Internal displacement distribution from in vitro loading of human thoracic and lumbar spinal motion segments: experimental results and theoretical predictions.

Small metal markers are implanted within intervertebral discs and their displacements in response to a complex load (flexion, compression, and anterior shear) are measured radiographically. These are contrasted to the displacements predicted by a finite element model (FEM) that uses 20 constant-strain triangular elements and is based upon a linear elastic isotropic material. The central portion of the disc (nucleus pulposus) moves posteriorly and oppositely to the FEM prediction. The anterior and posterior portions of the disc agree more closely with the FEM than the central portions of the disc. In general, the FEM predicts much more accurately the up-down displacement components than it does the anterior-posterior components. The measured displacements provide a new class of information concerning the function of the interior of the disc, and also provide a new basis for validation of FEMs that attempt to mimic real intervertebral disc behavior. Implications for understanding of disc function and pathology are discussed.