Understanding Equine TMJ Disease

You may have heard of TMJ disease in humans, but until fairly recently it wasn’t known that horses could get it as well. TMJ pain can be caused by trauma, arthritis, infection, inflammation or just wear-and-tear and can manifest as a number of symptoms including head tossing, head sensitivity, trouble eating, uneven wear on the teeth, and difficulty with bits and flexing at the poll.

The temporomandibular joints (TMJ) are two joints that connect the lower jaw to the skull, responsible for movements such as chewing and speaking. In humans, TMJ disorders can be known to cause pain, limitation of movement, and a clicking sound or grating sensation whenever you open your mouth or chew.

TMJ pain is relatively common in humans, with as many as 60 per cent of adults experiencing symptoms of TMJ disease during their lifetime. It’s also a common diagnosis in dogs, cats, and sheep – but it wasn’t typically seen in horses, something that Dr. James Carmalt found curious.

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“I thought, why are we not seeing this, given the fact that it’s so common in other species?” Carmalt said. “Every other joint gets arthritis. So why not this one?”

Carmalt is a researcher and professor with the University of Saskatchewan’s Western College of Veterinary Medicine. His research on the occurrence of TMJ disorders in horses has shown that horses often do show signs of cartilage degeneration and arthritis in their TMJs. Similar to other species, there is an age effect, with damage to joints accruing over time.

This research has opened the door to understanding TMJ disease in horses – to the extent that now it’s turning into a bit of a ‘trendy’ diagnosis, with possibly too many horse owners mistakenly believing their horse has TMJ disease to explain a plethora of behaviour issues.

Inside the temporomandibular joints

The majority of joints in the body consist of hyaline cartilage, a very specialized cartilage that helps bones move smoothly past each other and acts as a shock absorber. Once damaged, hyaline cartilage has a very limited potential to repair itself, as it doesn’t have a direct blood supply. Usually, a repair tissue is formed in its place, made up of fibrocartilage.

James Carmalt, Professor, Department of Large Animal Clinical Sciences, WCVM USask. (stobbephoto.ca)

The TMJ is fundamentally different from other joints, because it’s composed entirely of fibrocartilage, which is considered biomechanically ‘inferior’ because it lacks durability. That is a curious thing, considering the joint’s sole job is to feed the body system – a crucial function. “Why would you build an entire joint out of inferior cartilage?” Carmalt asks. “That makes no sense, from an evolutionary standpoint.”

He continues, “There has to be some benefit to having a joint that’s made up of inferior cartilage,” Carmalt says. “So then the question comes, what if it’s not inferior? Maybe the reason we’re not seeing a lot of disease is [because] it can actually do the job better than other joints.”

Early research

These questions led Carmalt to start looking into the prevalence of the disease, and try to build a story as to why we may (or may not) be seeing TMJ in horses.

Early studies between 2015-20 by Carmalt and his team started by looking at how the TMJ responded to trauma and found that it beats inflammation down much faster than a fetlock joint, for example. “It’s possible that the ability of the TMJ to respond to inflammation and respond faster and more completely is an evolutionary advantage,” Carmalt says.

Even when the TMJ was inflamed, however, “these horses still wanted to eat,” Carmalt says, something the researchers found puzzling. So they did another study where they put kinematic markers all over the horses’ heads and used an infrared camera on them while feeding them. It turned out that while the horses did eat with the joint inflammation, they adapted by chewing on the non-painful side.

“The drive to eat actually outweighs the pain, but they’re adapting,” Carmalt says. “But that didn’t explain why we’re not seeing clinical disease [disease with recognizable signs and symptoms].”

From there, he and several colleagues reviewed over 1,000 horse head CT scans. They found that the prevalence was actually very high, with approximately 40% of horses showing changes in their TMJs. But change isn’t necessarily the same as disease, Carmalt notes.

It was a case study on a dressage horse named Julio that eventually proved that there was clinically apparent TMJ disease in a sport horse. Julio was a normally well-behaved dressage horse who had become unpredictable and thrown his rider a couple of times. His owner went through several different vets, with one telling her Julio probably had TMJ disease. She wanted a definitive answer, so she shipped Julio to Carmalt and his team.

Julio arrived with hind end lameness, a separate issue that was being exacerbated by the TMJ pain. Carmalt’s team put the Lameness Locator on him and tried walking him in a circle on a loose rein each way, then gradually tightening the rein. As they tightened the rein, the hind end lameness got continually worse.

Then they blocked the TMJs and started over again, walking on a loose rein and slowly shortening them. The baseline lameness stayed the same, but this time didn’t get worse as the rein tightened.

“That doesn’t make sense,” Carmalt says. “Prior to the blocking, he was behind the bit. He was throwing his head around, he was not happy. After we blocked him, he was completely different.”

