Intervertebral disk appearance correlated with stiffness of lumbar spinal motion segments.

BACKGROUND AND PURPOSE Because it diminishes the stiffness of the intervertebral disk, disk degeneration results in abnormal motions of the spine. Therefore, disk degeneration associated with back pain may indicate spinal fusion. The purpose of this study was to correlate the MR appearance and stiffness of lumbar intervertebral disks. METHODS Eighty-two lumbar spinal segments were imaged with MR. The intervertebral disks were classified as: 1) normal, 2) having transverse or concentric 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-intensity zones in the annulus fibrosus or reduced signal intensity in the disk characteristic of radial tears, or 4) having advanced degeneration if MR imaging showed markedly reduced height, large osteophytes, or both. The rotation 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/degree for the disks with concentric or transverse tears; 1.7 Nm/degree for disks with radial tears; and 3.1 Nm/degree for disks with advanced degeneration. The differences were statistically significant. CONCLUSION Concentric, transverse, and radial tears of the intervertebral disk indicate reduced stiffness of the intervertebral disk and increased motions 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.