Ratcheting Behavior of Intervertebral Discs Under Cyclic Compression: Experiment and Prediction

Abstract

Objective: To evaluate the ratcheting behavior of intervertebral discs (IVD) by experiments and theoretical study. Method: The lumbar spines of sheep were obtained at a local slaughterhouse, and the IVD was processed with upper and lower vertebral bodies (about 5 mm) to ensure the mechanical state of the IVD in situ. The ratcheting tests of uniaxial cyclic compression loading for disc samples is carried out using the Electronic Universal Fatigue Testing System at room temperature. The effects of different stress variations, stress rates, as well as different segments on ratcheting behavior of discs were investigated. Results: The ratcheting strain evolution of lumbar IVD include stages of sharp increase and asymptotic stability. Both the ratcheting strain and ratcheting strain rate increase with an increase of stress variation (R = 0.962, P = 0.004) but decrease with an increase of the stress rate (R = −0.876, P = 0.019). Compression stiffness increases with an increase of the stress rate (R = 0.964, P = 0.004) or stress variation (R = 0.838, P = 0.037). Compared with L5–6, the L6–7 disc showed less ratcheting strain (P = 0.04), indicating that the disc at this segment was more resistant to the impact of the ratcheting cycle. In addition, ratcheting strain evolution was predicted using a ratcheting evolution constitutive equation, and the predicted results were in good agreement with experimental data. Conclusions: The ratcheting behavior occurs in IVD, and this cumulative deformation is consistent with the general ratcheting behavior. The constitutive equation can predict the ratcheting strain evolution of IVD very well. These results are of great significance for the analysis of defects and the development of repair in IVD.