How it Happens
Palmar osteochondral disease is a condition affecting the lower ends (condyles) of the cannon bone—an area subject to very high and repetitive forces during racing or race training. To understand why it occurs, we first need to understand how bones adapt to exercise.
“Horses in high-intensity exercise like race training adapt their bones to the repetitive loads placed on them—this is a normal process,” said Garrett. “If you want a bone to become strong at race training, you need to train the bone to know these are the forces being asked of it. Bones are incredibly smart, and they will adapt in response to these loads.”
This adaptation doesn’t happen overnight; she said it can occur over days to weeks to months. “Ideally, the bone makes those adaptations and can keep up with training load,” Garrett said. “This balance can be difficult to strike, and we don’t always get it right, unfortunately. POD develops when an imbalance exists between the work asked of a horse and the horse’s ability to adapt the bone quickly enough—essentially the bone can’t keep up, and a normal process gets out of hand.”
The result is chronic subchondral bone (which lies just beneath the cartilage lining the joint) fatigue and damage, which manifests in the horse as lameness.
“One thing that’s very frustrating is that there’s no clear answer as to when the bone isn’t keeping up, and when the adaptive changes cause lameness, because there is a ton of individual variation,” Garrett said. “You can’t just look at a set of (diagnostic) images and know whether a horse can keep training or needs a break, so you need to pay attention to the individual horse itself.”
Diagnosing palmar osteochondral disease starts by isolating the source of pain to the fetlock region through a lameness exam with nerve blocks. Then, veterinarians can move on to diagnostic imaging methods that identify changes in the bone.
“We’re looking for structural changes that we can find with radiographs, CT, and MRI, as well as functional changes (i.e., what the bone is doing) that we see with nuclear scintigraphy and PET scan,” said Garrett.
Different imaging modalities, of course, have different abilities to detect structural changes in the bone. “Radiographs are good but not great at detecting changes,” Garrett explained. “That has to do with two major factors: superimposition of adjacent bone—we’re getting a 3D image that’s been flattened into two dimensions—and less sensitivity to subtle changes than MRI or CT. MRI and CT are excellent at detecting these changes.”
Monitoring horses and intervening when they begin to show signs of POD is important, because severe damage can lead to permanent joint damage and generalized osteoarthritis in the fetlock. Otherwise, affected horses’ prognoses is typically very good, said Garrett, usually requiring just a break from training and plenty of turnout to keep the joints moving.