Dortmund’s defection from the Clark Handicap took a little spice out of the race, all because of a little crack in his hoof. Quarter cracks are relatively common among Thoroughbreds, and while they can take time to heal, they don't necessarily put a horse on the shelf for months, the way a soft tissue injury can.
A quarter crack is a vertically-oriented fissure in the side of the foot, often starting at the top of the hoof and working its way down. Some cracks are wider and deeper than others when they first appear.
The cracks occur as a result of strain on the hoof, particularly in the heel. Often, a horse with a quarter crack will have a long toe combined with a heel that is especially low to the ground, or one side of its heel could be a slightly different shape than the other side. Horses with a low heel have less hoof surface hitting the ground, so the force of each footfall becomes greater on the remaining hoof tissue. Quarter cracks also might occur because a horse places his foot on the ground asymmetrically, landing with one side and rolling the weight to the other.
In the case of the low heel, the tubules that make up the hoof wall material grow forward instead of up and down, which makes them weaker. The soft internal structures become compressed with extra force, creating “crushed heels.” Combine the weak hoof materials with the squished internal structures and the tough exterior gives way from the inside out.
In many cases, the farrier may be blamed when a horse develops a quarter crack as a result of hoof shape, but experts say it's not that simple. Quarter cracks are more likely to be a one-two punch of genetics and environment, says veterinarian and farrier Dr. Ric Redden.
“It is the rare horse that has matching feet inside as well as out,” Redden says. “One front foot is invariably a bit steeper and has the characteristic of increased deep digital flexor tendon tension while the other is lower profile inside and out.”
Those small, natural variations can put one foot under more strain than the other. Thoroughbreds have a long history of frail hooves compared to other breeds, with the layers of keratin that make up their feet often appearing thinner and more brittle. Conformational imbalances can exploit these weaknesses. Add to this the fact that they're bathed daily, with the constant wet/dry transition weakening the hoof wall, and it's easy to understand why a hoof might crack under pressure.
To learn more about quarter cracks, be sure to read the article titled, “Sheared Heels And The Correlation To A Quarter Crack” by Stephen O'Grady, an equine veterinarian and farrier, that was recently published in American Farriers Journal.