A resurgence of interest in the equine temporomandibular joint (TMJ) (which allows for opening and closing of a horse's mouth) led James Carmalt, MA, VetMB, MVetSc, FRCVS, Dipl. ACVS, ABVP (Eq), of the University of Saskatchewan's Western College of Veterinary Medicine, to discuss the relationship of TMJ disease to weight loss or behavioral changes in horses. At the 2010 Convention of the American Association of Equine Practitioners, held Dec. 4-8 in Baltimore, Md., Carmalt reviewed his study on how the TMJ responds to short-term inflammation as compared to the load-bearing fetlock joint.

Carmalt remarked that a horse grazes 18-21 hours per day, experiencing a staggering number of TMJ repetitive motions over time. Yet he said that the incidence of published, true TMJ disease in the horse is negligible, whereas human TMJ disease is relatively common.

Tooth floating has not been shown to consistently improve weight gain in the horse, therefore more horses are referred for TMJ disease, he noted.

"The feeling is that either this is a rare disease or we are not picking it up," said Carmalt. There are case reports of septic arthritis in the TMJ with secondary osteoarthritis or cases of post-traumatic inflammation, but there are no reports of primary equine TMJ disease; either horses are coping with the problem, horse owners and veterinarians are missing the condition, or it is not a real problem, according to Carmalt.

Previously published Japanese studies using mice challenged with medications to induce osteoarthritis demonstrated that mouse TMJ responds with less inflammation and cellular changes than what occurs in the mouse hock or stifle joints. Carmalt's study compared the equine TMJ response to short-term inflammation to that of the equine fetlock joint, looking to see if the TMJ has a different response than a joint that is routinely affected by osteoarthritis.

Investigators conducted thorough clinical, oral, and lameness exams in seven horses aged 5-10, along with taking radiographs of the fetlock joints. Then they injected lipopolysaccharide (LPS) into one TMJ and one forelimb fetlock of each horse to induce inflammation; the opposite of each joint was injected only with saline to serve as a control. Synovial fluid samples were taken over the next 24 hours and evaluated for inflammatory enzymes.

All joints injected with LPS were effusive (swollen), warm, and resistant to palpation, whereas only two control joints showed signs of inflammation. Interestingly, feeding behavior and chewing movements did not change--subjectively there appeared to be no effect of TMJ inflammation on eating. However lameness from the LPS-injected fetlock joints increased within several hours of injection (returning to normal by the end of the study). Carmalt suggested that despite probable TMJ pain, the horses kept eating. Carmalt concluded that inflammation appears to subside more quickly in the TMJ than in the fetlock joint.

Carmalt summarized saying, "The study suggests that TMJ disease may be rare in the horse and that rapid control of intra-articular inflammation within this joint may play a role (in its relative obscurity)."

About the Author

Nancy S. Loving, DVM

Nancy S. Loving, DVM, owns Loving Equine Clinic in Boulder, Colorado, and has a special interest in managing the care of sport horses. Her book, All Horse Systems Go, is a comprehensive veterinary care and conditioning resource in full color that covers all facets of horse care. She has also authored the books Go the Distance as a resource for endurance horse owners, Conformation and Performance, and First Aid for Horse and Rider in addition to many veterinary articles for both horse owner and professional audiences.

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