Mechanical function of facet joints in the lumbar spine.

Sections of human cadaver lumbar spines consisting of the L3, L4 and L5 vertebrae and the intervening discs and ligaments were tested mechanically to determine the effects of simulated spondylolysis and of facet joint fusion. Compression, flexion and extension, lateral bending, axial rotation forces and torques were applied to intact specimens and then after unilateral and bilateral division of the pars interarticularis, and, in a separate group of specimens, after unilateral and bilateral immobilization of the facet joints. Division of the pars interarticularis caused a large increase in axial rotation, and a lesser increase in lateral bending. Other motions were not statistically significantly changed. Facet fixation caused a statistically significant decrease in flexion and extension only. The average anterior or posterior shear motion across the intervertebral discs was less than 1 mm in magnitude in intact specimens, and none of the interventions produced statistically significant changes in this motion accompanying the angular motion.