After my replacements, I tried to hike and felt as if I couldn't breath. They sent me to an allergist who could find no reason for shortness of breath. Then nothing happened until they heard the heart murmur last fall, even though I kept telling them I felt the way my Mom seemed to be with her bad heart valve!
I stopped riding before my knees, really. Mostly it was that after they rebuilt my left shoulder in 1993 I never got my balance back and still sometimes I have some vertigo. The other part is that while I have some great horses, I did not have and still do not have one that has the miles on them for me to re-learn to ride and feel safe.
My rule since 2001 is "No more accidents!" I still keep wearing out body parts, but I can't control that.
Riding with replacements is fine. I have a friend who is 75, has had both knees replaced and her entire backbone worked on (fusions, plates, all sorts of things - all in one operation) and she still rides. She and her husband go for weekend trips, camp out and ride all day long.
She discussed it with her orthopedic surgeon ahead of time and told him if she couldn't ride, it wasn't worth doing. I have no idea if her implants are different than mine but she does great. I know when I was looking at the various implants, there are different ones and some allow more range of movement than others.
Take care with that knee - you need to do something before the muscles get weak from non-use or the ligaments aren't as flexible!