Unfortunately due to my career before the net, I've had 50% of the cartilage cut out of each knee. What's interesting, is that my knees give me more hassle than my uncle deals with on his knee that was replaced. (My knees were done over ten years ago and his was done within the last 6 months) The recovery time was considerably shorter as well. I've known a few others that have had the replacements done as well and all have had no issues with it.
Good luck to her
