F E A T U R E
Physitis In Young Horses Horse Care
by HEATHER SMITH THOMAS When a foal is young and growing, the leg bones lengthen swiftly. This growth is accomplished by changes and growth at the ends of those bones. The region of bone-growing activity is called the physis or growth plate. This is where new cartilage cells are formed and mature. After they mature they are replaced by bone. Growth plates at the ends of the cannon bone, distal radius and distal tibia, for instance, are instrumental in lengthening these bones as the foal grows larger and taller. Sometimes during this period there is inflammation in one or more of the growth plates and this condition is called physitis (often called epiphysitis in earlier years). Gary Baxter, VMD, MS (Hospital Director at University of Georgia in Athens, Georgia), says physitis is often considered one of the growth problems/abnormalities that we tend to group in the category termed DOD (developmental orthopedic disease) in young horses. “We don’t always know the exact cause of physitis; some foals have mild signs of this as they grow—as a natural progression of growth. This area where the bones are growing have a lot of cellular activity,” he says. These growth plates may be temporarily enlarged while the foal is growing swiftly. “Many of them enlarge for awhile and then the enlargement disappears as the foal matures. You might think a foal has physitis just because there is enlargement of the growth plate, but if there is no heat or pain, it may be just a normal condition. Most of the time it’s nothing to worry about; it is just a young, growing horse,” he says. “Some of these enlargements may be abnormal, however,
Enlarged knee joints—A varus deformity with crooked legs (out at the knees).
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and some may be due to improper nutrition. We’re not entirely sure what type and kind of nutrition contributes to this. There are also some cases due to trauma. It may be external trauma or too much exercise and resultant concussion on the delicate growth plate. Whether or not there is actual damage may depend on where the physis is, in the growth period,” says Baxter. “There may be times when the physis is more vulnerable to trauma during its growth period. If the foal overuses the leg when the bone is still soft and not as strong as it should be, there may be some crushing and damage to cells in the growth plate. This could create swelling and inflammation, with clinical signs of physitis.” In some cases this might occur if the owner has the young horse on a forced exercise program. “Exercise might not be as damaging by yearling age; the growth plate is most vulnerable during the first year of life and particularly when the foal is growing fastest—during the first six to nine months of age. There can be a nutritional component and/or a mechanical component when a young horse develops physitis. There might be regions of trauma in the growth plate where there is damage. There may be subchondral bone damage or microfractures in that area, leading to clinical signs of inflammation,” Baxter explains. “The foal may give you clues. If you see only one physis affected, you would suspect some type of mechanical or traumatic cause, whereas if you see multiple physes involved (such as young horses with enlargements on all four distal cannon bones) it might just be related to growth. The physes at the distal cannon bone close before the carpus and tarsus. In general, we see physitis involving the distal cannon in younger foals (knobby fetlock joints) and might see swelling at the knees and hocks in the older foals.” It is less common to see enlargements in the growth plate at the hocks. “It’s more common to see physitis at the knees and fetlock joints. Mechanical physitis problems, especially at the knees and fetlocks, can lead to crooked legs (angular limb deformity).” The bone is growing faster on one side than the other, due to the inflammation being more on one side—leading to asymmetrical bone growth. “Any foal with significant physitis may end up with damage to the growing cartilage on one side of the growth plate. If the damage is asymmetrical it can lead to different growth rates on each side, creating limb deformities. It is especially important to monitor this at the fetlock joints. If there is significant physitis in that region, causing asymmetrical growth, it usually causes a varus deformity—a
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