I had my left knee hyperextended (bent the wrong way) when sliding into second base playing softball as a sophomore in high school. I was probably 15 at the time; I didn't break anything in my knee, and was seen immediately by a physician. They didn't cast the knee or do surgery, and everything was fine...for a while. By the time I was 19, I had severe arthritis in my knee. Bad enough to require physical therapy so that I could walk to and from my college classes. It still bothers me when the weather is bad, but at least it'll hold my weight in the mornings now.
On the other hand, I knew a girl who was dropped on her knees to a gym floor from a cheerleading build-up; she was tossed in the air, they missed catching her, and (luckily) she landed on her knees. Both kneecaps were severely fractured. She didn't cheer the rest of that school year, but with a lot of hard work and determination, she cheered her next school year and enrolled in gymnastics classes. She doesn't limp at all now.
I think it depends on a lot of factors - how fast treatment is sought, patient determination, and the technical medical ability of the physicians.
WVWG
