There is some grey area with when you tell your AME.

You MUST tell the AME at your next scheduled physical.

But do you need to consult the AME before returning to the cockpit?

It depends...if your civilian doctor says you are returned to normal, or at least good enough to fly, that alone might be enough. In some cases if you get treated for something which resolves itself (flu, eye infection) you yourself make the determination when to go back without consulting any doctor. The catch here is that you don't want to fly with a conition which the FAA considers worthy of grounding...do you or your private physician know exactly what the FAA's theshold is? No. Some things are obviously OK for you to decide (flu, cold), others are obviously not (heart attack).

I had a broken ankle, I returned to flying on my own discretion when I could walk on it OK, but before it was 100% recovered (which took about a year). The AME had no issue.

I would suspect that you can return to flying on your own when the knee has healed to the point where you are in no danger of re-injuring it in the cockpit, and when you have enough strength and range-of-motion to operate the controls and walk on your own. If you're taking crutches to the airplane, better ask the AME first.

If in doubt, consult the AME. You'll need to tell him anyway in the long run.