IS THE NUCLEUS PULPOSUS A SOLID OR A FLUID - MECHANICAL BEHAVIORS OF THE NUCLEUS PULPOSUS OF THE HUMAN INTERVERTEBRAL DISC

Study Design. A new technique to measure the viscoelastic behavior of the nucleus pulposus in shear was used to assess its solid and fluid c haracteristics. Objectives. To review existing knowledge on mechanical behaviors of the nucleus pulposus,and to develop a new technique to s tudy the viscoelastic behaviors of isolated nucleus pulposus samples i n torsional (pure) shear under transient and dynamic conditions. Summa ry of Background Data. Numerous studies I have investigated the swelli ng behavior of the nucleus and found the swelling pressure to range ap proximately 0.05-3 MPa, depending on loading conditions. Very few stud ies, however, have investigated the loading deformational behaviors of the nucleus pulposus. Methods. Thirteen nondegenerate samples of nucl eus pulposus were harvested from lumbar discs and tested in torsional shear under transient and dynamic test conditions. A linear viscoelast ic law with variable amplitude relaxation spectrum was used to model t he stress relaxation and dynamic frequency sweep experiments. The coef ficients of the viscoelastic law were determined from the stress relax ation experiments, whereas the dynamic shear modulus and phase shift a ngle were determined from the frequency sweep. Results. The nucleus ex hibits significant viscoelastic effects in shear. Under transient cond itions, the stress relaxed to values near zero, which is indicative of the ''fluid-like'' behaviors of the nucleus. Under dynamic conditions , however, the material parameters for the nucleus, magnitude of the c omplex modulus (7-21 kPa), and phase angle (23-31 degrees) were more c haracteristic of a viscoelastic solid. The authors' proposed stress-st rain law exhibited excellent agreement with the viscoelastic data. Con clusions. In response to shear deformations, the nucleus pulposus exhi bited significant viscoelastic effects, characteristic of a fluid and a solid. Whether the nucleus pulposus behaves more as a fluid or a sol id in vivo depends on the rate of loading.