+1 on getting professional advice. Knee health is so closely tied to foot and ankle strength and flexibility, and with your history of ankle surgery, you just may not be able to come up with a foot strike that protects your knees.

Still, finding the right professional isn't always easy. So many doctors will categorically advise people not to run - against all the evidence of its general benefits for orthopedic health. If you can find a PT or osteopath who's also a Chi Running instructor or at least practitioner, that would be my first choice. While I'm a devout follower of Chi Running, I'm also well aware that they tend to overlook structural issues and preach that imbalances can be adequately addressed with running form only.