I suffered osteo-arthritis in my left knee for years. I changed to an automatic (Ford Focus at that time) for this reason about six years ago.
When we discovered Jazzes about five years ago it was therefore a no-brainer to go CVT, and we currently have a Mk1 and a Mk3 jazz both CVT.
I had a TKR (total knee replacement) last October. Three months later I can walk several miles without pain, without a stick and without a limp.
It's not all roses though. I have 110 degree bend which is good but not enough to cycle and it does make getting out of the car in tight spaces a bit awkward. Also some pain at night. Everyone says it takes a year to fully settle.
The only take away from my experience is that if you are thinking of changing your car for knee problems, with a TKR in your future, it might be worth thinking about one which is easier to get in and out. The Jazz is better than many cars but there are better, with higher level seats so you slide in rather than down. Don't get me wrong I am not going to change my Mk 3 for this reason, but if I was buying now it would be a factor.