The stiffness of lumbar spinal motion segments with a high-intensity zone in the anulus fibrosus

Study Design. Biomechanical and anatomic study of human cadaveric spinal motion segments. Objectives. To measure the stiffness of spinal motion segments by disc type and by load type (flexion, extension, axial rotation, or lateral bending). To compare stiffness in motion segments with and without a high-intensity zone or radial tear in the anulus fibrosus, Summary of Background Data. The high-intensity zone, that is a linear zone of high-intensity on T2-weighted magnetic resonance images corresponding to a radial lear in the anulus fibrosus, is a marker for a painful disc at discography. The high-intensity zone is hypothetically associated with diminished stiffness of the motion segment. Methods. Human cadaveric lumbar spinal motion segments with normal disc morphology or a high-intensity zone of the anulus fibrosus were selected on the basis of magnetic resonance imaging. The motion segments were subjected to incremental flexion, extension, rotation, and lateral bending torques. Rotation was measured with a kinematic system. Torque-rotation curves and stiffness were calculated for each motion segment and for each torque. The motion segments were sectioned on a cryomicrotome to verify the disc morphology as normal or as that of a radial tear. Results. In four motion segments with normal discs, stiff ness was greater in axial rotation (8.4 Nm/degrees) than in lateral bending (2.3 Nm/degrees), flexion (1.8 Nm/degrees), or extension (2.6 Nm/degrees). In 16 motion segments with a high-intensity zone, stiffness was 2.4 Nm/degrees in axial rotation, and less severely reduced in lateral bending, flexion, and extension. Stiffness in motion segments with a high-intensity zone was significantly less with smaller than with larger axial rotation loads. Conclusions. The presence of a high-intensity zone in the intervertebral disc is associated with reduced stiffness of motion segments. The reduction is greater in axial relation than in other torques. The reduction is more in smaller than in larger axial torques.