Fracture behaviour of cartilage-on-bone in response to repeated impact loading.

Cartilage-on-bone composite samples, of differing degrees of matrix stiffness of the cartilage, were subjected to repeated dynamic compression. In all but the most compliant cartilage, fractures always presented at 45 degrees to the articular surface and extended only to the boundary between the intermediate and deep zones of the cartilage matrix. Fractures were not initiated in cartilage of very low matrix stiffness, in which the articular surface and superficial zone are always poorly constructed, nor in high-stiffness cartilage in which the articular surface and superficial zone had been shaved off. It is postulated that an intact superficial layer is necessary for fracture initiation and that propagation of the fracture into the intermediate zone occurs along the planes of maximum resolved shear stress. Prolonged impaction can cause crushing of the underlying subchondral bone, the cartilage remaining structurally and mechanically unchanged. Thus under high rates of loading stress is transmitted to the bone, whilst the cartilage either remains unaffected or fails in a manner characteristic of a brittle material.