Osteochondritis dissecans
In any joint in the body, the smooth cartilage layer on the end of the bone can separate from the bone it is applied to. This often happens in discrete patches, can include a layer of bone with the cartilage and can happen either spontaneously or after an injury. The separated area can catch or even come away, floating in the joint. OCD is often best treated by arthroscopic (keyhole) surgery to remove the failed area (it usually fails to heal back on) and to improve the surface left.