(From the July 2009 “The Scope” By: Steven T. Tanaka, DC)
Smith LJ, Fazzalari NL. The Elastic Fiber Network of the Human Lumbar Intervertebral Disc. European Spine Journal April 2009; 18(4):439-448.
ABSTRACT: Elastic fibres are critical constituents of dynamic biological structures that functionally require elasticity and resilience. The network of elastic fibres in the annulus fibrosus of the intervertebral disc is extensive, however until recently, the majority of histological, biochemical and biomechanical studies have focussed on the roles of other extracellular matrix constituents such as collagens and proteoglycans. The resulting lack of detailed descriptions of elastic fibre network architecture and mechanical function has limited understanding of the potentially important contribution made by elastic fibres to healthy disc function and their possible roles in the progression of disc degeneration. In addition, it has made it difficult to postulate what the consequences of elastic fibre related disorders would be for intervertebral disc behavior, and to develop treatments accordingly. In this paper, we review recent and historical studies which have examined both the structure and the function of the human lumbar anulus fibrosus elastic fibre network, provide a synergistic discussion in an attempt to clarify its potentially critical contribution both to normal intervertebral disc behavior and the processes relating to its degeneration, and recommend critical areas for future research.
COMMENTS: Elastin is an important constituent of the disc. It is not as well discussed as the proteoglycans and other collagens. Elastic fibers have high linear elasticity and high extensibility and are found throughout the body in dynamic tissues, such as, arteries, skin, and lungs. These are all tissues that require material that are sufficiently elastic so that arteries can contract or expand to demand, skin can stretch and twist, and intervertebral discs can allow constantly stretch and torsion.
Elastic fibers are not amorphous as previously thought, but have a core of the protein elastin which is surrounded by structure-forming microfibrillar glycoproteins. Water is a key element to maintain an elastic quality. Without water, elastic fibers are a rigid solid and has been described as “glass-like.”
Elastic fibers occupy about 10% of the anulus fibrosus. It is known to be near vertebral bodies such as in Sharpey’s fibers and is also found in the anular laminae and in the transitional zone. The density of elastic fibers in the anulus is much greater in the outer anulus than the inner anulus and greater in the posterolateral region than the anterolateral region. Recently, it has been found throughout the disc. In the anular laminae, they are largely parallel to the collagen fiber bundles, while in the nucleus pulposus, they are oriented towards the peripherally and towards the endplates. The regional differences in density is thought to reflect the greater tensile stress in the denser regions as a result of bending and torsion.
Degradation of elastin is a major contributor to degenerative changes. In the skin which is a dynamic tissue as is the disc, loss of elastic fibers is a contributor to the changes in the skin from photo-aging. In the disc, an altered level of elastic fibers would compromise the mechanical properties of the disc. Tensile strength and tissue recovery from bending and twisting motions would be compromised.