MODEL RELEASED. Knee replacement surgery. Image 8 of 10. Surgeon inserting a hinge pin into a prosthetic knee implant during knee replacement surgery. The pin is inserted into the implant in the femur (thigh bone). Another implant (white) is fitted to the tibia (shin bone) to create 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 see M551/397-M551/406.