A human adult body has the ability to repair a bone on its own. Even the medical treatment that is provided to a patient usually is only to ensure that the process is faster and the new bone is not deformed in any way.
When a bone breaks, the blood vessels constrict to stop the flow of blood and a blood clot is formed. New tissues grow from each end of the fractured bone and unites with those from the other end. This tissue is then replaced by a form of tissue that can create the same structure as the original bone. It is a substitute for the original bone that was broken. Once the process is complete, the new bone created is as good as the original bone.