M.S. Thesis Presentation by Guilhem Denoziere
Tuesday, May 25, 2004

(Dr. David Ku, Chair)

"Numerical Modeling of a Ligamentous Lumbar Motion Segment"

Abstract

Eight out of ten people in the United States will have problems with low back pain at some point in their life. A significant amount of low back pain is related the degeneration of the spinal intervertebral discs (IVD), which is a natural process that starts as early as the second decade of life. Promoted by the continuous aging of the population, the treatments of the pathologies related to IVD degeneration have become a focal point of the biomedical industry. Many conservative treatments have been developed and are usually used in the initial treatment. However, in some cases for which conservative actions failed, it is necessary to consider surgical treatments. The most significant surgical operations can in general be distributed into two main groups of solutions: arthrodesis and arthroplasty. Arthrodesis consists of the distraction and the surgical immobilization of a joint, here of a functional spine unit (FSU), to alleviate pain and prevent mechanical instability. The treatment of arthroplasty consists of the plastic surgery of the FSU, in order to alleviate pain by restoring the relevant functionalities of a degenerated IVD. The objective of this study is to analyze and compare, relative to a healthy configuration, the alteration of the biomechanics of the lumbar spine after being treated either by arthrodesis or arthroplasty. In particular, four parameters were investigated: 1) the mobility of the segments in the different degrees of freedom, 2) the resistance of the intervertebral ligaments, 3) the force and pressure in the facet joints, and 4) the stress in the annular fibers of the healthy disc adjacent to the treated FSUs.

To perform the analysis, a three-dimensional finite element model of a ligamentous lumbar motion segment, constituted of two FSUs, was built and simulated through a static analysis with the finite element software ABAQUS. In the model, the intervertebral disc of the upper FSU was either healthy, treated by arthrodesis, or treated by arthroplasty. The three configurations were simulated in flexion, extension, right lateral bending, and axial rotation, under the same loading protocol appropriate to the upper limit of physiological conditions.

It was shown that the mobility of the model treated by arthrodesis was significantly reduced in all rotational degrees of freedom by an average of approximately 44%, relative to the healthy model. Against what was intuitively expected, the mobility and stresses in the healthy FSU, adjacent to the fused FSU in the model treated by arthrodesis, were only slightly different from the healthy model. Conversely, the mobility of the model treated by arthroplasty was significantly increased in all rotational degrees of freedom by an average of approximately 52%. The mobility and the stresses in the healthy FSU, adjacent to the restored FSU in the model treated by arthroplasty, were also significantly increased. The restored FSU itself showed a high risk of instability and further degeneration. Indeed, several components (the intervertebral ligaments (ISL & CL), the fibers of the residual annulus, the facet joints, and the cancellous bone of the underlying vertebral body adjacent to the prosthesis) of the restored FSU of the model treated by arthroplasty reached a stress that was close to or exceeded their ultimate failure stress.

In conclusion, in view of the devices modeled for the simulation of the treatments of arthrodesis and arthroplasty, and given the significant physiological loading conditions used, the solution of arthroplasty is more likely to show risks of instability and further degeneration on the treated level as well as on the adjacent levels.