Numerical investigation of lumbar disc injury under cyclic loading conditions

Approximately 75--85% of the United States population will be afflicted with some form back pain during their lifetime and of those, 50--75% will experience low back pain. Although epidemiological studies have suggested possible causes, the actual injury mechanisms resulting in pain and degeneration remain unclear. Hence, it is important to understand the relationship between disc degeneration and the subsequent change in intervertebral disc biomechanical properties. It is the unique interaction between the solid and fluid components that provide the intervertebral discs the strength and flexibility necessary to bear the physiological loading of the spine.; The first aim of this investigation was developing a numerical model of a lumbar disc motion segment including the fluid flow into and out of the intervertebral disc. The presence of the fluid is imperative in order to study the loading and unloading of the spine. This was accomplished by including the poroelastic behavior of the disc, the effect of the change in proteoglycan content in the disc, the effect of strain dependent permeability with respect to applied loads and regional variations in the material properties of the disc tissues. Muscle force vectors were included in order to simulate the loading conditions due to the global muscles surrounding the spine. Model validation was conducted by comparing, in a gross manner, the variation in height of human subjects obtained from the finite element model with those measured in vivo for three different loading conditions, circadian variation (over a 24-hour period), short-term creep loading and short-term cyclic loading.; The second aim was to use this model to identify where failures initiate and how they propagate through the disc in response to repetitive loading conditions. The hypotheses for this investigation were that for given forces and moments associated with varying lifting activities: motion would increase, nucleus pressure would decrease and disc height would decrease. The finite element model results confirmed all hypotheses except in the case of the nucleus pressure. The pressure within the nucleus increased with increasing load cycles. Meanwhile, failures were found to initiate in the endplates first and then in the posterior annulus.