LIMITATIONS OF THE STANDARD LINEAR SOLID MODEL OF INTERVERTEBRAL DISCS SUBJECT TO PROLONGED LOADING AND LOW-FREQUENCY VIBRATION IN AXIAL-COMPRESSION

The purpose of this study was to answer the following questions: (1) C an the standard linear solid model for viscoelastic material simulate the influence of disc level and degeneration on the ability of a disc to withstand prolonged loading and low-frequency vibration? (2) How we ll does the SLS model explain the relationship between the ability of a disc to resist prolonged loading and its ability to resist dynamic l oads and dissipate energy when subjected to low-frequency vibration? R esponses of human thoracic and lumbar discs were measured in axial com pression under a constant load, and for cyclic deformations at three f requencies. Parameters of the SLS model for each disc were determined by a least-squares fit to the experimental creep response. The model w as subsequently used to predict the disc's response to cyclic deformat ions. The SLS model was able to qualitatively simulate the effects of disc level and degeneration on the ability of an intervertebral disc t o resist both prolonged loading and low-frequency vibration. However, the model underestimated the stress relaxation, dynamic modulus and hy steresis of thoracic and lumbar discs subjected to low-frequency vibra tion. The SLS model was unable to explain the relationship between the ability of a disc to resist prolonged loading and its ability to resi st dynamic loads and dissipate energy when subjected to low-frequency vibration. Although in the lumbar discs the steady-state predictions o f the SLS model were significantly correlated to the experimental resp onse, the strength of model predictions decreased with increasing freq uency, particularly for hysteresis.