I believe atrophied is the wrong word. If they were atrophied, I don't think you would have been able to stand or walk for quite a while. Of course, your muscles in your knees weakened some because of your tkrs, but all of us have had to build ourselves back up. It took me months before I was even able to go up and down steps like an adult! And there are times when I'm extra tried my knee feels a little bit weaker. This recovery takes time!
I haven't had the left one replaced yet, just the right. I'm holding off asap! Call me chicken!