The flexion exercise you describe is very good. Another way to do this is to put on a pair of socks and sit straight up in a frame chair. Put your non-operated door/leg over the top of the foot on the TKR leg. Pull the TKR leg in as far as possible, until you feel the strain of resistance. Held for a bit, release and repeat 10 times, 3 times a day or as you can tolerate. If pain isn't bad, doing more of these can help.