It's not a knee, but I broke the s**t out of my left elbow in jr high school. Doc was a locally-reknowned ortho, and said it was unlikely I'd ever get full use again. But the day the cast came off and the pins came out, I was working on my range-of-motion. Years later, when I enlisted, the military docs gave my 4-inch scars the evil eye; but grudgingly noted that there was no impairment. Over the years, the only accommodation I've made was to give up high reps of push-ups. Otherwise, I can lift weights, canoe, chop wood... anything I want. I believe my use of the arm stems directly from getting my range-of-motion back as quickly as possible. Funny thing is, I remember bouncing a basketball with my left hand to stretch the joint; but my mother remembers me crying from the pain. I'm pretty sure she's making that part up. I guess pain is temporary and easily forgotten.