Knee replacement. X-ray of a prosthetic (artificial) knee joint in the leg of a patient after knee replacement surgery. The prosthesis consists of two implants; one attached to the head of the femur (thigh bone) and one attached to the head of the tibia (shin bone). 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. This knee is being replaced because of a valgus instability, which causes an inward facing knee. For a sequence showing the operation to fit this prosthesis see M551/397-M551/406.