How coffin bone fractures happen, and how to prevent them from becoming career-ending injuries
The coffin bone, also called the pedal bone or distal phalanx, is the terminal bone in the horse’s limb, encased within the hoof capsule. It serves as the foundation of the foot, where important structures ranging from the deep digital flexor tendon to the laminae attach. Despite its location behind the protective hoof wall, the coffin bone can fracture, resulting in one of seven types of breaks.
“Picture hitting an icicle with a baseball bat. If you swing like you are trying out for the Phillies, the ice shatters into a million pieces,” says Janik Gasiorowski, VMD, Dipl. ACVS, associate veterinarian and equine surgeon at the Mid-Atlantic Equine Medical Center, in Ringoes, New Jersey. “But if you were to push that same baseball bat slowly into contact with the icicle, it would only break into two clean pieces.”
The same holds true for bones. “All fractures happen quickly, but if you pause and ‘zoom in’ on the moment of impact, you see that the faster the force is applied, the more energy is stored in the bone, and the more violently that bone breaks,” Gasiorowski continues. “This difference accounts for ‘good’ versus ‘bad’ outcomes, with shattered bones carrying a worse prognosis than clean breaks.”
Indeed, shattering is a sign of high energy input at the time of fracture. “When the coffin bone is shattered into many pieces, we understand the energy input clearly because we see the resulting bone fragments,” he explains. “But things are not always so clear-cut.”
A coffin bone fracture can come with myriad invisible insults to surrounding structures. “Sometimes, much of the excess energy is absorbed by the soft tissues—cartilage, joint capsule, ligaments,” says Gasiorowski. “We cannot see squashed cartilage or torn ligaments on radiographs. Over time, these injured tissues scar or die. Damaged hyaline cartilage (the glassy smooth cartilage that lines joint surfaces) in the coffin joint gets replaced with (more rigid) fibrocartilage; the joint capsule and ligaments get thicker and stiffer. These changes cause arthritis.” Arthritis, as we’ll discuss in this article, can cast a dark shadow over an otherwise bright outlook for horse owners rehabbing a fracture.
Causes of Coffin Bone Fractures
If you consider the unique anatomy of the equine athlete, it could seem surprising that coffin bone fractures are a rarity rather than an inevitability. A half-ton animal landing at great speeds and with great force upon a single hoof at a time at the canter and gallop places a tremendous amount of pressure on the coffin bone.
Veterinarians and researchers (Kidd 2011, Morrison 2013) have found the most common causes of a broken coffin bone include:
- Trauma, which accounts for most fractures of the equine digit, such as kicking or colliding with firm objects, or racing injury.
- Improper shoeing, leaving the coffin bone to bear more weight and pressure than nature intended.
- Working on hard surfaces, inflicting repeated concussion on the bone.
- Stone bruises.
- Infectious conditions inside the hoof capsule, which can progress to osteomyelitis (bone infection) and fracture.
- Nutritional deficiencies—notably calcium and vitamins A and D—which weaken the coffin bone and predispose it to fractures.
Regardless of the type of coffin bone fracture, the most apparent clinical sign is generally lameness. To make a diagnosis veterinarians perform physical and lameness exams, including the application of hoof testers and possibly nerve blocks to locate the source of pain. Radiography (X ray) is the veterinarian’s go-to diagnostic imaging modality for suspected
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