Fatigue failure in shear loading of porcine lumbar spine segments.

STUDY DESIGN.: An in vitro study on porcine spinal segments. OBJECTIVES.: To determine the differences in mechanical behavior and fatigue strength in shear loading between intact spinal segments and segments without posterior elements, and between segments in neutral and flexed positions. SUMMARY OF BACKGROUND DATA.: Limited data are available on shear strength of spinal segments. Literature suggests that shear loading can lead to failure of the posterior elements and failure of the disc, when the posterior elements cannot provide adequate protection. METHODS.: In 2 experiments, 18 and 20 spines of pigs (80 kg) were used, respectively. Shear strength of the T13-L1 segment was tested, while loaded with 1600-N compression. L2-L3 and L4-L5 segments were loaded with a sinusoidal shear between 20% and 80% of the strength of the corresponding T13-L1 segment and 1600-N compression. In experiment No. 1, the posterior elements were removed in half the segments. In experiment No. 2, half the segments were tested in the neutral position, and half were tested in 10° flexion. RESULTS.: The group without posterior elements had failure earlier than the intact group. In the group without posterior element, stiffness increased on failure; in the intact group, it decreased. In experiment No. 2, no differences between groups were found. CONCLUSIONS.: Repetitive shear loading can induce failure of porcine spinal segments, likely caused by fracture of the posterior elements, and, although repetitive anterior shear forces can also induce disc damage, this appears not to occur in intact segments, not even when flexed close to maximal. ©2006, Lippincott Williams & Wilkins, Inc.