Please don't worry about your ROM yet. You are at 90 at less than a month after surgery. My surgeon doesn't expect 90 until about 6 weeks, so you would be ahead of the game in his eyes.
For what it's worth, expecting 90 degrees before you come out of hospital is impossibly optimistic. My knee only bent to about 5 degrees at that stage and my ROM is over 125 now.
Do you have a PT who keeps measuring your ROM and has set targets? That never helps, becasue it just gets you worried.
For what it's worth, your ROM (both flexion and extension) can continue to improve for a year, or even longer, after a knee replacement. There is no need to rush, no deadline by which you need to achieve a certain ROM or all is lost. Your knee won't set at what it is now. It will continue to improve, as long as you don't upset it by over-exercising.
Myth busting: the "window of opportunity" in TKR
Don't get trapped into thinking that you will achieve more ROM by doing more exercising. It doesn't work like that.
It's not exercising that gets you your ROM, it's time. Time to recover, time for swelling and pain to settle and time to heal. Your ROM is there right from the start, just waiting for all that to happen, so it can show itself.
TKR: work “smarter” and not “harder”
[URL='https://bonesmart.org/forum/threads/bonesmart-philosophy-for-post-op-therapy.37103/']BoneSmart philosophy for sensible post op therapy[/URL]
I've had both knees replaced. I did a minimum of PT on one and none at all on the other. Both knees are strong and reliable. They straighten fully and both bend to over 125 degrees.
I travel from New Zealand to England and back again every year. I don't need any ehlp and don't use any walking aids. I can swim well and still love to go snorkeling.
Where you are now is not where you will end up. You are very early in this year-long recovery.
Your knee is still a work in progress.