A biomechanical profile across the patellar groove articular cartilage: implications for defining matrix health

Macroscopically normal articular cartilage across all bovine patellar grooves studied exhibited a smooth gradation in mechanical properties. Rigorous standardisation of microtensile and microcompressive testing showed that stiffness in tension of the deep matrix, its tendency to rupture and compressive stiffness all dropped progressively across the medial margin and trough of the groove, and reached their lowest values at approximately one quarter of the distance up the lateral margin. The changes in mechanical properties were correlated with ultrastructural differences. The deep matrix of very stiff tissue from the medial margin showed a dense arrangement of fibril segments orientated with varying degrees of obliquity about a radial mean. The more compliant tissue had a markedly less dense fibrillar array with a pronounced radial orientation. It is suggested that the gradation in mechanical properties results from differential loading of the joint surface. From the available evidence it seems likely that the compliance of the cartilage increases proportionately with the reduction in load. The results are discussed with reference to Broom's (1986b) model of the fibrillar architecture of cartilage. It is proposed that increasing compliance is related to a graduated reduction across the joint surface in the number and/or strength of the interfibrillar bonds, resulting from differential loading. A proportionate number of fibrils would have a reduced number of short-period lateral deflections and thus an increasingly overall radial orientation. This would result in a concomitant graded reduction in the degree of constraint exerted by the three dimensional fibrillar network on the hydrated proteoglycans.