Michael,
I can't imagine that happining, but I know it does. One of my uncles is 6'3" and useta hurdle fences in his nieghborhood, without a problem. Then one day, he jumped off his deck (about a 16" drop) and BAM!!!!! No more knee, hes been battlin' that problem for 10 years now, and still wears a brace occasionally. I've been lucky, a few dirt bike wrecks, mountains bike ooops's, general stupidity, and still have intact knee's. Now that I'm creepin' up on old age, I've been tryin' to be a little more careful, however, "careful" doesn't always matter.
Later, Rob