Effect of lumbar disc degeneration of spine biomechanics and trunk muscle recruitment patterns

Effect of lumbar disc degeneration on spine biomechanics and trunk muscle recruitment patterns was studied using the follower load concept to mimic physiological loading conditions. To demonstrate that the follower load is a physiologically relevant principle, the capability of the trunk muscles to generate a follower load was studied using both experimental and computational modeling. The experimental study consisted of a real-time simulation of a frontal plane mechanical spine analogue performing lateral bending tasks via the control of pneumatic muscles. The computational modeling part of the study consisted of (1) a non-linear 3-D finite element model and (2) a 3-D musculoskeletal model of the trunk with elastic joints and 154 muscle fascicles with physiological lines of action.;After demonstrating both experimentally and computationally that a follower load is a mechanism by which the spine can support loads of physiological magnitude, the follower load concept was used to study the effects of disc degeneration on spine biomechanics and muscle recruitment patterns. Mild and moderate disc degeneration at the L4-L5 lumbar level was simulated in a non-linear 3-D finite element model of the lumbar spine by changes in geometry and associated material properties. The observed changes in the stiffness of the intervertebral discs, and location of the instantaneous center of rotation were then used to simulate moderate degeneration at the L4-L5 lumbar level in a 3-D musculoskeletal model of the trunk.;Disc degeneration caused a decreased in segmental motion and a shift in the load transmission from the nucleus to the annulus at the degenerated L4-L5 level. However, at the segments adjacent to the degenerated level nucleus pressure remained the same, while stress in the annulus increased. Changes in the quantity and quality of motion were most prominent at the degenerated disc and the level above, while changes in loading patterns were more significant at the level below the degenerated disc. Degeneration caused an increase in the activation of most muscles, though activation strategies were posture dependent. Overtime the increased stress concentration and motion re-distribution observed at the adjacent segments may lead to damage accumulation and subsequent progression of the disease.