Elastic Fibers Enhance the Mechanical Integrity of the Human Lumbar Anulus Fibrosus in the Radial Direction

The anulus fibrosus of the human lumbar intervertebral disc has a complex, hierarchical structure comprised of collagens, proteoglycans, and elastic fibers. Recent histological studies have suggested that the elastic fiber network may play an important functional role. In this study, it was hypothesized that elastic fibers enhance the mechanical integrity of the extracellular matrix in the radial orientation, perpendicular to the plane containing the collagen fibers. Using a combination of biochemically verified enzymatic treatments and biomechanical tests, it was demonstrated that degradation of elastic fibers resulted in a significant reduction in both the initial modulus and the ultimate modulus, and a significant increase in the extensibility, of radially oriented anulus fibrosus specimens. Separate treatments and mechanical tests were used to account for any changes attributable to non-specific degradation of glycosaminoglycans. Additionally, histological assessments provided a unique perspective on structural changes in the elastic fiber network in radially oriented specimens subjected to tensile deformations. The results of this study demonstrate that elastic fibers play an important and unique role in the mechanical properties of the anulus fibrosus, and provide the basis for the development of improved material models to describe intervertebral disc mechanical behavior.