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.