An Analysis of the Effect of Artificial Disc Replacement on the Mechanical Response of the Human Lumbar Spine

Abstract

The objective of this project is to develop a simplified, two-dimensional mathematical model of the lumbar spine for the purpose of studying the behavior the lumbar spine when affected by degenerative disc disease. Several hypothetical treatment options, including fusion and different types of artificial disc replacements (ADR) were examined. The cases presented consisted of three one-degree of freedom artificial discs, three two-degrees of freedom artificial discs, one ideal three-degrees of freedom artificial disc, a degenerated disc, a fused disc and a healthy spine. The equations of motion were generated for a healthy lumbar spine using Lagrange's equations and numerically integrated using Matlab®. Results were obtained for all cases at two different levels, L4-L5 and L5-S1 in response to an impulsive force of 100N applied at L3 in the posterior anterior direction. In the 1-DOF ADR cases at the L4-L5 level, the shear ADR performed better than the other two ADR, while at the L5-S1 level, the rotational ADR performed better than the other two ADR, and significantly better than the fused vertebrae case since it matched the behavior of the healthy spine much more closely. All the other 1-DOF ADR provided little or no improvement when compared to the fused case. In the 2-DOF ADR cases, the shear rotational ADR behaved very similarly to the healthy spine when implanted at both levels, L4-L5 and L5-S1, showing a behavior that varied by less than 1% in the posterior anterior direction and flexion extension rotation and less than 10% in the axial displacement when compared to the behavior of a healthy spine. Overall, the results of this thesis indicate that implanting an artificial disc to replace a damaged disc offers more benefits for the spine than fusion since this allows the spine to behave closer to the natural healthy spine, and hence most likely cause less damage to adjacent discs.