What I have concluded is that I have never, until this surgery, heard so much about ROM and the need or obsessiveness to achieve this until I had a tkr and of course this then puts the patient (you and me) in that state of panic to get it. Many Doctors and PT push us for the numbers and even sometimes we push ourselves to hard because we compare ourselves to other people's numbers.
I don't know who started the idea, or why, but a lot of surgeons and PT therapists seem to think that you need to rush to get ROM. You don't, because ROM can continue to improve for a year, or even longer, after a TKR.
Myth busting: the "window of opportunity" in TKR
As you've realised, all this emphasis on ROM does cause worry - and it's so pointless. Your knee will bend when it's ready to bend - and that is when the swelling has gone down enough and your knee is getting over the trauma of the surgery.
I think the PTs get obsessive about ROM, because it's a measurable thing that they can use, to show the insurance companies that their ministrations have been effective. It's a shame, though, that all the measuring causes so much worry.
I was fortunate, because my surgeon never asks about ROM. He just looks at my knee and says "Show me what you can do."
So now I am here with a tkr and am slowly recovering. I will not have the ROM that others have because my knee cannot physically do that because of my patella and that is ok. This surgery was so much more extensive than my other surgeries so if it takes 6 months to a year for all this soft tissue to heal than so be it. I am writing this to remind myself that this is a journey not a race and why should I think I can run before I walk. The mantras on this site are true and sound and my past recoveries are proof of that. ROM is there it's just waiting for the knee to heal so it can reveal itself.
You're very wise to realise that your knee may need longer than most, because of the previous surgeries. I'm sure it will get there, even if its journey is a little slower.
Here's a little picture for you: