Confusion.....

I had my right knee replaced about 10 years ago. Total recovery took about 3 months give or take a week or two. Therapy was around 6 weeks ( Pay particular attention to range of motion exercise ) I can't kneel on the replacement knee at all . I don't recall a difference in the way I walked but I don't have the Back problems you do.
Just curious...why can't you kneel on the aftermarket knee? Pain?