Occurrence and etiology
"During the last few years, we have gained a better understanding of the disease," Bramlage says. "In general, we find these
fractures in the early stages at a much higher rate than we used to." That is because technology such as bone scans and, more
recently, MRI have contributed to our understanding of the disease and to our understanding of what we are seeing on radiographs.
"Through the use of MRI, it's become apparent that when we can see the fracture on radiographs, it is already well into the
disease process. MRI can show a whole lot more pathology than we ever knew was there," says Bramlage.
"We've known for some time that this disease process occurs because of the repetitive trauma to the bottom of the cannon bone
that a horse sustains while it's training," Bramlage says. The key thing to understand is that the horse's skeleton has to
adjust to the stress of training. "The musculoskeletal system of a yearling is just not capable of withstanding the kind of
stress that a racehorse will put on it. As training progresses, the skeleton adapts."
The major places where this adaptation is seen are in the front cannon bones and the tibias. These bones have to get much
bigger and stronger. "If the tibias get behind in this adaptation, the horses will develop a stress fracture," Bramlage says.
"If the cannon bones of the front legs get behind, the horses will get bucked shins or eventually a stress fracture if they
These two bones have the advantage of being able to change both their structure (what they are made of—they can get denser)
and their shape (the cannon bone and tibia enlarge). "That adaptation also has to take place on the bottom of the cannon bone
at the fetlock joint surface," says Bramlage. "The big difference is that the surface at the bottom of the cannon bone doesn't
have the luxury of changing its shape. It is a joint surface. The only option for the bottom of the cannon bone is for the
bone to change its structure, which becomes denser and denser."
"There are two kinds of vulnerabilities associated with that," says Bramlage. "One is the original process of making that
bone continually stronger in response to training, which can create a condylar fracture. The second vulnerability is the extreme
density. If you change the bone away from normal porous or spongy cancellous bone to more dense bone, it becomes brittle,
allowing fractures to propagate much more aggressively.
"This can be likened to a porcelain coffee cup that has gotten a small crack," Bramlage says. "It still holds coffee, doesn't
separate and never seems to be a problem, though if you hit it just right or load it at just the right location, the cup falls
apart." In condylar fractures, the bone sustains microdamage over time and continually repairs itself as the bone adapts.
If the horse loads a vulnerable area unevenly or heavier than normal, it might result in a condylar fracture.
Bramlage notes that anecdotal data indicate artificial racing surfaces are much more consistent than dirt tracks. Horses do
not hit irregular spots in the base on artificial surfaces as frequently as they do with dirt, so the number of condylar fractures
is reduced with these new surfaces. As long as a horse never hits a vulnerable area just right, it can withstand the microdamage
and eventually repair itself. But if it finds an irregular spot in the base of the track or makes an awkward foot plant for
whatever reason, a condylar fracture may result.
All horses have this process of damage and repair going on at all times. "The fact that the bottom of the cannon bone is incapable
of altering its shape to minimize that stress makes it much more vulnerable throughout the horse's career than the shins,
which adapt their size and shape to neutralize the force, or the tibias, which eventually adapt their size and shape and are
no longer vulnerable to stress fractures," Bramlage says. "It's a big demand to ask of the bottom of the cannon bone."