Thoroughbreds can sustain several different types of fractures during racing and training

ALTHOUGH most Thoroughbreds typically are seven times the weight of a human runner, the legs that carry them over the track are rarely bigger than those of their human counterparts. At peak speed, a galloping racehorse lays down one hoof at a time and exerts more than seven tons of force on each leg as he completes his stride.

A bad step, an uneven surface, or a conditioning program that does not allow the body's repair mechanism to stay ahead of the continuing damage of training and racing set the stage for bone failure. Depending on the factors that contribute to that failure, the horse may sustain an injury ranging from a hairline fracture to a catastrophic breakdown that ends his career or worse.

Fractures are separated into four main groups: closed or open; complete or incomplete; simple or comminuted; and displaced and nondisplaced:

• A fracture that protrudes through the skin is an open fracture; one that does not produce a skin wound is a closed fracture;
• A complete fracture breaks the bone into two or more pieces; an incomplete fracture starts on one side of the bone but does not break completely through it;
• A simple fracture causes a single fracture line that separates the bone into two pieces; a comminuted fracture breaks the bone into several pieces; and
• When a fracture is displaced, the broken pieces are spread apart, and the fracture can be seen clearly on an X-ray; a nondisplaced fracture allows the bones to remain in alignment, such as in a hairline fracture, so it may be difficult to detect on an X-ray.

Here are some of the more common types of fractures seen in racehorses.

Condylar fractures

Condyles are the knuckles at the end of the long bones that help to form the various joints. A condylar fracture occurs when a crack begins near the joint surface and extends outward through the bone. In racehorses, condylar fractures occur most often at the lower end of the cannon bone where it forms the fetlock joint.

The collateral ligament that stabilizes the joint and keeps it in alignment may prevent a condylar fracture from becoming displaced. So, even if a horse exhibits lameness, the fracture may be overlooked or misdiagnosed as inflammation of the joint, a pulled ligament, or other joint ailment.

However, in severe cases when the fracture becomes displaced, the break will extend to the outer surface of the bone, essentially breaking off the knuckle and a portion of the bone.

According to one of the leading researchers into catastrophic breakdowns, Susan Stover, D.V.M., Ph.D., a professor at the University of California at Davis, repetitive trauma causes condylar fractures. Her research has shown that it is very likely that how much impact a horse's bones have experienced in the preceding three months is a significant factor in the development of condylar fractures.

"If we train too hard and race too often, it catches up with us," Stover said. "One of the factors could be how intensely they are trained, but also things that affect how much force is transmitted to the limb with each stride, and that could include shoeing and track surface conditions."

Fractures of the knee

The horse's knee is made up of the lower end of the radius (forearm), the upper end of the cannon bone, two rows of small carpal bones sandwiched in between, and a small, oval-shaped bone at the back of the knee called the accessory carpal bone.

Besides condylar fractures, the knee may sustain a chip fracture, a slab fracture, or a sagittal fracture.

A small sliver of bone that breaks off one of the carpal bones or off the end of one of the long bones is called a chip, which floats freely in the joint or eventually reattaches to the parent bone. A slab fracture occurs when the front breaks off one of the carpal bones. Both chip fractures and slab fractures damage the joint surface when the separated fragment interferes with the smooth gliding of the joint. Chip fractures also are prevalent in the fetlock joint.

A sagittal fracture is a vertical fracture on the front of one of the carpal bones--most commonly the largest bone, the third carpal bone--that penetrates through the top and bottom surfaces of the bone, as well. Usually, it does not extend through the rear surface of the bone.

"Almost every horse that has had a slab fracture of the third carpal bone, which is a common injury in a Thoroughbred racehorse, had pre-existing, repetitive stress injuries," said Dean Richardson, D.V.M., chief of surgery at the University of Pennsylvania's New Bolton Center. "They have changes in the bone associated with repeated pounding."

Richardson urged horsemen to be vigilant about clinical signs, such as heat, swelling, tenderness, or subtle variations in gait--all of which could indicate the need for a closer look with diagnostics.

To preserve the health of the joint, it is essential that bone fragments are removed, a common procedure especially in two-year-olds that is performed arthroscopically. During arthroscopic surgery, a half-inch incision is made on one side of the joint into which a rigid endoscope with a fiber-optic cable and light source is inserted. The joint is inflated with sterile saline solution to aid visibility within the joint to make it easier to locate the chip. Once the chip is found, an instrument is inserted through another small incision at the opposite side of the joint that is used to grasp and extricate the chip and any other debris that the surgeon finds in the joint.

Bucked shins and saucer fractures

David Nunamaker, V.M.D, chairman of the Department of Clinical Studies at the New Bolton Center, has spent decades studying why bones break. During his research, he discovered an association between bucked shins and catastrophic breakdowns.

Bucked shins are an inflammation of the periosteum, the membrane that covers the shin where the tendons attach to the cannon bone. Although extremely painful for the horse, the condition usually is not career-threatening unless it is accompanied by a saucer fracture of the cannon bone at the shin. A saucer fracture occurs when the cannon bone is put under stress and microfractures--small, horizontal fractures in the lattice-like structure of new bone cells that form on the outside layer of the shin bone--cause the bone to give way. Simply put, the constant flexing of the immature cannon bone causes stress cracks at a rate greater than the horse's ability to repair them.

Bone undergoes almost constant turnover when it is stressed. During the process, small cavities occur in the bone where portions of it have resorbed, and those gradually are filled by new bone during bone remodeling.

"These resorption cavities make the bone look like Swiss cheese," Nunamaker said. "If it doesn't remodel, it breaks. Horses that don't get stress fractures don't fracture bones. If you reduce the incidence of bucked shins, you reduce catastrophic injuries."

During the 1990s, Nunamaker worked with John Fisher, D.V.M., a veterinarian and trainer based at Fair Hill Training Center in Maryland, to develop a training protocol to prevent bucked shins by stimulating bone remodeling while controlling microfractures. The three-stage program, called the Maryland Shin Program, prescribes short breezes twice per week, but it reduces the miles galloped.

The training program promotes bone density in the cannon bones of two-year-olds by conditioning them through short, frequent bursts of speed. Because bone density increases in response to stress, the program's aim is to stimulate the activity of the periosteum to strengthen the front cortex of the cannon bone (shin).

The program, which is conducted during a six-day week with Sundays off, is begun when a young horse reaches the stage in training where he easily can gallop a mile. Long, slow gallops are interspersed with one-furlong, 15-second works. Over a 16-week period, the length and intensity of the works are gradually increased until the horse is working a half-mile in :52 every third day. Once the training reaches this peak performance level, the workouts are scheduled every five days, with conventional training in the interim days.

"Institution of an effective training program for young Thoroughbreds to significantly decrease the incidence of bucked shins will, therefore, significantly decrease the incidence of fatal musculoskeletal, mid-cannon bone fractures during racing and training," Nunamaker concluded.