I have just been able to kneel on a hard surface recently, nearly two years on from my first TKR, and only momentarily at that. Getting some of your weight onto your hands as quickly as possible helps. I am going to try having a bath using one of those firm foam kneeling pads you get from gardening shops. You won't damage anything, I checked this with my OS.
Here's a tip for de-sensitising your scars after TKR. Tap your knees and just below, on and around your scars, in time with the music if you are listening to any. Make it hard enough to be slightly uncomfortable but not hard enough to hurt. Only do this for a minute or two at a time, but several times a day. When I am driving and get to a red light, I tense my quads for a count of 10 (separately) then start tapping my scars until the lights change to green. It seems to have worked over a period of time.