Biphasic rheology of different artificial degenerated intervertebral discs

Simulation of the intervertebral disc (IVD) degeneration in animal models is of great interest towards exploration and evaluation of potential regenerative therapeutics. Hence, the objective of this study was to investigate the biphaisc response of the IVD for different artificial degeneration models. Fifty motion segments were dissected from juvenile sheep lumbar spines. The specimens were assigned equally into 5 groups (i.e., (1) intact (I), (2) punctured with a 16-G needle (P), (3) punctured with a 16-G needle combined with fatigue loading (PF), (4) denatured by injecting 0.5 ml 0.25% trypsin solution (T), and (5) denatured by injecting 0.5 ml 0.25% trypsin solution combined with fatigue loading (TF)). Specimens were mounted in a chamber filled with phosphate buffered saline and underwent a stress-relaxation test. Based on linear biphasic theory, the aggregate modulus (H_A) and permeability (k) were extracted. Aggregate modulus decreased in P and T discs but increased in PF discs as compared to intact ones. The difference of the aggregate modulus between TF and intact discs was not significant. Permeability decreased in T, PF and TF discs. The permeability of both PF and TF discs was significantly lower than F and T discs, respectively. It is concluded that proposed artificial degeneration models can be used to investigate potential regenerative therapeutics.