In vitro torsion-induced stress distribution changes in porcine intervertebral discs.

STUDY DESIGN: A cadaveric porcine spine motion segment experiment was conducted. OBJECTIVE: To test the hypothesis that small vertebral rotations cause increased stress in the anulus while decreasing stress in the nucleus through stiffening of the anulus. SUMMARY OF BACKGROUND DATA: Stress profiles of the intervertebral disc reportedly depend on degeneration grade and external loading. Increased stress in the anulus was found during asymmetric loading. In addition, depressurization of the nucleus combined with an instantaneous disc height increase was found when small (<2 degrees ) axial vertebral rotations were applied. METHODS: Seven lumbar porcine cadaveric motion segments consisting of two vertebrae and the intervening disc with ligaments were loaded in the neutral position with 340 N of compression. Stress profiles were obtained in the neutral position, then after 0.5 degrees and 1 degrees axial rotation of the bottom vertebral body. The distribution of compressive stress in the disc matrix was measured by pulling a miniature pressure transducer through the disc along a straight path in the midfrontal plane. Stress profiles were measured in vertical (0 degrees ) and horizontal (90 degrees ) orientation. RESULTS: Deformation of the anulus by small axial rotations of the lower vertebra instantaneously decreased the horizontally and vertically measured stress in the nucleus while increasing stress in the anulus. A 1-hour period of creep loading decreased the stresses in the nucleus and the anulus 20% to 30%, depending on the orientation, but the effect of an increasing stress in the anular region after axial rotation persisted. CONCLUSIONS: The compressive Young's modulus of the composite anulus tissue increases instantaneously when small axial rotations are applied to porcine spine motion segments. This is accompanied by decreased stress in the nucleus pulposus, increased stress in the anulus fibrosus, changes in the stress profile superimposed on and independent of prolonged viscoelastic creep and dehydration, and changes in stress distribution independent of horizontal and vertical orientation.