Localized Stresses in the Intervertebral Disc Resulting From a Loose Fragment

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Abstract
Study Design. A theoretical analysis to estimate the stresses generated in the anulus fibrosus by a loose fragment. Objectives. To test the hypothesis that a fragment can generate stress concentrations in an intervertebral disc that could lead to the progression of a fissure, and to provide a theoretical foundation for the laboratory model of lumbar disc protrusion, which proposes that a fragment is formed first and that prolapse is the final event of a chronic process. Summary of Background Data. A laboratory model was developed by Brinckmann and Porter in 1994, which indicated that introducing a fragment into an otherwise intact disc resulted in rapid failure of the disc under physiologic loads and flexion. Identical in vitro surgery with no reintroduction of fragments did not result in failure. Methods. A theoretical model was developed in which a spherical fragment was impressed against a plane surface, representing the inner surface of the anulus. The stiffness of the sphere and that of the surface were varied, and the stresses generated in the anulus were calculated as a function of applied load. Results. The contact stress and the shear stress increased as the stiffness of the fragment approached and exceeded that of the matrix. Increases in stiffness to more than four times that of the matrix resulted in little further increase in stress. Peak shear stresses are developed in the bulk matrix beyond the point of contact between the fragment and the anulus. For relatively small forces on the fragment, these stresses were comparable with those found in diarthrodial joints. Conclusions. It is shown that a fragment in an intervertebral disc produces localized areas of increased stress. Fatigue over long periods at these stresses could result in fissuring and premature failure of the tissue.