Just got this info from an equine appraiser:

You know in racing it is not uncommon for trainers/owners to unnerve a horses lower leg area so they don't feel pain or fatigue. This allows more stamina for the race. By what I have read is sounds as though she may have had some stress fractures in her splint bones which would cause pressure on the coffin bones. If that happened the bones turn to dust with all the weight. For both of her front splints to give out she had to be pounding the ground with heavy shoulders. When this happens a horse will usually be lame, and go to pasture never to be raced again. In Belles case she more than likely crushed the coffin bones thus no support for a 1200 pound horse and a 100 pound Jockey.
It is a shame how money plays a big part in decisions about racing young horses.
In the knee of a horse there is a joint called the epithasile. This joint is not fully developed in a horse until they are 4 or 5. Quarter horses have been known to be developed by the age of three but none the less should be treated lightly until 4 or 5. Money rules the minds of these trainers and owners to the point of abusing these animals. Most race horses don't make it to a 5 yr old. It is the futurities,<(horses 4 years and younger) that bring in the big money with no real regard to the horses condition. As long as they can make it to the Derby trainers will play it as it comes. It is sad that a high percentage of fillies never make it to being a mare or even having foals because there legs are shot. A filly is 4 years or younger. I like racing because horses are born racers and love to run, it is the way trainers (mostly men) wear these horses out early in their life. In the earlier days race horses like Man-O-War,War bond Leo, Dash for Cash were treated like the majestic creatures they were meant to be. Now it is about the mighty dollar.