Estimation of shear load sharing in moderately degenerated human lumbar spine

Shear load sharing between intervertebral discs and apophyseal joints was investigated experimentally in human lumbar motion segments with moderately degenerated intervertebral discs. 'Motion-Segments' (21-42 years, n=6) and 'Disc-Segments' (22-42 years, n=6) were subjected to shear in 0° flexion, using a modified materials testing machine, while immersed in a Ringer bath at 37 °C. Initially, two cycles of anterior and posterior shear loading up to 200 N (50 N/s) were applied, to evaluate stiffnesses in both directions. Specimens were then exposed to 15 mm of anterior displacement at a rate of 0.5 mm/s. A physiological compressive load of 500 N was applied throughout. The initial 5 mm of the load-displacement curves were approximated with 6th order polynomials for evaluation of the mean behaviour in each group. 'Disc-Segments' were 66% (p=0.002) and 43% (p=0.026) less stiff than 'Motion-Segments' for anterior and posterior shear directions, respectively. 'Disc-Segments' exhibited 44% lower peak shear load (p=0.015) than 'Motion-Segments'. All specimens in the 'Disc-Segments' group showed damage either at the interface between the endplates and the disc. The intervertebral disc contributes 38% to initial anterior shear load-bearing, increasing to 66% at 5 mm displacement. Some over-estimation of disc load-bearing might have been caused by the comparison of segments from different levels. The apophyseal joints make a substantial contribution (65-55%) to anterior shear load-bearing over the initial 2 mm of shear displacement but this decreases with increasing shear displacement.