Aftermarket knees

Who here has one? Wondering about kneeling on it. Doc I am seeing has told me that they have patients that do. Have talked with a few people who said it ain't happening. Any input would be greatly appreciated.
My father had both knees replaced. On the first one he followed all of the Docs advice and did his P/T religiously, but on the second...not so much.

The second knee is two years "newer" than the first, but guess which one he can't kneel on as long, and has less range of motion?

So to answer your question: My 89 year old father can kneel on his knees, but he says it does hurt if he's on them too long.