Why a broken leg is a death sentence for a horse

The sight of a racehorse being put down by the side of the track is one of the most emotive and controversial in sport, but it is one that still occurs regularly.

And the debate over the treatment of injured horses is sure to continue.

So why is a broken leg so often a death sentence for a horse?

Weak bones

There can be no real comparison between a broken leg for a human and a broken leg for a horse because their bones are so different.

Racehorses have been selectively bred for centuries and are designed for speed, vet Jenny Hall tells The Guardian. "Their bones have become lighter. They're very strong to carry their weight, yet they're light for them to be able to go fast. So, unfortunately, sometimes when they break they just shatter."

To make matters worse, in the moment before the bone snaps it bends, resulting in "plastic deformation".

"Even if it were possible to put the pieces back together, you would end up with a badly bent bone," says The Guardian.

The horse's lower limbs have little soft tissue and that means bones often pierce the skin when broken.

Complex anatomy

The complex bone structure of a horse's leg adds to the problems.

"Out of the 205 bones that make up a horse's entire body, 80 of them are located in its legs," explains website How Stuff Works. "The complex system of joints, bones, ligaments, tendons, cartilage, lubricant, laminate and hooves that contribute to a horse's amazing speed can also be the cause of its downfall."

Rebuilding a horse's leg is an almost impossible task.

Prey animals

A horse is a prey animal, which means it has evolved to remain on its feet at all times.

Treatment for a broken leg requires the weight to be taken off the damaged leg, not something a horse can handle.

What about slings?

Slings can take the weight off the feet but can cause sores and also end up compressing a horse's internal organs. Knocking a horse out so it is lying down can have the same effect and allows fluid to accumulate on the lungs, causing pneumonia.

Sheer weight

If a dog, for example, breaks a leg it can support itself on its remaining three feet, not so a horse. The problem lies with the hooves.

"These are strong enough to support the horse's weight when that burden of 500kg or so is shared across four legs. But when a horse breaks one of those legs and tries to support itself on the other three, the increased burden creates serious problems for the laminae at the base of those legs."

A quick diagnosis

What often shocks racegoers and the public is the speed with which the decision to euthanise a horse is taken.

But Hall tells The Guardian that there are trained medics at the racetrack who get there fast and the decision is often an easy one to take.