Warm-up for knees

I am returning to Aikido after a three year break. I had knee surgery a bit over a year ago for a torn ACL, torn PCL, and other ligament damage. Needless to say I am out of shape and a bit nervous about my return. I know to take the training part slow. I spoke with my sensei about needing to relearn how to roll, pivot, etc over again and may not be able to do swari waza. My ROM is excellent (-10 extension, flexion can get my knee to my butt and a bit more: equal to other knee)and one lession i HAVE learned though PT is to listen to my body. However, does anyone have good suggestions for warm up stretches to get myself ready?
thanks so much

Wow! I can't get my knee to my butt with intact ligaments--damn femur gets in the way.

I'm not a medical professional, so take the following with a grain of salt and consult with your favorite MD or PT. I might suggest warming up with slow, aikido-relevant range-of-motion exercises, rather than stretches. As I recall, Michael Friedl from Aikido of Ashland put together a DVD a couple years ago with solo katas of aikido movements done at ultraslow speed. He used such katas as part of the warm-up at a couple of his seminars at the University of Iowa. They seemed nice for developing range of motion, stability through motion, and relevant strength.

welcome back michelle.
I strongly recommend that you leave suwariwaza out of your repertoire. with the internal damage yo have sustained, there are no doubt microtears both in your menisci and in your articular cartilage, and suwariwaza will create further friction/wear and tear that you just don't need.
my suggestion is to focus warmup on slow evenly paced gross muscle movement (like range of motion), which is a true 'warm up' for the muscles and once warmed up do very gentle stretching; save the real stretching for after class. what charles describes is pretty typical for dojos that came from a ki society/tohei background: warmups that are slow repetitive versions of the footwork and turning we do in our common techniques (for instance, stepping into ikkyo, pivoting into kokyunage, sliding laterally into sokumen iriminage, etc). i agree that these are a good and safe warmup.