Fused knee joint. Coloured X-ray of a knee joint showing fusion of the bones. The femur (thigh bone, top) is touching the tibia (shin bone, lower right), and the bones have grown together. This leads to immobility of the joint. At lower left is the fibula, the smaller lower leg bone. The fusion of a joint can be a symptom of severe arthritis. It usually occurs in the back (where it is known as ankylosing spondylitis), but can occur in the knee or hip joints, as well as others. Arthritis (joint inflammation) can have many causes. Treatment includes anti-inflammatory drugs.