THE VISCOELASTIC BEHAVIOR OF THE NONDEGENERATE HUMAN LUMBAR NUCLEUS PULPOSUS IN SHEAR

The viscoelastic behavior of the nucleus pulposus was determined in sh ear under transient and dynamic conditions and was modeled using a lin ear viscoelastic model with a variable amplitude relaxation spectrum. During stress-relaxation tests, the shear stress of the nucleus pulpos us relaxed nearly to zero indicative of the fluid nature of the tissue . Under dynamic conditions, however, the nucleus pulposus exhibited pr edominantly 'solid-like' behavior with values for dynamic modulus (\G \) ranging from 7 to 20 kPa and loss angle (delta) ranging from 23 to 30 degrees over the range of angular frequencies tested (1-100 rad s(- 1)). This frequency-sensitive viscoelastic behavior is likely to be re lated to the highly polydisperse populations of nucleus pulposus molec ular constituents. The stress-relaxation behavior, which was not linea r on a semi-log plot (in the range tau(1) much less than t much less t han tau(2)), required a variable amplitude relaxation spectrum capable of describing this frequency sensitive behavior. The stress-relaxatio n behavior was well described by this linear viscoelastic model with v ariable amplitude relaxation spectrum; however, the dynamic moduli wer e underpredicted by the model which may be related to non-linearities in the material behavior. (C) 1997 Elsevier Science Ltd.