HEALING OF A HERNIATED INTERVERTEBRAL DISC AND THE INFLUENCE OF EXERCISE The extent to which exercise can influence the homeostasis (including both repair and maintenance) of biological tissue has been evaluated through a number of research efforts over the years.38-42 Most of the studies on tissue response to exercise have been performed on animal models. Homeostasis of both bony and soft tissues is maintained through the appropriate balance of activity and rest.43An understanding of how exercise influences biological tissues can enhance a clinician’s exercise prescription through staging of healing as well as his/her analysis of how/why certain favorable or unfavorable responses develop. Therefore, the following information presented on tissue biomechanics will examine what is known about how these tissues respond to the loading that occurs during therapeutic exercise. Reversing disc degeneration and affecting healing involving the inner annulus and NP appears to be an extremely slow process, if possible at all.44 The low cell density appears to be prime reason for poor healing of the NP,45 with mathematical extrapolations predicting that turnover of the highly cross-linked collagen network of the matrix and collagen half-life would require more than 100 years to complete.46 Given that the NP is an avascular structure, homeo
stasis is largely managed by diffusion and bulk fluid flow, according to O’Hara et al.47 The extent of flow across the NP is influenced by a patient’s physical activity level. Given that the nuclear matrix is composed largely of Type 2 collagen, which resists the forces of compression, homeostasis would appear to be maintained best by intermittent compression and relative decompression. Theoretically, these forces would be uniformly generated by rotation around the longitudinal (y) axis of the body as the annular fibers impose some measure of compression and decompression via reciprocal (by layer) tightening and loosening of annular fibers. However, this model is somewhat speculative since the avascular NP has such low metabolic turnover (i.e., protein synthesis). Like articular cartilage, it is uncertain whether or not the innermost portions of the annulus can successfully repair following an injury. Nonetheless, Guehring et al48 recently demonstrated that distraction of the disc promotes its rehydration, stimulates extracellular matrix gene expression, and increases the number of protein-expressing cells in rabbits. These are essential elements for homeostasis of biological tissue. Excessive compression of the disc can lead to a decrease in proteoglycan synthesis while a modest amount of increase in hydrostatic pressures can have the opposite effect.47,49 Connecting the key points from the findings of Guehring48 and Ishihara49 establishes a basis by which the authors suggest a program that incorporates carefully applied axial rotation to a lumbar spine whose discs have reasonable regenerative capacity, and that this program may provide the stimulus for tissue healing.
The outer annulus, but not the inner annulus, appears to demonstrate good healing potential in animal models.45,50,51 In as little as six weeks of healing, the annulus is able to resist significant hydrostatic pressure within the NP.52 This may be due to the outer annulus sharing similar cells and matrix composition to tendons and ligaments, although tendons and ligaments are also surrounded by collagenous sheaths.53,54 Rotation may also provide the basis for normal homeostatic mechanisms for the annular fibers of the disc. As noted earlier, the annular fibers of the disc are oriented in alternately oblique fiber directions. As the spine is exposed to y-axis rotation in one direction, one-half of the annular fibers are made taut while the layers alternately oriented to the tautened fibers become relatively lax. Obviously, contralateral rotation produces the exact opposite response. The fibers that are tightened will produce an approximation of the adjacent vertebral bodies to which the annular fibers are attached. The result is translation along the y-axis, or compression and relative decompression. The annular fibers are precisely and uniquely oriented to offer control to rotation about the y-axis. There have been numerous studies over the years which have demonstrated the ill effects of immobilization on collagen production and glycosaminoglycans (GAG) in ligaments.55-57 Conversely, application of appropriate levels of tension along the lines of fiber orientation has been shown to support the homeostatic mechanisms of collagen production.58,59 Finally, Adams et al26 suggest that controlled rotation may aid in reducing excessive scar tissue formation within the annulus during the phases of healing after an injury.
Although the frequent occurrence of vertebral end plate fracture has been noted5 and implicated in disc degeneration,34 minimal research is available regarding healing of this structure. In animal studies, endplate fracture stimulates trabecular bone growth and propagation of cartilaginous tissue at the site of injury, similar to the presence of a Schmorl’s node.60 The vertebral endplate and its trabeculae are metabolically active and remodeling may occur in response to altered mechanical loading of the bone due to outer annular injury.61,62 Antonious et al demonstrated9,63 that a healing response may be present based on an increased level of collagen turnover at the endplate in the presence of disc degeneration. In a possible attempt by the body to improve metabolite delivery, increased vascularization of the cartilaginous endplate near the peripheral disc lesion was also noted.64 Vascular and sensory nerve proliferation has also been noted at the endplate in degenerated human discs providing another potential representation of healing.65
