Knowing nothing about horses, question -- when a dog breaks a leg they put a cast on it - hip replacements are common as well. They've even fitted an elephant with a prosthesis. Good Year Rubber fitted a seal with a prosthetic rear flipper/tail. Accepting the horse will never race again....what's with horses they must be euthanized for a fractured leg? Why?
I'm gonna repost some bits I wrote earlier to answer the same question....
Unlike a person, or even a dogs, the lower leg of a horse has practically no soft tissue, no muscle that is rich with blood supply, it is all bone and ligament, and the leg and hoof does not get the blood supply it needs to function, let alone heal, unless it can move. Movement is circulation to a horse.
The goal will be to get the leg stabilized enough so he can be turned out to pasture to move as soon as is possible. A horse's health deteriorates rapidly from inactivity.
Horses are too big to roll around in wheelchairs, and cannot lay down for long periods and have normal function either. To digest food, they need to move. To feed their legs with oxygen, they need to move, to breath, they need to move. A horse will get pneumonia merely from standing too long unable to lower their head to drain their sinus. They fear laminitis, a debilitating foot condition, in his "good" back leg, because while it's supporting all the weight, it is nearly as immobile as the bad leg. It's very complicated to rehab a horse from an injury this serious.
Blood circulation is a big deal in a horse's leg- they don't have much blood supply. Also sheer weight on not only injured limbs but extra on good limbs when severe injury. Horses are hard to immobilize, and if immobilized then other life threatening issues can occur. It is a tightwire act to keep them from moving enough to reinjure yet moving enough to keep their system working properly. Horses are a lot more delicate than they appear to be. They are not always cooperative patients either, some will relax and accept treatment & rehab- others will fight against it.
Even with all the strides made in equine treatment, trying to save a horse with severe leg injuries is an uphill battle, throughout the whole process.