Mechanical behaviour of articular cartilage under tensile cyclic load

Although fatigue has been implicated in cartilage failure, there are only two published studies in this area, by the same author. However, in these previous studies cartilage was tested in the direction parallel to that of collagen orientation in the superficial layer, where it possesses greater tensile strength. In the present work, articular cartilage was also tested along the direction perpendicular to that of the collagen. Furthermore, the study investigated topographic and zonal variations in the fatigue behaviour of cartilage from the human knee.

Specimens were tested in a specially constructed apparatus that allowed the number of cycles at specimen failure, as well as the load and elongation of the specimen, to be monitored for each specimen. To date, some 72 specimens have been tested, all from the same knee joint, though from different sites and at different depths within the cartilage layer.

The most impressive of the outcomes of this study is the scatter of the data. Considering all the specimens used, the range of number of load cycles to failure was between 2 and 1.5 million. The zonal variation in fatigue behaviour was similar to that in tensile modulus reported previously; the surface and deep layers seemed to have better fatigue properties whether tested in the direction parallel or perpendicular to that of the collagen in the superficial layer. The middle layer was far weaker, suggesting that highly packed and ordered fibres in the surface and deep zones have better mechanical properties than the more random and loose fibres in the middle zone. The variation in fibre organization through the cartilage thickness was also reflected in the differences observed in the elongation of the specimen during the test. The surface and deep zones had a higher stiffness than the middle zone. Cartilage had better fatigue resistance when the specimen was loaded in a direction parallel rather than perpendicular to the collagen within the surface layer. This was true whether specimens were harvested from the superficial, intermediate or deep layer. There were many factors that confounded attempts to estimate the likely fatigue life from the data obtained in such a study.