Disc lesions and the mechanics of the intervertebral joint complex.

Abstract

Correlations between tears in the disc and the mechanics of both the intervertebral joint and vertebral body bone were analyzed. To examine the effect of disc degeneration on the mechanics of spinal motion segments. Degeneration of the intervertebral disc results in changes to the mechanics of the spine. The actual effect of tear type and size on the mechanics of the intervertebral joint is unknown. Thirty spinal specimens (median age, 68 years) were divided into T12-L1, L2-L3, and L4-L5 motion segments. Mechanical tests recorded stiffness in flexion, extension, and torsion. Disc morphology was ascertained by taking three transverse sections of the disc and mapping and measuring the concentric tears, radial tears, and rim lesions. The severity of each tear type within each disc then was quantified. Bone cubes from the adjacent vertebral bodies were tested in compression to determine the elastic moduli and tested to failure in the longitudinal direction. Groups with tears were older and had reduced bone elastic moduli than groups without tears. Extension stiffness for the intact joint tended to increase with increasing tear severity. A decrease in torsional stiffness was present with increased severity of rim lesions at both L2-L3 and L4-L5. Tears in the intervertebral disc are reflected in a reduction in vertebral bone elastic modulus and in changes in the mechanics of the intervertebral joints in flexion, extension, and torsion.