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Finite–Element Simulation of Changes in the Fluid Content of Human Lumbar Discs: Mechanical and Clinical Implications

SHIRAZI–ADL, A, PhD

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The effect of alteration in the nucleus fluid content on the mechanics of a lumbar motion segment was analyzed by a finite–element model. Various combined loads were applied on the lumbar segment and were then kept constant while the disc–confined fluid was changed incrementally to a maximum of 12% gain or loss in its original volume. Change in the nucleus volume directly affected the intradiscal pressure. Loss of nucleus fluid content increased the contact forces on the facets and diminished tensile forces in the anulus fiber layers. The inner anulus layers were more affected than the outer ones. Reverse trends were computed when the nucleus fluid content was increased. Except in combined extension and compression loading, fluid gain increased the segmental stiffness while the overall stiffness lessened with loss of fluid content. Loss of the nucleus fluid caused inward bulge at the inner anulus layers and altered the stress distribution in the vertebral bodies. The nucleus material normally carries a portion of the applied compression and stresses and supports the surrounding anulus layers. A loss in the nucleus–confined fluid disrupted the normal mechanical function of the nucleus, whereby disc anulus layers were predisposed to lateral instability and disintegration, and hence to further degeneration; facets were subjected to significant additional loads; and vertebral bodies underwent a markedly different stress distribution.

From the Applied Mechanics Section, Department of Mechanical Engineering, Ecole Polytechnique, Montreal, Quebec, Canada

© Lippincott-Raven Publishers.