, which extend posteriorly and posterolater-ally and can eventually lead to fissures through the annu-lus. Microscopic fragmentation of annulus fibers has been observed, leading to a degeneration of individual fibers. Vertebral rim lesions, annular tears at the corners of the vertebral body separating the annulus from the bony attachment, are commonly present after the age of 50 years. Concentric cracks and cavities and radiating ruptures of the annulus are often present. At the disc boundaries cartilage endplate fissure formation, horizontal cleft formation, death of chondrocytes, vascular penetration, and Schmorl's nodes are observed. Disc thinning occurs due to loss of water content, conversion of the nucleus tissue to a highly organized collagenous tissue, gradual ossification of the endplate and protrusion of disc tissue. While the cartilage endplate and annulus are normally sufficiently strong to contain the nucleus, even under great stress, degeneration of the disc can lead to potential weak points in the sub-chondral bone and in the posterior and posterolateral segments of the annulus, which are thinner and less firmly attached to the vertebra.

Age- and degeneration-related changes to disc tissue material properties have been extensively evaluated. Based on measurements of the viscoelastic properties of the human nucleus pulposus, Iatridis et al. [14] concluded that changes to the mechanical properties suggest a shift from a "fluid like" behavior to a more "solid like" behavior with degeneration. Due to its crucial role in the containment of the nucleus, changes to the properties of the an-

nulus fibrosus have also been the subject of several studies. An increase in the elastic modulus with progressive degeneration has been shown, likely the results of an increase in tissue density due to water loss. This suggests a shift in the load carriage mechanism of the disc with increasing degeneration from fluid pressurization to elastic deformation of the annulus fibrosus [15]. Although dramatic changes in annulus fibrosus morphology and composition have been documented with aging and degeneration, the tensile mechanical properties of the annulus are not substantially affected by degeneration. A far more important factor for the tensile properties, especially in the radial direction, is the position within the annulus, and this relationship does not change substantially with age or degeneration [1, 7]. Significant changes to the ligamen-tous structures of the spine with aging have been reported. For example, the elastic modulus of the main substance of the anterior longitudinal ligament increases twofold, while the modulus of the ligament insertion decreases threefold, between 20 and 80 years of age, and the strength of the bone ligament junction decreases twofold with aging [25].

The fluid content of the disc is important for determining its mechanical response. Hydration depends on the proteoglycan content of the disc and also on the balance between external load and the internal swelling pressure of the disc. The influence of age, spinal level, composition and degeneration on disc swelling pressure has been measured for human discs [37]. The natural swelling pres-

Fig. 3 Typical stress profiles for grade-1 disc (top) and for a grade 4 disc (bottom). In the healthy disc, a hydrostatic pressure is developed in the nucleus, as indicated by the plateau in the stress plot. For the degenerate disc, nuclear pressure is lower, and stress peaks in the annulus fibrosus are observed. A Anterior; P posterior. (Adapted from [2])

Fig. 3 Typical stress profiles for grade-1 disc (top) and for a grade 4 disc (bottom). In the healthy disc, a hydrostatic pressure is developed in the nucleus, as indicated by the plateau in the stress plot. For the degenerate disc, nuclear pressure is lower, and stress peaks in the annulus fibrosus are observed. A Anterior; P posterior. (Adapted from [2])

sure for human discs was found to be approximately 0.10.2 MPa. Proteoglycan content decreased with age, and was lowest at L5-S1, but no substantial change in collagen content was found. Therefore the relationship between equilibrium hydration and swelling pressure could be predicted based on proteoglycan and collagen content, while age and degree of degeneration were not significant factors.

Aging and disc degeneration have a profound effect on the mechanism of load transfer through the disc. Using the technique of "stress-profilometry," it has been shown that age-related changes to the disc composition result in a shift of load from the nucleus to the annulus [2]. A reduction by approximately 50% of the central hydrostatic region of the disc was observed, and a corresponding 30% reduction in pressure for degenerate discs (Fig. 3). The width of the functional annulus increased by 80% and the height of the compressive stress peak in the annulus by 160% with degeneration. While age and degeneration were closely related, the state of degeneration had the most profound influence on the measured stress distributions. Therefore structural changes in the annulus and endplate with aging may lead to a transfer of load from the nucleus to the posterior annulus, which may cause pain and also lead to annular rupture.

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