On the Compressive Stress Relaxation Response of Cartilage as Influenced by Early Degenerative Change

Abstract

It has recently been proposed that the stress relaxation response to indentation of articular cartilage can be divided into two relaxation responses: an initial, primarily viscous, decay and a latter, primarily elastic, decay. Along with the use of a specialised indentation method and imaging of the deformation response, this convenient division of the stress relaxation curve allowed for the mechanical behaviour to be more directly linked with the microstructural response. The aim of this new research is to apply these methods to characterise, and gain new insight into, the mechanical behaviour and microstructural response of cartilage over the course of its early stage degeneration. Cartilage-on-bone samples were extracted from 44 bovine patellae representing a range of tissue states, from healthy to early degenerative. Each sample was subjected to an indentation stress relaxation protocol, involving three sequential steps of increasing nominal strain and relaxation time. The loading rate was 0.02 mm s-1. Each mechanical response was divided into three components for analysis: (I) a driven response (modulus obtained from the ramp phase), (II) an initial relaxation response, and (III) a latter relaxation response. Cartilage microstructure was characterised using differential interference contrast (DIC) microscopy, as was the microstructure of the osteochondral junction. With increasing tissue degeneration the driven modulus decreased, indicating an increased compliance. However the following initial relaxation time was not significantly affected by tissue degeneration. It is suggested that the increased compliance during the ramp phase may have caused this ‘offset’. Together with the microstructural data, these findings thus demonstrated the interrelationship between initial pore pressure and permeability in the immediate viscous response of cartilage during stress relaxation. Finally, in the degenerate tissue, there was an increased loss of fibrillar interconnectivity that was associated with a decreased τ2 in the latter relaxation response. It is suggested that the de-structured fibrillar network results in less restriction to the movement of solid phase components approaching final equilibrium of the compressed cartilage matrix.