Dynamic fracture characteristics of the osteochondral junction undergoing shear deformation.

This paper presents a biomechanical study of the dynamic fracture response of the osteochondral regions of both immature and mature cartilage-bone laminates induced through shear loading. An instrumental impact machine, providing both mechanical and real-time macro-photographic fracture data, was used to load the osteochondral region in shear by means of a direct compressive force applied to the cartilage layer only, and in a direction parallel to the plane of the osteochondral junction. This force was applied at a known velocity of 2500 mm/s. Our results show that under these conditions of shear loading the dynamic mode II fracture toughness of the osteochondral region of the mature tissue is approximately 1.5 times that of its immature counterpart, and that the derived dynamic shear stiffness of the immature tissue is about 4 times that of the mature. The structural studies demonstrated that the mature tissue delaminates within a well-defined tidemark region whilst the immature fractures through the subchondral bone into which fingers of articular cartilage penetrate. We suggest that this pseudo-brittleness of the immature tissue could explain why there is an increased susceptibility to osteochondral failure in the younger human joint. Finally, we note that the strength-to-toughness relationship, in which stiffer engineering materials are known to exhibit lower fracture toughness, also applies to the cartilage-bone system.