It does not affect the internal organs. The cause of knee osteoarthritis is not known. It is thought to be simply a process of wear and tear in most cases. Some conditions may predispose the knee to osteoarthritis, for example, a previous fracture that involved the joint, or by lesser injuries that may have torn ligaments or menisci. Abnormalities in development of the knee bones, such as bow legs, may cause the knee to wear out sooner than normal.
Bare bones grind against each other and cause mechanical pain .
Fragments of cartilage floating in the joint may cause inflammation in the joint lining , and this is a second source of pain. X-rays show the joint space to be narrowed and irregular in outline. There is no blood test for osteoarthritis. The cause is not known. It eventually destroys the joint cartilage.