The big thing to add to the cost of having a human-sourced knee is being on immunosuppressant drugs for the rest of your life, which is a significant health risk (but significantly less than, say, not having a functional critical organ). There are processes to clean some transplants, but I'm not certain they work on anything other than the simplest of structures (e.g. pig heart valves)

Corneas are the exception: they don't provoke an immune response.