Structural relationship between articular cartilage and subchondral bone - A critical frontier in joint homeostasis and osteoarthritis.

The osteochondral unit is formed by the articular cartilage, calcified cartilage and subchondral bone. They structurally and functionally communicate with each other in a mechano-adapted fashion, in joint homeostasis and osteoarthritis (OA). The tibial plateau is in the focus of many anatomical and translational studies because it enables knee stability and function and is frequently affected by OA. The normal human tibial plateau has a thin and porous subchondral bone plate, covered by a considerably thicker articular cartilage. Fundamental topographical relationships exist between many components of a normal ovine osteochondral unit in the central region. The absence of such correlations in the submeniscal (peripheral) region highlights the clinical importance of the meniscus within this osteochondral-meniscal unit. In OA, pathological remodeling of the subchondral bone is associated with enhanced articular cartilage degradation in a location-dependent fashion. Early OA established in sheep following a defined meniscus injury reflects the important clinical fact that human knee OA is spatially heterogeneous. Many previously normal correlations within an osteochondral unit are reduced in OA. Varus alignment of the leg in the frontal plane is characterized by site-specific effects of OA on the osteochondral unit of the lesser loaded lateral compartment. Therapeutic surgical unloading has shown its value both in translational models and clinical studies to be capable of shifting the osteochondral phenotype towards normal, restoring several disturbed osteochondral correlations. This concept of spatial osteochondral patterning emphasizes the importance of understanding how its individual parts are interconnected at different levels in joint homeostasis and OA.