Mechanical response of the lumbar intervertebral joint under physiological (complex) loading.

Nineteen fresh, intact lumbar intervertebral joints (two vertebrae and the intervening joint) from twelve spines were loaded in a materials testing machine to determine their mechanical behavior. The loads applied were pure axial compression and complex loading conditions simulating physiological states. The measurements made included vertical load deflection, disc bulge, and strains in both the anterior and the lateral aspects of the vertebral body and in one lamina. The results showed that the posterior elements transmit considerable force during quasistatic complex loading, particularly in extension and frontal shear. When a healthy specimen is subjected to complex loading, "yielding or failure" occurs in the vertebral body and not in the annulus fibrosus of the disc.