The articular surfaces of the knee joint are covered with a layer of cartilage. If this gradually wears out, the diagnosis is: knee osteoarthritis. Doctors also refer to this joint wear as gonarthrosis. It is one of the most common signs of wear on the joints. The cartilage damage in the knee increases the pressure on the underlying bone. To compensate, the articular surface increases and bony peaks form at the edge. As a result, the knee joint becomes less flexible and the cartilage damage increases. The bones move closer together and the joint gap narrows further. When the bones lie almost unprotected on one another in the late stage and the joint gap has completely disappeared, the knee becomes immobile and stiff.