Maureen...
Tough luck, friend. Sounds like a nasty diagnosis.
You might want to check out a recent thread on the "Ask Cathe" section called "Knees and Workouts". Theres a lot of discussion of knee pain, various exercises, knee braces, etc. There are a number of us out here in Cathe-land who have been through knee pain and survived. Are you taking pain meds? Has your doctor talked to you about the benefits of glucosamine/chondroitin supplementation? How about physical therapy? Has he discussed the potential for steroid injections into the knee or other therapy? Does he know why you've developed a necrosis of the bone? And what are his plans after your two-month imposed non-weight bearing time period?
Sometimes, having a better idea of what the future holds helps to operate positively in the present.
Personally, with my diagnosis of chondromalacia, I found quad strengthening for better muscular support of the joint helped immensely. Ask your doc about physical therapy, but make sure his mode of treatment is carefully laid out so the Physical therapist plans your exercise regime appropriately. As far as cardio and impact workouts, I found starting slowly on an elliptical machine was great for getting my heart rate up but minimized the dgree of flexion in the knee while allowing me to exercise without impact.
You have to listen to your body, make note of what behaviors bring on the knee pain, what makes it feel better or worse; then have a list of questions for your doctor next time you go in to see him. And, no matter what you think, no question is too silly or insignificant. The best patients are armed with knowledge, because it allows you to take some control over your progress and recovery. (I'm a nurse; believe me, I love teaching people how to manage their own health care, especially when they're motivated by their own need to know).
Good luck, and keep us posted on your progress...
Hugs,
Lynn Finn