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 200N (50N/s) were applied, to evaluate stiffnesses in both directions. Specimens were then exposed to 15mm of anterior displacement at a rate of 0.5mm/s. A physiological compressive load of 500N was applied throughout. The initial 5mm 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 5mm 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 2mm of shear displacement but this decreases with increasing shear displacement.
