Not to rain on your parade, but I completely agree with Jo Burble, and I have the MRI's to prove it. :think:
I was off the bike standing beside it when I blew my ACL. It was just an awkward position, supporting an overweight bike (then a KLR-250, now sold) on a slippery hill.
No way of knowing if it was the fact that the boots immobilized my ankle that transferred the force to the ACL and popped it, or if it was just an inevitable fluke. I am not a doctor.
But that was my first and last ride in boots that immobilize my ankle. Even if I do catch a toe on a stump or something, if there is that much force, I want *both* the knee *and* ankle (and for that matter the hip) to rotate as far as possible. Trying to immobilize one or the other seems to invite something to break sooner... it's not like the stump will break because my boots are strong.
Take this with a grain of salt though. When bad things happen, it's really easy to look back and find "the reason" and convince yourself of it after the fact. I'm sure I am doing that to some degree, and some people do great with the boots and never have any knee problems (and save a lot of crunched toes).