Intervertebral disk appearance correlated with stiffness of lumbar spinal motion segments

BACKGROUND AND PURPOSE. Because it diminishes the stiffness of the interver tebral disk, disk degeneration results in abnormal motions of the spine. Th erefore, disk degeneration associated with hack pain may indicate spinal fu sion, The purpose of this study was to correlate the MR appearance and stif fness of lumbar intervertebral disks. METHODS: Eighty-two lumbar spinal segments were imaged with MR, The interve rtebral disks were classified as: 1) normal, 2) having transverse or concen tric tears of the annulus fibrosus if MR imaging showed only these changes, 3) having radial tears of the annulus fibrosus if MR imaging showed high-i ntensity zones in the annulus fibrosus or reduced signal intensity in the d isk characteristic of radial tears, or 4) having advanced degeneration if M R imaging showed markedly reduced height, large osteophytes, or both, The r otation occurring from the application of a 6.6-Newton-meter (Nm) moment of axial rotational torque was measured kinematically. Average stiffness, in Nm/degree, was calculated as the ratio of the torque to the rotation. RESULTS: Stiffness averaged 7.0 Nm/degree for the normal group; 1.9 Nm/degr ee for the disks with concentric or transverse tears; 1.7 Nm/degree for dis ks with radial tears; and 3.1 Nm/degree for disks with advanced degeneratio n. The differences were statistically significant. CONCLUSION: Concentric, transverse, and radial tears of the intervertebral disk indicate reduced stiffness of the intervertebral disk and increased mo tions for a unit of applied torque. The most severely reduced stiffness was found in disks with radial tears of the annulus fibrosus. With collapse of the disk space, stiffness increases.