Osteoarthritis is the most common cause of pain and disability in knees. In the knee joint, smooth articulate cartilage, called surface cartilage, covers the ends of the femur (thigh bone) and tibia (shin bone). Between the two bones sits a second type of cartilage, called menisci, which acts as a shock absorber. Joint fluid also lubricates the knee joint.
Osteoarthritis (OA) starts as the lack or loss of surface cartilage, progressively involving the surrounding bone, tissues and synovial fluid. In OA, your knee cartilage may thin in spots or disappear completely, resulting in areas of exposed bone.