Leg extensions are really good for building the muscle around the knee. Be careful with those squats though. I've never had knee problems, but I've heard that squats can be hard on the knees. Leg press machines may be a little safer.
I would do partial squats with light weight and lots of reps. alo straight leg lifts with ankle weights. I do 3 sets of 20. I am recovering from a full lateral release. Do not do leg extentions, my ortho told me they were a big no, hardest thing on the knees. Ask you dr for some exercises to do.