You mentioned "heavy" squats. What about higher rep/lighter weight squats? And let me be clear that I'm speaking of old fashioned barbell squats. I've had three knee surgeries,( one due to a car accident) and I perform barbell squats and leg extensions, and both exercises have played a big part in my total knee rehabilitation. But I stay away from hack squats and smith machine squats. And I also make sure to put equal effort into building the hamstrings with leg curls, as well as the calves with standing calf raises. The knee joint needs to be equally supported on all sides by these other muscle groups. otherwise, you can create an imbalance. In addition to this, I would suggest trying weighted lunges, (as long as you already have a decent degree of quad development built up from these other exercises).