If it is done higher on the leg (like for example in this book The Running Dream, Jessica's prosthetic leg was also for her thigh where it had been cut off.) Then they would have to have a bigger or longer leg.
When the limb is affected it can be either broken or in need of amputation. Jessica got in a terrible accident so she had to get her arm amputated but that doesn't always happen. She was lucky she didn't have death as her result.