The Osteochondral Unit: The Importance of the Underlying Subchondral Bone

Abstract

The term osteochondral unit reflects the tight functional association of the articular cartilage, calcified cartilage, and the subchondral bone. Due to its special composition of collagen type II, aggrecan, and water, the extracellular matrix of the articular cartilage provides exceptional tensile strength and compressive resilience to the tissue. A radiologically denser, discrete band of mineralized cartilage, the tidemark separates the articular cartilage from the calcified cartilage. The hydroxyapatite-containing calcified cartilage forms an undulating transitional zone, attaching the cartilage to the subchondral bone. The subchondral bone plays a key role in mechanically and metabolically supporting the articular cartilage, maintaining the joint shape, and absorbing shock. It consists of two regions: the subchondral bone plate is a dense bony lamella which merges into a network of trabecular bone called the subarticular spongiosa. The cavities of the subchondral bone contain blood vessels which are important sources of nutrients and signaling molecules for the deeper layers of cartilage. Several diseases affect the osteochondral unit. The most well-known of them is osteoarthritis (OA), a disease of the entire joint including the cartilage, subchondral bone, and other structures. Avascular necrosis and osteochondritis dissecans are diseases primarily affecting the subchondral bone, often leading to OA and focal cartilage defects. In these diseases and during the repair of osteochondral defects, phenomena like subchondral bone cysts, bone marrow edema, and intralesional osteophytes are common findings in both patients and translational animal models. Currently all “cartilage restoration” strategies now recognize the importance of considering the entire osteochondral unit during implementation, to avoid chondral deterioration or even delamination.