Date of Award

12-2008

Document Type

Thesis

Degree Name

Master of Science (MS)

Program

Biomedical Engineering and Imaging

Research Advisor

Denis DiAngelo Ph.D.

Committee

Richard Kasser, Ph.D. Brian Kelly, Ph.D. Gladius Lewis, Ph.D.

Abstract

Recent advancements in biomaterial technologies have fostered growth in alternative surgical procedures to fusion surgery for treatment of early stages of degenerative disc disease. One application of immediate interest is that of nucleus arthroplasty (NA). Novel materials are being developed to better match the nonlinear biomechanical properties of the native tissue. The effects of changing the motion segment unit (MSU) properties via surgery (nucleotomy) or placement of a nucleus arthroplasty material, changes the effort or work required to move the altered spine condition through a prescribed kinematic path relative to the intact spine condition. The closer the loading mechanics of the altered spine are to the intact spine condition, the better the likelihood the device will restore the native properties. The objective of this research was to use a new testing protocol to evaluate different designs used in nucleus replacement devices and compare their restorative characteristics to the native tissue.

Seven human lumbar MSUs were tested in the harvested, nucleotomy, compliant implanted, and non-compliant implanted spine conditions. The spinal segments were mounted in a spine robot and tested in flexion and extension about six fixed points of rotation located along the centerline of the disc and 5 mm below the endplate. The spinal MSUs were rotated about the designated fixed points of rotation until a target bending moment of 8Nm of flexion or extension was reached, or the compressive or shear forces exceeded 500N. Measurements for all test conditions included the MSU axial force normal to the plane of the disc, shear force along the plane of the disc, sagittal rotation, and sagittal bending moment.

During flexion testing, greater MSU rotation occurred for the nucleotomy condition compared to the harvested and implanted spine conditions for all points of rotation. There were no differences between the harvested and compliant implanted spine condition in the MSU rotations, compressive load, or shear load for all points of rotation. The non-compliant implanted spine condition caused greater compressive and shear forces at the posterior points of rotation. Compared to the other three spine conditions, the nucleotomy spine condition had significantly greater rotation in flexion. In extension testing, greater shear and compressive forces acted on the MSU for the nucleotomy spine condition compared to harvested and implanted spine conditions at central and posterior points.

Denucleating the MSU led to a more destabilized spine condition with greater MSU rotation in flexion and greater disk compression in extension. After implantation of a compliant implant, variation was reduced and the response profile moved towards the harvested state for all test points. Implantation of a non-compliant implant caused an increase in the shear and compressive forces acting across the joint. Since spinal discs and compliant nucleus replacement technologies do not have a prescribed axis of rotation, evaluating the kinematic response at multiple locations of rotation may more effectively characterize the restorative effect of these technologies compared to more traditional in vitro test methods. Overall, this method offers new insight into thoroughly understanding the kinematics response of all types of nucleus arthroplasty technologies.

DOI

10.21007/etd.cghs.2008.0274

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