On the Mechanical Properties of the Human Intervertebral Disc

The human intervertebral disc is a highly inhomogeneous fiber composite pressure vessel. Interest in the mechanical properties of the disc started from a desire to develop a non-invasive diagnostic technique to assess disc damage based on X-ray and computer-aided image enhancement. These would be important in gaging the X-ray detected deformations of the disc under various loads. The water content of the disc material was found to dominate its mechanical behavior. From a study of three-layer specimens, several important aspects of the mechanical properties were established. First, the relaxation behavior is very sensitive to moisture content. Second, water diffuses slowly in the layers. The water apparently acts similar to a solvent in a polymer, effecting a change in the relaxation times. Increasing water content caused a shortening of relaxation times, drying, the opposite effect. Data covering a wide spectrum of relaxation times are presented that include all time scales experienced by the human body. This mechanical characterization provides an estimate of how discs respond to different rates of deformation and loading conditions. The incidence of disc problems with advancing age is explained in terms of the decrease in the moisture content of the disc along with possible changes in the nature of its mucopolysaccharides. (Author)