Signs of a fracture to this bone are easily confused with other hoof injuries.

The summer of 2008 was a hard one for Gigi Carter. She couldn’t pinpoint the exact time that her intermediate event horse, Inka Dinka Do, started showing signs of lameness, but she does remember that it was a dry summer with hard ground.

“He kept coming up a little bit off after hard work, and I just knew that something wasn’t right, even though he was just a little off,” said Carter, 48, of Round Hill, Va. “So I decided to be safe and get him checked out.”

Carter called Piedmont Equine Practice’s Joe Davis, DVM, who took X-rays. When nothing showed up and “Ink” was still off, Carter decided to go a step further and get an MRI.

“I could just tell something wasn’t right, even if the X-rays weren’t showing anything,” she said. “For the longest time I thought it was a persistent stone bruise.”

The MRI revealed a coffin bone fracture. Even though it was a serious injury, Carter was relieved to finally have a diagnosis. Luckily, the fracture wasn’t too severe as to require any other therapy besides rest and time. Davis prescribed four months of stall rest with only hand walking.

“Ink hates being inside!” Gigi said with a laugh. “We had to buy a tiny round pen so he could at least be outside the barn.”

After the four months of stall rest, Carter faced another problem, bringing an event horse back into controlled work in January. “I’m a sales and marketing manager for a non-profit in D.C., and I travel a lot in the winter,” Carter said. “That made it very interesting to bring Ink back into work.”

Carter got the Argentine Sport Horse, who is now 11, when he was 5. “He had competed some novice, but I took him back down to beginner novice to start,” she said.

She took him up through the levels and was competing successfully at intermediate before the injury. Carter couldn’t say for certain that the fracture happened at an event.

“Bringing him back was slow. We spent a month at the walk, then added in the trot for a month, then the canter, and so on,” Carter remembered. “It wasn’t until the summer that he was ready to compete again.”

Between the stall rest and bringing him back into work, Ink spent a year recovering from the injury.

“I thought we could just jump right back into it at the level we were doing before. It didn’t quite work out that way,” Carter remembered. “It took a few events to get back into the groove of things. But it all worked out in the end.”

Carter ended 2009 on a good note, winning the preliminary The Chronicle of the Horse Adult Team Challenge at the Virginia Horse Trials. And the fracture hasn’t changed her long-term plans. “It is still my goal to advance to the level of our collective abilities,” she said.

But she has become more selective about which events she enters. “Footing is going to be a major factor,” she said. “I’m going to have to get used to scratching him if the conditions aren’t good,” she said.

“I just feel so lucky that Ink was able to fully recover,” she added. “We’ve been together for so long.”

Contributing Factors

A coffin bone fracture can cause the same clinical signs and pain of a hoof abscess. Yet because bruises and abscesses are far more common, a fracture can be easily overlooked. And a delayed diagnosis may worsen the break and lengthen healing time and recovery.

Coffin bone fractures are not particularly common among show and sport horses and recreational mounts.

Generally, they’re associated with race horses who exert excessive stress on their limbs at high speed and are at a higher risk of breaking a bone while galloping on uneven or hard surfaces. Young horses, whose bones are soft and still developing, also appear more prone to coffin bone fractures. Still, a horse of any age or discipline can break a coffin bone by simply kicking a fixed object or running over rocks or hard footing.

The coffin bone, also called the pedal bone or the third phalanx (PIII), is encap-sulated within the hoof, just below the short pastern bone and in front of the navicular bone. A break to this bone can occur when trauma or stress is applied repeatedly and forcefully. Most coffin bone fractures occur in the front legs, but horses can break the bone in the hind leg by kicking out.

“This is really a concussive injury due to repetitive pounding, which is probably why we see it more in race horses than any other sub group,” said Jim Hamilton, DVM, of Southern Pines Equine Associates (N.C.). He sees about five to eight coffin bone fractures a year.

Hamilton estimates that 75 percent of all coffin bone fractures occur along the “wings,” where the edges of the bone flare out. In fewer cases, a chip breaks off the edge of the bone (known as a solar margin fracture) or near the extensor tendon (extensor process fracture). In rare occurrences, the body of the coffin bone splits into two pieces. Fractures involving the coffin joint cause the most concern, as they carry the added risk of arthritis.

Trimming, shoeing and footing conditions, as well as conformation, may play a role in fractures. “Depth of sole can have bearing on whether the coffin bone is traumatized,” said Jerry Black, DVM, of Pioneer Equine Hospital in Oakdale, Calif. “It seems that thin-soled, platter feet that have no concavity are probably prone to coffin bone pain.”

Other contributing factors to coffin bone injury may include conformation flaws such as underrun heels and long toes, which shift a disproportionate amount of weight to the back third of the foot.

