Creep characteristics of the human spinal column.

Thirty-two static compression tests were carried out on 52 intervertebral disks and their posterior articular facet joints. The spinal units were tested within a uniquely designed test apparatus. Following the experiment, each intervertebral disk was examined macroscopically to assess the degree of disk degeneration. Three experiments were carried out at suitably chosen load levels, which were statically appropriate for a given vertebral level and its loading history. In the first experiment a constant load was applied to a spinal segment and its decrease in height was measured as a function of time. The results were as follows: Under the application of a constant load, the vertebral unit suffers a gradual change in its dimensions. This typical behavioral response pattern is identified as creep. Creep may be defined as the time dependent part of the deformation that accompanies the application of a load to the disk. The measured creep did not continue indefinitely but decreased with time until the vertebral specimen was equilibrated. The time to equilibration was a function of disk grade. The rate of creep was found to be dependent upon the condition of the nucleus pulposus. Creep is of considerable importance because of its influence on the kinetics and kinematics of the vertebral unit. The effect of creep is to increase the modulus of elasticity with time; hence a reduction occurs in the compliance of the spinal unit. In the second experiment, incremental loads were applied to a spinal unit. The unit was allowed to equilibrate and an additional load was applied. Its purpose was to further delineate spinal unit mechanics in terms of constant load and creep behavior. It was concluded from these tests that the nucleus plays an important mechanical role in influencing the behavioral response of the vertebral unit to constant compressive load. As nuclear maturation occurs, the stiffness characteristics of the spinal unit were noted to increase as dis its deflection characteristics. Spinal unit geometry no doubt also plays a role in influencing these results. In the last experiment, load-deflection curves were determined for the vertebral unit. It was found that load-deflection curves are not uniquely defined by the relationship between the variables of load and deflection. The time each value of load is maintained must also be taken into account. The physiological and biochemical factors responsible for the observed spinal unit behavior are outlined, and the importance of the articular facet joints in governing spinal unit deflection stressed. Although it has long been known that the intervertebral disk suffers a slow and continuous deformation when subjected to mechanical load, the concept of spinal unit deformation as a time dependent function of load is comparatively new.