Being at the university, I get a lot of difficult cases sent to me. Some of the challenges concern how to replace bone and how to treat fractures around the artificial components.
Infections are another concern.
Although they happen infrequently, getting rid of infections in these joints usually takes multiple operations. Only a small percentage of hip replacements result in problems, but since half a million of them are done in this country every year, even a small percentage of problems is a lot of problems. As treatment gets better and better, the problems become more challenging. So there's always a new problem to solve. I'll think I've seen it all, and the next week I'll see something new.