Bucking the strain of bucked shins
"Bucked shins are one of the most frustrating hurdles for Thoroughbred trainers to overcome," says Patty Hogan, VMD, DACVS, of Hogan Equine at Fair Winds Farm, Cream Ridge, N.J. "They're a natural phenomenon, as they occur due to the timing of bone development during early training of young racehorses. The natural way a bone adapts during training is at such an accelerated rate, it doesn't allow for the making of healthy bone."
Hogan says that older horses can sometimes develop a sequelae of this syndrome, although with deeper changes in the bone, leading to stress fractures. If, by chance, older horses (i.e., 3- or 4-year-olds) enter training for the first time, they may develop bucked shins, too.1According to Daniel J. Burba, DVM, DACVS, professor of veterinary surgery at Louisiana State University's School of Veterinary Medicine, 70 percent of young Thoroughbred racehorses in training develop the problem, usually during the first six months of their training. Although it can be treated fairly easily, 12 percent of horses that develop bucked shins go on to develop stress fractures later in their careers.
Make the diagnosis
Burba says it was thought that the periosteum of the bone is affected, resulting in a subperiosteal hematoma. It forms because the periosteum is torn away from the surface of the bone, resulting in microbleeding of the capillaries. As the periosteum is pulled away from the bone's surface, pain receptors in and along the bone surface are affected, resulting in a very painful event for the horse.
In a presentation at the American Association of Equine Practitioners in 2002, David Nunamaker, VMD, DACVS, countered that the classical hypothesis of subperiosteal hemorrhage and microfractures seems to be the wrong interpretation of the bone's morphology.1 "The high-strain events of bending of the MCIII (cannon bone) induce bone formation on the dorsal surface of the MCIII. Classical training will induce this bone on the dorsomedial surface," said Nunamaker. "Lamellar bone formation can accrue on surfaces up to 1 to 2 microns per day. Faster bone formation may occur using fiber bone that forms the periosteal new bone that is vascular and porous. This normal periosteal formation is interpreted by the clinician as bucked shin, and therefore disease."1
Although both forelimbs may be affected, it's often the left forelimb that contracts the condition first, since Thoroughbred racehorses in the United States race counterclockwise, thus incurring the stress of higher loading of the left leg while traversing the track's turns.
Nunamaker writes that if Wolff's law is strictly applied, it follows that a bone that adapts to a particular peak tensile strain may not be adequately prepared to resist far larger peak compressive strains in the same location.1
When an animal is breezing, its leg is at a much different angle than when the animal is jogging or even galloping, and the bone has to adapt to that to be structurally sound, according to Hogan. "Most horses are not trained to breeze frequently, and mentally it's very difficult to give a horse the right amount of breeze work to actually train the bone properly. Many horses have very long jogging regimens—many miles at a lesser speed—so bone is very strong for that," Hogan says. "But when then asked to do the fast work, the bone is radically affected—the angle is so different that it creates a stress concentration on the front of the cannon bone." Therefore, the horse develops a very painful reaction, subperiosteal inflammation, as the bone has not yet remodeled to withstand the demands.
Hogan compares good training to building a house of brick and mortar—both require a very slow process that builds sound structure. "If you want to build quickly, you use fiberboard," she says. "That's sort of what happens with bucked shins. It's the body's way of putting something down very fast to adapt to the request for strength and speed. Therefore, it lays down this 'fiberboard bone,' which is structurally not very sound."
That's when you see periosteal reaction on the front of the cannon bone: frail, linear bone vs. very strong, lamellar bone, Hogan says.