Why does a broken leg mean death to a racehorse?

What would be a relatively simple, if painful, injury for a human often proves fatal to a horse. Jake Port explains why.  

Dunaden edges out Red Cadeaux to win the 2011 Melbourne Cup.
Scott Barbour/Getty Images

Tragedy struck last year’s Melbourne Cup, Australia’s most famous horse race, when the much-loved English stayer Red Cadeaux was euthanised after breaking his left foreleg during the race.

Sadly, the 10-year-old’s misfortune is not an isolated incident and the all-too-common deaths of thoroughbreds are used by racing’s opponents to demand an end to the sport.

So why do these big powerful animals break limbs so easily? And why is an injury that is painful but usually easily treatable in a human, a death sentence for a horse?

Anatomy is a big factor

It is the very power of the animals, combined with light – but dense and strong – bones, that cause the biggest problems.

“Being very dense bone under huge load, when it breaks it breaks in a big way sometimes," says Chris Whitton, an equine orthopaedic researcher at the University of Melbourne in Australia.

In more severe cases the bone doesn’t break but shatters. Reassembling the shattered fragments sometimes just isn’t possible.

Making things even more difficult is the fact that horses have very little soft tissue in their lower legs, a prime area for injury.

That means a compound fracture, where the skin is broken, is much more likely – as well as damage to the blood supply to the limb.

In the case of Red Cadeaux, for example, initial treatment by vets appeared to be working. It was the loss of blood flow to the leg that sealed his fate.

“When they fracture they are often open to the environment. So contamination becomes a big problem,” Whitton says.

Then there is weight

It is not just the damage itself that makes a broken bone such a catastrophe for a horse. Assuming the bone could be set and immobilised, weight would have to be taken off the injured leg for many weeks as it healed. And therein lies another problem.

To do this, the half-tonne or so a horse weighs would have to be distributed over its three remaining good legs.

A horse’s hoof is attached via a special tissue known as “interdigitating laminae”, and laminae do not cope well with added weight. Almost inevitably it would lead to the extremely painful inflammation of the laminae known as laminitis.

Laminitis also affects blood flow to the injury further reducing the body’s ability to heal.

Immobilising the animal is not an option

A logical answer is to immobilise the animal until the injury heals, but this only leads to other problems. Lying down inhibits the natural process by which a horse clears fluid from its lungs. Without this, pneumonia quickly sets in.

In the past, vets have tried taking the weight of a horse’s legs using a sling, but this has also proved impractical. The weight of the animal pressing against the sling causes bed sores and damage to internal organs.

What about prosthetic limbs?

Attempts to replace the injured leg has been met with little success, with the horse’s weight again becoming an issue. The weight of the horse must be equally distributed across all four limbs as soon as possible to avoid the risk of laminitis. But the weight of the stump pressing into the prosthetic leg also leads to pressure sores.

And that’s even if you can make a suitable prosthetic.

“Generally we use plates and screws to put the bits of bone back together,” Whitton says.

“But if you can’t reconstruct the bone, it’s unlikely that you have an implant strong enough to take the load of a 500-kilogram animal.”

So how do vets decide if a horse can be treated?

To minimise suffering, racing and other equine events will have a group of veterinarians on site to make quick clinical assessments where possible.

In many cases the injury requires a more detailed assessment, with CT scans used to accurately determine how the bone has been injured.

“3-D imaging allows you to reconstruct and plan your surgery and we certainly have found it to be a huge boost for us in planning how we do the surgery, maximising the efficiency of the construct,” Whitton says.

Future research may lead to equine specific implants as well as new measures that will mitigate the chance of an injury.

  1. https://cosmosmagazine.com/biology/cure-second-biggest-killer-horses
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