Well not anything that I have experience with either personally or in close family, but a friend just had HALF one knee replaced, several decades after the first one she had done years ago when her OH was employed by HM Gov there, having knackered em playing hockey to a very high level some good few years previously. She's been suffering a good while so because it would be at least another year before she could get it done by the NHS, and they like all of us weren't getting any younger, she had this one done privately in order to try and enjoy her future with less pain.