I read (in yesterday's thread?) that sometimes a horse can even survive with just 3 legs. So I wonder with severe cases why they couldn't amputate above the fracture and put on some type of prosthetic replacement leg below, so that they could begin walking as normally as possible almost immediately. I would guess such an artificial limb would be a complicated device if it could get close to recreating a horse's natural leg movements, but with today's technology it would seem possible.
For a horse to survive on three legs is so rare as to be nearly unheard of. I didn't see that post, and don't think they'd survive well. One FReeper vet relayed a story about a groundbreaking prosthetic on a horse done at a University. It cost a million bucks. It's not a practical solution.