The interaction of the zone of calcified cartilage and subchondral bone in osteoarthritis.

The zone of calcified cartilage (ZCC) forms an important interface between cartilage and bone for transmitting force, attaching cartilage to bone, and limiting diffusion from bone to the deeper layers of cartilage. The height of the ZCC is a relatively constant percent of articular cartilage and the height is maintained by a balance between progression of the tidemark into the unmineralized cartilage and changing into bone by vascular invasion and bony remodeling. During its formation, the cells that form the ZCC have properties similar to the cells of the growth plate. In the adult, the ZCC becomes quiescent but not inactive. The ZCC may be reactivated in osteoarthritis and may progressively calcify the unmineralized cartilage. This might contribute to cartilage thinning which would increase the concentration of forces across the uncalcified cartilage leading to more damage. Although the subchondral bony plate remodels extensively in osteoarthritis, there is little evidence that a change in the biomechanics of the plate directly initiates the osteoarthritic process in cartilage. However, increased repair by endochondral ossification of vertical cracks in the ZCC that penetrate into the marrow space could contribute to progression via changes in the ZCC.