In some cases infection weakens the coffin bone and opens the door to a stress fracture. “The coffin bone is very porous,” said Hamilton. “It has massive vascular channels that run up and down it. Sometimes an area of the bone demineralizes—it loses bone density—in relation to an infection. That defect leaves the bone vulnerable to injury.”

Diagnosis

Clinical signs of a fracture are similar to other hoof injuries and may involve any of the following symptoms: lameness, heat in the hoof, swelling in the coronary band, a digital pulse, and focal or diffuse sensitivity to hoof testers.

Lameness varies with the location of the break. “Wing fractures start out with less lameness and can look more like heel pain,” said Black.

Horses with more severe fractures may appear shocky and resistant to put any weight on the limb. Affected horses may or may not respond to hoof testers or show pain over the entire sole.

Because signs of a fracture mirror those of an abscess, said Hamilton, a description of when and how the lameness occurred can help in diagnosis and further evaluation.

“I want to know when the horse turned up lame and what he was doing,” he said. “Though there are exceptions, you usually don’t see the pasture-kept horse suddenly appearing lame from a coffin bone fracture. The typical scenario is a horse who finishes his race or going cross-country. He’s sound and is taken back to the barn, cooled out, and then comes up lame. Usually there’s a lag time between injury and lameness. Those details help assess the problem.”

Coffin bone fractures are confirmed with radiography, but fractures are usually not visible from standard views; they require oblique views. Even so, some fracture lines cannot be seen until demineralization of the bone occurs seven to 10 days after trauma. “There’s not much separation at the fracture because the bone is encapsulated by the hoof,” said Black.

In uncertain cases, nuclear scintigraphy can help confirm or dismiss a suspected fracture. “Sometimes it’s difficult to confirm a fracture line that parallels the natural vascular channeling in the coffin bone,” said Hamilton. “In that case, a bone scan gives us an absolute diagnosis of the PIII.”

Problem Solving

Treatment of coffin bone fractures involves stabilizing the foot and allowing the fracture time to heal.

Veterinarians recommend stall rest to restrict movement and a bar shoe and clips or other therapeutic shoeing to minimize hoof expansion. A horse may require stall rest for anywhere from six to 12 months.

“The hoof is dynamic and it expands during impact, so the goal is to stabilize it,” said Black. “Usually, we recommend a bar shoe with up to five clips or a specialty shoe with a rim welded around it to keep it from expanding.”

Once the fracture has healed, veterinarians recommend gradually returning a horse to light work and using bar shoes and clips for a least one year after recovery to minimize the risk of re-injury.

Young, growing hooves in horses 2 years and younger heal successfully with minimal therapy. Horses under 1 year may not even require stall confinement and typically show no signs of lameness or developmental problems later in life.

In rare cases of a complete fracture through the body of the bone, surgery may be recommended for internal fixation using screws to secure the two bone fragments.

Said Black, “This procedure requires an experienced surgeon and special technology and imaging to drill through the hoof capsule and screw the two pieces of bone together.”

Prognosis for a full recovery from a coffin bone fracture varies according to the horse’s age and location and type of fracture. Young horses have the best prognosis for full recovery. Nonarticular wing fractures also resolve well with supportive therapy. However, some horses who return to full soundness retain fibrous callous—scar tissue—along the fracture line. The scar tissue doesn’t impede the horse’s activity, but it is visible on a radiograph.

Articular fractures offer a more guarded prognosis because of the high risk of degenerative joint disease. Hamilton said that he treats articular fractures with arthritis in mind. “You want to take an aggressive stance using medications and therapy and treatment to address the likelihood of degenerative joint disease,” he said.

Added Black, “Most of the time the horses who become arthritic never get totally sound from an injury. The fracture is stable, but the horse isn’t sound.”

While articular fractures offer a grave prognosis, wing fractures that don’t involve the joint typically heal well, and most horses with these injuries return to their former careers.

A Fracture In A Class Of Its Own

An extensor process fracture constitutes a coffin bone break, but it’s not a stress fracture. The extensor process is an outcropping of bone at the front of the coffin bone near the coronet band. When a fragment chips off the bone in this area, it’s known as an extensor process fracture.

“This is different because it’s not at the ground surface [of the bone]. It’s a fracture aggravated by dynamics of motion, not weight bearing,” said Jim Hamilton, DVM. “It probably occurs as a result of poor foot conformation—long toe and low heel—which results in a misalignment of the pastern and the foot. And if there’s any pre-existing weakness in the extensor process, then trauma to that area is more likely.”

Fragments of the extensor process may require surgical removal, though some are managed with special shoeing to increase the heel of the foot and alleviate discomfort. Hamilton added that he may use shockwave therapy to minimize extensor tendon inflammation that often accompanies this injury.