Histological Assessment of Cartilage Repair: A Report by the Histology Endpoint Committee of the International Cartilage Repair Society (ICRS)

Physical injury frequently causes tissue damage, including laceration. Repair of the damage usually results in the formation of a scar; complete anatomic healing and true regeneration are rare. Connective tissues tend to heal naturally and successfully only if the injury is minor. If the damage is more severe, then a good functional result can be achieved only if Nature is assisted by surgical intervention. The efficacy of such measures has been established in the cases of bone and tendon injuries but not in the case of cartilage damage 1. In the latter situation, we are still prejudiced by Hippocrates' opinion that "ulcerated cartilage is universally allowed to be a troublesome disease." 2 In addition, our view is necessarily colored by the scarcity of successful therapeutic modalities 3. Articular cartilage is a narrow layer of specialized connective tissue that permits smooth, frictionless movement of diarthrodial joints. It is comprised of a relatively small number of cells (chondrocytes) embedded in an abundant extracellular matrix 4. The latter consists predominantly of type-II collagen, proteoglycans, and water, along with smaller amounts of other collagen types and noncollagenous proteins. Histologically, articular cartilage is divided into three zones, which are distinguished by the shape of the chondrocytes and the arrangement of type-II collagen fibers. The superficial zone is characterized by flattened disc-like chondrocytes, a low proteoglycan content, and densely-packed, horizontally-arranged collagen fibrils of uniform diameter. This layer has been described as a tension-resisting diaphragm 5 by virtue of its tendency to curl when the articular cartilage is released from the subchondral bone 6. In the middle zone, chondrocytes attain a more rounded profile, proteoglycan content increases, and the collagen fibers decussate to provide an oblique transitional network between the superficial tangential zone and the deep radial zone. The deep radial zone is …