One of the most common fractures that can happen to complicate a knee replacement is a fracture in the femur, or thigh bone. Many patients who have such a fracture don't have any pain or experience any problems with their knee. In fact, often the fracture is only found by the doctor during a routine check up and x-rays.
If your father's fracture isn't causing any pain or discomfort and the knee is working well, the doctor takes that into consideration when deciding on treatment. As well, when the fracture is found on the x-ray, it is possible that it has already healed and it is an older fracture that the doctor saw. In this case, there is no reason to perform any repairs.