Knee replacement. Coloured X-ray of a side view of a prosthetic (artificial) knee joint (red) in the leg of a male patient. The prosthesis consists of two implants; one attached to the head of the femur (thigh bone, top) and one attached to the head of the tibia (shin bone, bottom). This creates a hinge joint that articulates in exactly the same way as a normal knee joint. Joints may need to be replaced after injury, disease, for example arthritis, or deformity.