Bucking the strain of bucked shins

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Although fairly prevalent in 2-year-olds, this common inflammatory condition of the cannon bone can affect any Thoroughbred racehorse in training and lead to more serious injury further on in a horse's career.

Rolling Fog won the Del Mar Futurity on Sept. 5, but just two weeks later he was sidelined with bucked shins, a common ailment in 2-year-old Thoroughbreds.

ANJA HILD, GETTY IMAGES

"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.1

According 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

Bucked shins can be diagnosed easily on physical examination by palpating heat, pain and swelling on the dorsal surface of the forelimbs. An affected horse may be short-strided or lame. Radiography of the cannon bone will confirm the diagnosis; you'll notice increased bone formation on the periosteal surface and thickening of the front of the cannon bone (Photos 1 and 2). However, it's not unusual for clinical signs to be present but for radiographs to appear normal in the early stages of the disease.

>>>A lateral radiograph of the left front cannon bone in a 3-year-old Thoroughbred racehorse. Note the increased irregular thickness to the entire dorsal surface of the shin. This horse has experienced a focal cortical stress fracture as sequelae of chronic bone remodeling of the shin.

"Bucked shins—a high-strain, repetitive-motion injury—is due to the intense training that young racehorses go through," says Burba. "As a result, the bone hasn't had the time to adapt fast enough, so you get an inflammatory process of the dorsal cortex of the third metacarpal, called dorsal metacarpal disease."

>>>An oblique radiograph of the left front cannon bone in a 3-year-old Thoroughbred filly. Note the obvious bone inflammation along the dorsal aspect of the shin. This is an extreme example of the intense bony remodeling that can occur in some situations in which the bone is attempting to quickly compensate for the inability to handle the exposure to increased speed exercise.

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.

Treatment and resolution

Rest, with controlled activity, is the primary treatment, and no matter what else is done during that time (e.g., ice treatment, shock-wave therapy), reduction in strenuous training will allow the bone to heal and the condition to be resolved. "Time is best to resolve the condition," says Burba.

But Hogan counters that extended rest is actually contraindicated. "It's not that rest isn't a good idea, but rather it's the degree or grades of rest given," says Hogan. "The rule of thumb: Once bucked shins occur, give the horse only what exercise level it can handle without experiencing any discomfort so that it can develop the right bone structure without causing pathology."

Nunamaker hypothesized that to adequately adapt for racing, during training the MCIII, or cannon bone, should be exposed to strains of the actual magnitude and direction experienced during racing. Generally, horses just need a reduced training regimen. Treatment involves therapies aimed at cooling them down and reducing pain and inflammation (e.g., ice, topical medications, systemic anti-inflammatories).

Shock-wave therapy in horses with bucked shins probably relieves pain and may have some effect on the surface of the bone, but not on the deeper tissue. The technique involves applying high-energy sound waves repeatedly at tissues to encourage an increase in blood vessel formation in the area, thereby increasing blood flow to aid the healing process.

The good news, according to Nunamaker, is that once the condition occurs and resolves, horses do not experience this problem again. But evidence shows that future, more serious injury is possible.1

Nunamaker also states that catastrophic complete midshaft fractures of the MCIII can occur when these horses are exercised at speed or raced.

Burba states that many horses get bucked shins but never get stress fractures. As the horse gets older, the level of training intensity is increased, and perhaps if a particular animal develops a stress fracture, it is because of its individual training regimen, not because of previous bucked shins.

"It's a balance of training to get the horse ready to race but not pushing it to a point beyond where it becomes detrimental and possibly leads to a fracture," Burba says.

Prevention

According to Moyer, Fisher and Nunamaker,1,2 horses that train on a harder track surface seem to remodel their bone at a faster rate than horses exercised on a more compliant surface. Also, classically trained horses that exercise on a harder surface seem to have a higher incidence of bucked shins than horses training or racing on a more compliant surface.

Nunamaker suggested that an exercise regimen could be designed to optimize the shape of the MCIII. This in turn should influence (i.e., decrease) the incidence of bucked shins in the Thoroughbred racehorse model and diminish the problem within the industry. Noted Nunamaker, "Adaptive exercise has been shown to change the geometric properties of MCIII, to influence bone modeling and remodeling and to reduce the incidence of fatigue [bucked shins] of MCIII."1

If everyone followed the Nunamaker-Fisher program for training,1,2 there probably wouldn't be any bucked shins, states Hogan. "They figured out why horses get bucked shins and discovered that it's really a very simple phenomenon. They studied the physics of the way the horse's foot hits the ground and determined the best way to train young horses to help eliminate the condition."

Nunamaker found that increasing the number of short distance works (breezes) from once every seven to 10 days (as done with classical training programs) to three times a week produced large changes in the modeling, remodeling and inertial property measurements of MCIII.1 Some trainers may suggest that, mentally, it's difficult to get a 2-year-old to gallop out the last furlong a couple of times a week. Nunamaker and Fisher used sensors applied to the cannon bone and worked out the physics, coming up with the best training modality. Nunamaker showed with the modified training regimen that only 9.3 percent of the horses trained had bucked shins during the five-year training period when the modified training program was implemented. Compliance, then, is the issue.

To extrapolate from their work, Hogan suggests that when adding breezes to a young horse's schedule, at the end of their gallops once or twice weekly, push them the last furlong to run at a 12- to 13-second clip, so the bone gets the signal or request for higher speed that allows it to adapt to that kind of speed. "Then the bone is prepared for that increased speed as it begins to lay down strong lamellar bone. But in this scenario it has enough time to do so," he says.

Conclusion

As with most musculoskeletal injuries in the racehorses, an increased awareness and understanding of the physiologic requirements of bone's adaptation to modern training practices has led to the early detection of many stress-related injuries. Bucked shin syndrome is just a precursor to what could become a potentially deleterious breakdown in the microstructure of the cannon bone. Early recognition of the problem, coupled with the knowledge of how to use exercise judiciously to temper the changes occurring in the bone in response to training, has resulted in decreased morbidity and a properly-modeled cannon bone better suited for multiple years of safer competition. All good news for Rolling Fog.

Ed Kane, PhD, is a researcher and consultant in animal nutrition. He is an author on nutrition, physiology and veterinary medicine with a background in horses, pets and livestock. Kane is based in Seattle.

References

1. Nunamaker DM. On bucked shins. Milne Lecture: Equine Orthopedics, in Proceedings. American Association of Equine Practitioners, 2002;48-76.

2. Moyer W, Fisher JRS. Bucked shins: effects of differing track surfaces and proposed training regimens, in Proceedings. American Association of Equine Practitioners, 1992;541.

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