Osteoarthrosis (OA) is a joint disease with a high prevalence, especially among elderly people. In an osteoarthrotic joint, articular cartilage undergoes structural changes leading to impairment of the mechanical behavior of the tissue. Subsequently, joint motion becomes gradually impaired and this is accompanied by an increase of pain. The finite element (FE) technique has been used to reveal structure-function relationships in normal and osteoarthrotic cartilage. This technique has increased our knowledge of cartilage function as a loadbearing tissue and of the changes in the mechanical properties of cartilage in OA. However, most of the previous studies have utilized a simplified tissue structure and failed to model the more realistic, inhomogenous structure or anisotropic properties of the cartilage.

In this study, inhomogenous and anisotropic poroelastic models of articular cartilage were developed and applied for the prediction of the mechanical response of normal or experimentally degraded bovine or canine articular cartilage. In the FE analyses, experimental mechanical measurements were simulated in unconfined compression and indentation geometries by using different loading protocols and loading rates. Special emphasis was paid on the clarification of the significance of superficial cartilage and the compression-tension nonlinearity for the mechanical response of the tissue. Polarized light microscopy was used to provide structural information about the collagen network for the models. The FE technique was also applied for the validation of a novel configuration of the arthroscopic indentation instrument.

Experimental tests showed that, by using the elastic isotropic theory, the equilibrium Young's modulus of articular cartilage was significantly higher when measured in indentation that in unconfined compression geometry. The FE predictions suggested that the high tensile stiffness of the superficial fibrillar network parallel to the articular surface accounted for this difference. In addition, the thickness of the superficial cartilage zone with a high transverse Young's modulus and shear modulus modified significantly the indentation response. Fibril reinforced poroelastic model was able to predict the experimentally found stress-relaxation behavior of cartilage after specific degradation of solid matrix constituents, i.e. collagen and proteoglycans. Due to the nonlinear fibrillar network, the model could depict the compression-tension nonlinearity in the direction perpendicular to the articular surface. The transversely isotropic poroelastic model suggested that the novel arthroscopic indentation instrument was particularly sensitive for detecting early degenerative changes in the superficial tissue.

To conclude, the inhomogenous structure and the anisotropic mechanical properties of articular cartilage can significantly modify the response of the tissue under compression, as shown and highlighted by the fibril reinforced and transversely isotropic poroelastic models. The models which encompass compression-tension nonlinear capabilities are able to predict closely the structure function relationships of cartilage under different loading geometries and protocols, and can be of assistance in understanding the mechanical consequences of the cartilage degeneration which takes place during the development of OA.