Friend had hip repl & diag of pigmented villonodular synovitis. chances of reoccur/prevention or known causes? is synovectomy usually perf at op?

Yes: In a total hip replacement, both sides of the hip are replaced, essentially removing the entire joint including the synovium. As such, it would be very unlikey it would recur after a total hip replacement. Thank you for your question.

Possible: If the diagnosis was known at the time of joint replacement, then the surgeon would likely have performed as complete a synovectomy as possible. However, it is not possible to remove all of the synovium, even when a hip replacement is done, and therefore many orthopaedic oncologists recommend adjuvant radiation therapy, which could adversely affect the hip replacement. An expert is necessary!