How changes in fibril-level organization correlate with the macrolevel behavior of articular cartilage

The primary structural components of articular cartilage are the zonally differentiated interconnected network of collagen fibrils and proteoglycans, the latter having the potential to bind large amounts of water. Both components exist in a coupled relationship that gives rise to its remarkable mechanical properties. The response of cartilage to compression is governed both by the degree to which the hydrated proteoglycans are constrained within this fibrillar network and the ease with which the matrix fluid can be displaced. The functional properties of cartilage are therefore closely linked to the integrity of the fibrillar network. Our current understanding of this network has been derived via studies conducted at the macro, micro, and ultrastructural levels. Of particular interest to joint researchers and clinicians are issues relating to how the network structure varies both directionally and with zonal depth, how its integrity is maintained via mechanisms of fibril interconnectivity, and how it is modified by ageing, degeneration, and trauma. Physical models have been developed to explore modes of interconnectivity. Combined micromechanical and structural studies confirm the critical role that this interconnectivity must play but detailed descriptions at the molecular level remain elusive. Current computationally based models of cartilage have in some cases implemented the fibrillar component, albeit simplistically, as a separate structure. Considering how important a role fibril network interconnectivity plays in actual tissue structure and mechanical behavior, and especially how it changes with degeneration, a major challenge facing joint tissue modellers is how to incorporate such a feature in their models.