Thoroughbreds are much more delicate than they used to be.
Article in Wikipedia about Ruffian tragedy goes into it.
Ruffian. The greatest female racehorse in history.
It was a "battle of the sexes", a match race between Ruffian and that year's Kentucky Derby winner, Foolish Pleasure
http://en.wikipedia.org/wiki/Ruffian_%28horse%29The first quarter-mile (402 m) was run in a blazingly fast 22 1/5 seconds, Ruffian ahead by a nose. Little more than a furlong (201 m) later, Ruffian was in front by half a length when both sesamoid bones in her right foreleg snapped. Her jockey, Jacinto Vasquez, tried to pull her up, but she ran on for another 50 yards (46 m), apparently unwilling to give up the race.
She was immediately attended to by a team of four veterinarians and an orthopedic surgeon, and underwent an emergency operation lasting 12 hours. Tragically, when the anesthesia wore off after the surgery, she thrashed about wildly on the floor of a padded recovery stall despite the efforts of numerous attendants, breaking the cast and causing even more catastrophic damage to her injured leg. The medical team, realizing that further surgery was useless, euthanized her shortly afterwards.
Ruffian's legacy is complex. Her breakdown and tragic death led to a public outcry for more humane treatment of racehorses. One result was that medications, such as Lasix for bleeding and corticosteroids for inflammation and pain management, came into common use in racehorses. While helping the horses in the short term, the increased use of medications at the track had a downside, as many horses were raced while injured. It can be argued that thoroughbreds are becoming more delicate as a result; racehorses today run only half as many starts before retirement as did their counterparts 50 years ago. Some of this effect is likely also due to breeding practices that select for horses likely to have short, brilliant careers—like Ruffian's—instead of the traditional racing career which might have lasted several years. Indeed, Ruffian's bloodline may be considered at least partly to blame for her broken leg; her sire, Reviewer, suffered three breakdowns.