Osteoarthritis, or “wear and tear” arthritis, is a common degenerative disease often seen in older people. It can affect any of the body’s joints, but is most common in load-bearing joints like the knees and hips. Osteoarthritis of the knee happens when the cartilage that cushions the bones of the knee joint wears down or tears, causing pain and stiffness and making it hard to bend or straighten the leg. Aging is one cause, but repetitive sports and injury often are at fault.