Cartilage exists at the surface of the ends of the bones and provides joints with a gliding surface and shock absorber during activities of daily living. Osteoarthritis causes the cartilage layer to break down and wear away, exposing raw bone. It also causes changes in the bone underlying the articular cartilage (subchondral bone) and makes this bone stiffer and less adaptable to load. In addition this progressive wear leads to recurrent inflammation of the inner lining of the joint (synovium). This inflammation can cause some of the pain felt with osteoarthritis, and it also produces a lot of fluid in the joint that, in itself, is detrimental to the remaining articular cartilage. If there is a lot of fluid buildup it can lead to cysts that form, often in the back of the knee. These cysts are called a “Baker’s Cyst”.
As a result of the cartilage loss, the progressive wear of the bone and in response to the inflammatory changes there will be additional bone formation along the edges of the joint. These so-called “osteophytes” may be the bodies’ attempt to increase the weight bearing surface of the joint (and thus decrease the maximum load transmitted through the joint) but it is also the main reason for the progressive stiffness that develops during the course of osteoarthritis. Other changes occur throughout the course of this disease that affect the subtlety and flexibility of the joint capsule, the muscle and tendons around the affected joint and contribute to the, often-felt stiffness.