Why do they euthanize racehorses who break their legs?

There is a lot to enjoy for a new fan introduced to thoroughbred racing. There is the beauty of the animals, the thrill of watching them move and the joyful feeling of outsmarting the other horseplayers and cashing a bet.

But there is a cold fact about the sport that can be hard for fans — and impossible for critics — to accept: Sometimes a horse gets hurt, and sometimes it is euthanized, often right on the track.

Earlier this month, seven horses died at Churchill Downs in the lead-up to the Kentucky Derby, including four that broke down while racing or training. And the victory of National Treasure, trained by Bob Baffert, in the Preakness was clouded by the collapse and euthanasia of another Baffert racehorse earlier in the day at Pimlico Race Course.

People who oppose horse racing on principle often point to such occurrences while making their case. Even for racing fans, the disquieting reality of breakdowns can raise the question: Does something as apparently simple as a broken leg have to lead to a horse dying? The unfortunate answer, veterinarians say, is often yes.

Horses are just different from many animals, even other equines. "They can run really fast," said Dr. Scott E. Palmer, the equine medical director of the New York State Gaming Commission. "And because they weigh about 1,100 pounds, the forces that are acting on their legs are really profound."

Palmer continued: "All their muscles are up high. When you get down into the lower part of the leg, there is literally skin and bones and tendons and blood vessels and nerves. If something breaks, the circulation of the area can be easily compromised by the injury."

As a result, horses are vulnerable to breaking their legs; it happens running on the racetrack, or running in a pasture, or kicking a stall door. The problem is that it is very difficult to heal a broken leg on a horse.

To fix a broken bone on any animal, the break must be immobilized. But immobilizing a horse brings a host of challenges. Horses are restless and skittish. Thoroughbreds are bred to run. Keeping them in one place for an extended period is difficult.

Horses also spend almost all of their time on four feet, even when sleeping. So all four of their legs bear their weight. If suddenly three legs have to support that weight, the uninjured legs can quickly develop problems.

Most commonly and dangerously, horses can get laminitis, a painful condition that develops in the tissue between the hoof and the bone. "The hoof is attached to the bone by organic fasteners like a Velcro system," Palmer said. "If those little hooks become swollen, they become unhooked. That is impossible to take care of."

The entire treatment experience can bring severe pain for a horse who, of course, cannot understand what is going on the way a human undergoing painful treatment would.

The pain for the horse is consideration "No. 1, No. 2 and No. 3," Palmer said.

Horses cannot simply lie down for extended periods to avoid putting weight on their legs. Lying down for more than a few hours will cause muscle damage, restricted blood flow and blood pooling in the lungs.

Any elaborate or unusual process to try to repair a badly broken bone can cost thousands of dollars. Few horse owners are willing to spend that kind of money on a painful treatment process that might not work and probably won't get the horse back to the racetrack. Euthanasia is the unfortunate choice most of the time.

When the 2006 Kentucky Derby winner, Barbaro, broke his leg in the Preakness two weeks later, his owners Roy and Gretchen Jackson decided to try to save him.

His injury was serious: The leg bone was broken into 20 pieces. He had five hours of surgery to insert 27 pins and a stainless steel plate.

Palmer was on the scene the day of the injury. "I said: 'The fracture is horrible, but none of the wounds came through the skin. Because of that, I believe that surgery is possible.' I honestly thought that was the very best chance he had for survival."

"We just reached a point where it was going to be difficult for him to go on without pain," Roy Jackson said. In the end, the extraordinary efforts lengthened his life by only eight months.

"From a purely surgical perspective, it was extremely unsatisfying because he didn't make it," Dr. Dean W. Richardson, the surgeon, said at the time. "Professionally, I think we did the best we could."

The dazzling filly Ruffian in 1975 had 12 hours of surgery after a bad break. Upon waking, she began thrashing around in her stall, causing another break and leading to her euthanasia.

If euthanasia is the only option, the horse is sedated, then a barbiturate solution is administered, generally behind a screen to block the view of spectators.

Strides have been made in the last decades in treating horses, including development of better antibiotics and the aluminum splint and improvements in understanding laminitis.

After a string of horse deaths at Aqueduct in 2011 and 2012, Palmer and others made recommendations, including improving the racing surface, changing claiming and purse rules and strengthening drug regulation. Those have helped the number of racing deaths come down and stay down.

Palmer has hope for Fitbit-type devices — biometric sensors that can spot horses with gaits that might lead to injury before those injuries happen. A trial at Saratoga Race Course last year was promising, he said.