Modelling creep behaviour of the human intervertebral disc.

The mechanical behaviour of an intervertebral disc is time dependent. In literature different constitutive equations have been used to describe creep. It is unsure whether these different approaches yield valid predictions. In this study, we compared the validity of different equations for the prediction of creep behaviour. To this end, human thoracic discs were preloaded at 0.1 MPa for 12h, compressed (0.8 MPa) for 24h and finally unloaded (0.1 MPa) for 24h. A Kohlrausch-Williams-Watts (KWW) model and a Double-Voight (DV) model were fitted to the creep data. Model parameters were calculated for test durations of 4, 8, 12, 16, 20 and 24h. Both models described the measured data well, but parameters were highly sensitive to test duration. The estimated time constant varied with test duration from 3.6 to 17h. When extrapolating beyond test duration, the DV model under-estimated and the KWW model over-estimated creep. The 24h experiment was still too short for an accurate determination of the parameters. Therefore, parameters obtained in this paper can be used to describe normal behaviour, but are not suitable for extrapolation beyond the test duration.