Most horses that break a leg do not recover successfully and are euthanized. Horses rely heavily on the use of their legs, which contain 80 of the animal's 205 bones. They stand much of the time, even when they are sleeping.
The chances of recovery are low because several possible life-threatening or painful and debilitating complications could occur. The likelihood of an infection is high, and the horse's large size makes treatment with antibiotics difficult.
Not all horses with broken legs are killed, though. A horse's chance of a successful recovery is increased if it suffers only an incomplete fracture, is younger or breaks a bone in the upper leg.