Effects of disc injury on mechanical behavior of the human spine.

The effects of injury to the intervertebral disc were investigated using three-dimensional flexibility and creep measurements of functional spinal units from fresh cadaver lumbar spines. The techniques utilized were accurate and the data had a high degree of reproducibility. An injury to the annulus and a removal of the nucleus significantly altered the mechanical properties of the spinal unit. Not only were the main motions affected but also the coupled motions. Sagittal plane symmetry was disturbed, resulting in asymmetric facet joint movements. These effects of injury could be measured because of the three-dimensionality of the experiments. Previous studies, utilizing only axial compression loading, claimed to observe no changes due to the disc injuries and are, therefore, in conflict with the present findings.