Background

Diurnal loading on the human spine consists of 16hrs loading and 8hrs rest. After an initial load increase, due to rising in the morning, an axial loading is maintained throughout the day. As a consequence subsidence of the intervertebral disc (IVD) occurs during the day while disc height recovers during the night. This behaviour is time dependent (non-linear). In literature different constitutive equations have been used to describe creep. A stretched exponential (Kolraush-Wilson-Watts, KWW) and a double Voight (DV) model have both been used to quantify the creep behaviour. Using these models, time constants and the deformation at equilibrium are estimated. It is unsure whether these different approaches yield to valid predictions. In this study we compared the validity of different equations for the prediction of creep behavior.