hi,
I had the same problem, my bucket handle tear slipped into the joint therefor causing it to lock (not fully extend. I also had a problem before my acl recon but doc said it would be ok because after they do it they bend it and straighten it a bunch of times, while your out of course

and then the physical therapy after helps with that alot. If you know you don't have cartiladge that slipped into your joint you can lay straight on your back and take a long band of something, like a scarf and wrap the middle of it around your foot, crisscross it over the front of your foot, extend the leg up and pull the foot towards you (while keeping it straight) this will help straighten you leg. i did 10 reps for about 30 seconds each 2 times a day.