Arthritis. Artwork of a side view of a human knee joint affected by arthritis. The area of pain and inflammation is highlighted in orange. The knee cap (or patella) is at upper centre, between the thigh bone (or femur, upper left), and the lower leg bones, the tibia and fibula (smaller, below tibia). There are many types of arthritis. The two most common are rheumatoid arthritis, in which the body's defences start attacking the soft joint tissues, and osteoarthritis, which is generally caused by wear and tear. Both cause pain and joint stiffness in varying degrees. Treatment is aimed at reducing the symptoms with painkillers and anti-inflammatory drugs.