Injuries that disrupt articular cartilage and the underlying subchondral bone are referred to as osteochondral fractures.
Mechanical injury that disrupts bone as well as articular cartilage causes hemorrhage, fibrin clot formation, and inflammation.19 Soon after creation of the osteochondral defect, a fibrin clot fills the injury site and inflammatory cells migrate into the clot. Injury to bone and subsequent clot formation release growth factors and proteins that influence multiple cell functions, including migration, proliferation, and differentiation. Bone matrix contains a number of growth factors, and platelets release at least two important growth factors: platelet-derived growth factor and transforming growth factor-β. Cells within the bony portion of the osteochondral defect form immature bone, fibrous tissue, and cartilage with a hyaline matrix. This bone formation restores the original volume of injured subchondral bone but rarely, if ever, progresses into the chondral portion of the defect. In general, by 6 months after injury, the subchondral bone defect has been repaired with a tissue that is primarily bone but also contains some regions of fibrous tissue and hyaline cartilage. In contrast, the chondral defect is not repaired completely and does not restore the normal structure, composition, or mechanical properties of an articular surface. In most injuries, the chondral repair tissue begins to show evidence of fibrillation and loss of chondrocytes and hyaline matrix in less than 1 year. However, the fate of cartilage repair tissue is not always progressive deterioration. Occasionally, the repair tissue persists and appears to function satisfactorily as an articular surface for a prolonged period of time. The reasons why some repair tissue persists for a prolonged period of time while most repair tissue deteriorates remain unknown. Because repair tissue can provide a functional surface in some instances, many orthopaedic surgeons will drill, puncture, or abrade below the tidemark and stimulate bleeding of the bone in an attempt to restore the articular surface. Of course, the repair tissue is fibrocartilage, not hyaline cartilage. Fibrocartilage is a form of cartilage that is less resilient, less smooth, and less suited to load bearing.