Measurement of cadaver lumbar spine motion segment stiffness

Prospective. To measure lumbar spine motion segment stiffness and relate it to the degree of disc degeneration. The association between the instability of the lumbar spine motion segment and disc degeneration remains unclear. The traditional method for determining motion segment instability at the time of decompressive surgery is a manual test performed by the surgeon. To quantify instability of the lumbar spine, a vertebrae distractor was developed in the authors' laboratory to measure motion segment stiffness by applying a defined load at a constant rate. Lumbar stiffness was measured by subjecting cadaver lumbar spine motion segments to a constant rate flexion-traction load and recording the magnitude of the resistance to distraction versus the range of motion. Disc degeneration was measured by pressure-volume discography and by grading of disc morphology. Motion segment stiffness decreased with the initial stages of disc degeneration and then increased with severe disc degeneration. This measure of motion segment stiffness correlated well with a manual stiffness measure. The observed results follow an accepted hypothesis of motion segment instability associated with disc degeneration.