"Forelimb conformation receives lots of attention because of perceived predisposition to injury (with various conformational problems)," said Liz Santschi, DVM, Dipl. ACVS, clinical associate professor at the University of Wisconsin-Madison.

"Angular limb deformities (limbs crooked when viewed from the front), rotational deformities (such as toeing in or out), offset deformities (bench knees), and combinations of these deformities are very common," she reported, at the Western Veterinary Conference held Feb. 20-25, 2005, in Las Vegas, Nev. She discussed a study of foal conformation that she recently completed and will submit for publication.

Limb conformation has bearing on procedures used to alter that conformation including periosteal elevation, transphyseal bridging, hoof augmentation, and changes in management. Conformation also affects selection of sale and breeding stock. "Yearlings with carpal offset (bench knees) are now often subjected to surgery to make them valgus (knock-kneed) as well based on some observations that horses that are slightly offset and valgus (bench and knock knees) have less injury than horses that only have offset knees," she stated. "I imagine that it is confusing to clients as for years we've been telling them that carpal valgus needed correction, but now we're telling them it's desirable.

"Adding to recent controversy, some have suggested that periosteal elevation doesn't work (to fix crooked limbs)," she went on. She referenced a few studies that found no difference in correction success and speed between periosteal elevation and no treatment at all.

"What do we think, what do we know, and what can we prove (regarding conformation and correction)?" she asked the audience. "I'm trying to increase the proven information."

In her study, racing-bred Thoroughbred foal forelimb conformation was evaluated shortly after birth and repeatedly as the foals matured. She found that nearly all foals were carpus valgus (knock-kneed) at birth, many toed out, and very few had offset knees. But the incidence of conformational deviations changed quite a bit as the foals grew up. As long yearlings, nearly two-thirds had offset knees and the incidence of knock knees had dropped by a large margin. Less than 10% of foals were considered to be completely straight in the forelimbs at any age, and more than half had more than one conformational deviation.

Another finding was that heavier birth weight of the foal was associated with offset carpal conformation at every age. While the debate over nature vs. nurture controlling development rages about multiple species, Santschi opined that the concept of genetics alone controlling physical characteristics is "too simple."

She quoted another study saying the following: "Limb conformation is determined by a genetic template that is impacted by developmental processes and environmental influences tempered through biological constraints."

To look at the genetic component of conformation, her study also evaluated conformation of the foals' sires and dams whenever possible. She found several associations between carpal conformation of the parents and that of the offspring. "So it took me a long time to prove what everyone already knows," she said with a laugh.

Conformation Correction
Santschi also evaluated the efficacy of conformation correction in some foals. "My opinion on procedures is that they (especially hoof augmentation) reduce fetlock toeing in, and transphyseal bridging for severe early carpal valgus is useful, but that avoiding bone crushing (from unbalanced loading of incompletely ossified knee bones) is at least as important (as surgical therapy)," she said. "Periosteal elevation may accelerate self-correction, but is mostly unnecessary with regards to ultimate conformation. However, it may have value to prevent crushing carpal bones (by correcting poor conformation more quickly and thus reducing the amount of time carpal bones undergo unbalanced stress)."

She compared carpal bone crushing to scoliosis in humans, in which asymmetrically loaded vertebrae become wedged in shape; with more loading, these bones become more and more crooked, resulting in progression of bony deformities. "I think this is what happens with foal legs," she said.

"How do we get misshaped bones?" she asked the audience. "These can result from abnormal forces on normal limbs (compensatory overload) or normal forces on abnormal limbs (hypoplasia). Limbs become permanently and severely deformed when abnormal forces are placed on abnormal limbs, resulting in progression of deformity.

"What do we know about load on foal bones?" she continued. "There is normally two times the compressive strain on the medial (inside) aspect of the cannon bone than on the lateral side. Lateral (hoof) wedges alter strain for about 10 days, then the limb returns to higher medial strain. I think this is why hoof augmentation can work, because it changes the loading on the bone."

To further illustrate how loading changes bones, she showed the audience a computed tomography scan of the lower end of a toed-in foal's cannon bone. (The scan shows bone density, which increases with increased strain.) "See this very dense spot at the dorsal medial corner of the fetlock?" she pointed out. "That's where all the strain is; it becomes denser due to the increased strain. The overload reduces growth in this local area, and everything else grows around it. This changes long bone growth of the forelimbs and makes the foals look like bulldogs." She also showed several radiographs of foal bones before and after correction, illustrating how asymmetrical bone density in response to asymmetrical load (before correction) changes to more uniform density after correction.

"What we start with are foals that at birth have various degrees of overall musculoskeletal maturity, and occasionally have focal immaturity," she summarized. "When these tissues (bones and soft tissues) are less mature, they are more susceptible to load. If we can keep them from pounding (asymmetrically) on those soft little bones, they'll be OK.

"Most limb deviations in foals are a normal developmental process, or the result of musculoskeletal immaturity," she summarized. "Some require more specific management, but will improve after careful evaluation and management changes (such as restriction of exercise). And a small number of foals will require more specific therapy such as hoof trimming and augmentation, external coaptation (casts and braces), and/or surgery."

About the Author

Christy M. West

Christy West has a BS in Equine Science from the University of Kentucky, and an MS in Agricultural Journalism from the University of Wisconsin-Madison.

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