Arthritic knee, computer artwork. Knee of a patient with rheumatoid arthritis. At top is the femur (thigh bone), at bottom is the tibia (shinbone), with the smaller fibula to its left. The patella bone (knee cap) sits in front of the femur. Areas coloured orange-brown indicate joint inflammation caused by rheumatoid arthritis. It is caused by the immune system attacking the body's own tissues, causing progressive joint and cartilage destruction. As the cartilage is worn away, new bone grows as part of the repair process. This causes stiffness and deformity of the joint. Treatment is with anti-inflammatory drugs and physiotherapy. In severe cases the joint may be replaced with a prosthetic knee.