The death of the filly Eight Belles after a freak injury in the Kentucky Derby on Saturday continues to raise complicated questions about the nature of horse racing.

Yet for people unfamiliar with horses, a simpler question remains unanswered: Why are horses euthanized after leg injuries?

Dr. Shawn McCoole, a veterinarian at Cedar Ridge Veterinary Clinic in Atchison, Kan., explained how the anatomy of a horse makes leg injuries so severe.

Horses have an ankle joint similar to a human’s wrist, Dr. McCoole said, but where a human has a complete set of hand and finger bones, a horse has only one of each to form the foot and the hoof. If a horse breaks a bone in its foot, it does not have other bones to rely on. When Eight Belles broke both front ankles during its post-race gallop, the veterinarians were left with no option but to euthanize the animal.

The 3-year-old racehorses that compete for the Triple Crown face additional issues, Dr. McCoole said. Selective breeding has produced speedy animals with massive amounts of muscle supported by a fine bone structure. A lack of physical maturity also adds injury risk.

“The bigger the horse’s frame, the longer they take to mature,” Dr. McCoole said. “A lot of those horses will grow until they are 4 or 5 years old.”

Michelle Hinkle, show chairman for the Pony Express Saddle and Bridle Club of St. Joseph, said that in her years of showing, breeding and riding quarter horses, she has never had to deal with such an injury. She now has “only” six horses for recreational riding. Even with fewer animals, the chance of injury exists.

“If the decision needed to be made for the well-being of the animal, I would be prepared to make that decision,” she said.

Ms. Hinkle described the reasoning she would go through before deciding to euthanize an animal. Horses are not designed to be stationary animals, she said. So if the animal had to be put in a sling, could it recover from the atrophy that would follow? And when the horse returned to its feet, would it have the reasoning or the ability to stay off a weak leg? The answer: probably not.

Even a world class horse like Barbaro could not recover from a broken leg.

“What’s really humane? Putting a horse through months of difficult rehab when he will probably never fully recover, or putting him down,” Ms. Hinkle said. “Leg injuries are almost impossible for a horse.”

Clinton Thomas can be reached at clintonthomas@npgco.com.