Final Report to the Air Force Office of Scientific Research on Hygroviscoelasticity of the Human Intervertebral Disc

Abstract

In order to gain an improved understanding of the behavior of human intervertebral disc material under various kinds of loads the viscoelastic properties of small specimens excised from human L4-L5 discs were examined. Excisions were made from donated spine segments procured a few hours after death and then frozen. Material examined was in the form of single lamellar specimens as well as specimens containing several lamellae. Tensile relaxation tests were performed on single lamellae prepared such that the collagen fibers were a) aligned with the tension axis, b) normal to the tension axis and c) at an angle of about 30[degrees] with that axis. The multi-lamellar specimens were excised from the disc such that one set produced the tensile axis to run parallel to the disc circumference (surrounding the spinal axis) while another set caused the tensile axis to run parallel to the spinal axis. It was found early in the study that the water content of the disc material has a profound effect on its mechanical response. Consequently the diffusion and swelling characteristics of the material in different water environments were studied. Primarily air of differing relative humidity and various concentrated solutions of NaCl were used to provide for different water concentrations in the material. For the relaxation studies the same environments were used. This allowed achieving water concentrations ranging from virtually dry to in-vivo conditions. The main findings of this work are: 1) Water affects the relaxation time in a sensitive way. A few percent change in water content can change the relaxation time by an order of magnitude or more. This fact is important when one is concerned with laboratory testing without being able to control the water content at all times. 2) Diffusion is a surprisingly slow process taking place over several hours (2-3) in specimens only 1/2 mm thick. The amount of water take-up is controlled by the environment (distilled water destroys samples into fissures) and by constraints offered by the fiber structure and the stresses it engenders (see point 5) below). 3) The stiffness of the disc material is on the order of 3 to 10 x 10^-6 N/m^2; relaxation occuring, on the whole rather slowly, i.e., on the order of 10% per decade. The relaxation modulus at right angles to the collagen fibers is about 1/3 that of the modulus along the fiber direction. 4) The relaxation modulus of multi-layer specimens cut from the disc in a circumferential and a spine-axial direction are very nearly the same; the modulus for the circumferential material is slightly higher. The consequence of this would be that although one would expect a markedly anisotropic behavior of the disc material on the basis of its fiber and lamellar construction, the disc material does not deviate very strongly from isotropic response. (Isotropy as-referred to a plane that is tangent to the disc perimetric surface.) 5) Stress affects the equilibrium concentration of water in the material. The implication - and test result - is that once the stress has changed the equilibrium of water content with the environment is disturbed and the concentration changes. That change, in turn, causes a change in relaxation behavior such that tensile stresses cause an acceleration of relaxation or creep (softening), while compression would cause the opposite affect (hardening). The material responds thus to stress as a system that is open to the environment.

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