Are TMJ Variations Normal in Horses?

Researchers found that “an unexpectedly large number of normal horses” had temporomandibular joint (TMJ) variations.

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In 2008, researchers used computed tomography (CT) to examine the temporomandibular joints (TMJ, which are located on either side of the horse’s jaw and allow the mouth to open and close) in eight young horses. That study showed that CT can produce good images of the TMJ, said James Carmalt, MA, VetMB, MVetSc, FRCVS, Dipl. ABVP, AVDC, ACVS, “but we can’t rely on eight horses to provide information for the entire population. We have to find out what’s out there. What can we define as normal” for these joints of increasing interest to the equine veterinary community.

To that end, Carmalt, professor of equine surgery at the University of Saskatchewan’s Western College of Veterinary Medicine, in Saskatoon, and colleagues recently took a more widespread look at the equine TMJ in hopes of characterizing age-related features. He presented those findings at the 2015 American Association of Equine Practitioners’ Convention, held Dec. 5-9 in Las Vegas.

The TMJ is a unique joint, Carmalt said. It’s a diarthrodial (the site of the junction or union of two or more bones), synovial (movable), load-bearing joint that undergoes countless repetitive motions. It has a meniscus, and the joint surfaces are covered with fibrocartilage (made up mostly of fibers like normal connective tissue) rather than hyaline cartilage typically seen in limb joints. And, TMJ injuries or ailments appear relatively uncommon compared to other joints.

Carmalt said he and colleagues hypothesized that TMJ variations on CT would be rare, but they wanted to characterize the ones they identified. So, they collected CT scans of 1,018 horses’ TMJs (for a total of 2,036 TMJs) from eight clinics in North America and Europe. None of the horses included were suspected of having TMJ problems, he said

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Erica Larson, former news editor for The Horse, holds a degree in journalism with an external specialty in equine science from Michigan State University in East Lansing. A Massachusetts native, she grew up in the saddle and has dabbled in a variety of disciplines including foxhunting, saddle seat, and mounted games. Currently, Erica competes in eventing with her OTTB, Dorado.

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