The reason a broken leg is such bad news for a horse.

After finishing in second place in Saturday's Kentucky Derby, Eight Belles fell to the track with two compound ankle fractures. The horse, the first filly to run at Churchill Downs in nine years, was immediately put down. Two years ago, when Barbaro broke his leg at the Preakness, Daniel Engber explained why such an injury is so devastating for a horse. The full article is reprinted below.

Barbaro's veterinarians say the champion racehorse has a 50 percent chance of survival after breaking his leg at the start of the Preakness. He may not recover even after a successful five-hour surgery on Sunday, during which he had almost two dozen screws implanted to stabilize his bones. Why is a broken leg so dangerous for a horse?

There's a high risk of infection, and the horse may not sit still long enough for the bone to heal. Infections are most likely when the animal suffers a compound fracture, in which the bones tear through the skin of the leg. In this case, dirt from the track will grind into and contaminate the wound. To make matters worse, there isn't much blood circulation in the lower part of a horse's leg. (There's very little muscle, either.) A nasty break below the knee could easily destroy these fragile vessels and deprive the animal of its full immune response at the site of the injury.

Barbaro was lucky enough (or smart enough) to pull up after breaking his leg. If he'd kept running—as some horses do—he might have driven sharp bits of bone into his soft tissue and torn open the skin of his leg. Though his skin remained intact, he still faces the possibility of infection; any soft-tissue damage at all can cut off blood flow and create a safe haven for bacteria.

It's not easy to treat a horse with antibiotics, either. Since the animals are so big, you have to pump in lots of drugs to get the necessary effect. But if you use too many antibiotics, you'll destroy the natural flora of its intestinal tract, which can lead to life-threatening, infectious diarrhea. You also have to worry about how the antibiotics will interact with large doses of painkillers, which can themselves cause ulcers.

If the horse manages to avoid early infection, he might not make it through the recovery. First, he must wake up from anesthesia without reinjuring himself. Doctors revived Barbaro by means of "water recovery." That means they suspended him in a warm swimming pool in a quiet room and then kept him there for as long as possible. Not all horses are willing to sit around in a sling, and the antsy ones can thrash about and break their limbs all over again. (In 1975, the filly Ruffian managed to break a second, healthy leg in the process.)

If Barbaro starts favoring his wounded leg post-surgery, he may overload his other legs, causing a condition known as "laminitis." If that happens, the hooves on the other legs will start to separate from the bone, and his weight will be driven into the soft flesh of the feet. He may also develop life-threatening constipation as a side effect of the anesthetic.

Doctors will often put down a horse that develops a nasty infection, reinjures its broken leg, or develops laminitis in its other hooves. (A horse that's unable to stand will develop nasty sores and can be expected to die a slow and painful death.) A few horses have had broken legs amputated and replaced with metal, but the equine prostheses don't have a great track record.

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Explainer thanks Rick Arthur of the American Association of Equine Practitioners and Carl Kirker-Head of the Cummings School of Veterinary Medicine at Tufts University.