There are several causes for knee replacement surgery. Osteoarthritis is the most common cause for a knee replacement. In this case, the knee joint simply wears out -- the cartilage on the ends of the bones wears away and allows the bones to rub against each other causing pain. Previous fractures or mechanical abnormalities can also call for a knee replacement, even if it may be years since the initial problem.
When it has been decided that a knee replacement is necessary, a knee prosthesis is put inside the leg in the place of the knee joint. This is done through an incision in the front of the knee to reach the joint. After reaching the knee joint, a prosthesis made of three parts -- the tibial component (bottom knee portion), the femoral component (top knee portion) and the patellar component (the kneecap).
In this procedure, the knee is measured and cutting guides are used to make perfect cuts for the femoral and tibial components to fit. Some components of the knees are press fit, much like total hips, while others are cemented. Both methods are accepted forms of fixation to bone for stabalization and function.