CT scans showed that Julio had huge mandibular condyle cysts. They spent six months developing the surgical technique to go in arthroscopically and debride the cysts. After the surgery, Julio did fine for nine months before getting worse again.

Carmalt’s team published the study in 2015 as a case report. “It showed us that there could be clinically apparent TMJ disease in a sport horse,” Carmalt says.

In another study, Carmalt and Dr. Nathalie Reisbig, a clinical associate in equine field service at the WCVM Veterinary Medical Centre, put horses on a treadmill. They used kinematic markers all over the horse’s heads, rein sensors, and kinematic cameras to try to work out exactly how the horse was moving with TMJ pain, since in humans there is a connection between TMJ disease and balance disorders. The horses were given a lipopolysaccharide injection on one side, which causes mild joint pain and inflammation for 24 hours with no long-term effects. Reisbig long-reined the horses from the side of the treadmill to see if the difference could be felt through the reins.

“The theory was if these horses are reacting differently because they’ve got some pain in their TMJ, maybe the rider will be able to appreciate the difference,” Carmalt says. The study found that “the long and short of it is, yes, you can tell,” Carmalt says. When there was pain in the TMJ, Reisbig was able to feel it via the long reins, even if it was subtle: the horses would avoid the bit tension or pressure on the injected side. They also picked up that the horses were behind the bit the entire time when they had TMJ pain, and avoided bit contact as much as possible afterwards. “Now you can say that inflammation in the TMJ can cause that, which has opened everything up now because now all the riders are blaming any bit contact issues and chewing issues on TMJ disease.”

After spending 15 years questioning whether or not horses could suffer from TMJ disease, and then proving that they could, Carmalt now finds himself telling people that their horses don’t have TMJ. “Everybody’s jumping on the bandwagon,” Carmalt says, adding that it’s causing a major issue with over-diagnosis and people who think their horse has TMJ disease may give it steroid injections to treat it. Steroids have a body-wide effect, however, far beyond the joint that’s been injected. “So if the horse gets better after you inject the joint, everybody’s saying, ‘Oh, I got the diagnosis right,’ when in fact, that’s not necessarily true,” Carmalt says. “So a lot of people are making poor diagnoses and probably missing other stuff, to be honest.”

The over-diagnosis issue prompted them to write a free-access paper, published in November 2023, which describes “a logical, stepwise approach to excluding common causes of poor performance before investigating the potential role of the TMJ in cases of poor performance.” For example, a horse may have another issue (i.e., back pain) and TMJ. “If you want to return to top performance, you better make sure you diagnose all of the problems and treat them sequentially rather than just cherry-picking today’s sexy diagnosis.”

Getting horseowners involved

Current options for treatment of TMJ disease are fairly limited and include antibiotics if infection is present, corticosteroids, hyaluronic acid or combination joint injections, or even corrective dental work to ease discomfort. Many horse owners swear by complementary therapies such as chiropractic, massage, or acupuncture. Carmalt says that nobody has seen enough cases yet to say whether any treatment has a higher success rate.

Currently, Carmalt and Reisbig are working on a study to compare “the basic cellular metabolisms of the fibrocartilage and the hyaline cartilage along with their cellular responses to inflammation and anti-inflammatory treatments,” according to the WCVM website. Simply put, Reisbig is doing benchtop work to see how primary cell lines from different joints respond to treatment.

“Even on the human side, nobody has done the basic research,” Carmalt says. The hope is that this work will help determine whether certain treatments have a more positive or longer-lasting effect than others.

Meanwhile, on the clinical side, Carmalt’s team is trying to build up the number of definitive TMJ cases.

“Until we start getting a larger number of horses that are definitively diagnosed, we’re not going to be able to get much further forward,” Carmalt says. “Because it’s very difficult to come up with a treatment protocol that you can expand to the population at large.”

One of the steps they’ve taken to find cases is put out the call on social media through a public group on Facebook called Equine TMJ Disease. The group has grown to over 1,800 members from around the world. Horse owners and practitioners can use the group to discuss both suspect and diagnosed equine TMJ disease in order to help the research team understand an approximate idea of the prevalence of equine TMJ worldwide. “It is amazing,” Carmalt says. “We’ve got cases in Russia, Italy, South Africa, New Zealand, Australia, the UK.”

Carmalt stresses that the page is not meant to replace people communicating with their veterinarians, or to replace a veterinarian’s professional opinion. “I’m very careful not to second-guess the vets,” Carmalt says, “because I’m not there, I didn’t see that horse move.”

For free, however, he will listen to people’s experiences as it pertains to TMJ, look at the CT scans, and if he thinks it could be a TMJ case, he may offer to speak to the person’s veterinarian. “I can interpret the CT images, if they send them to me, in conjunction with their vet who saw the horse,” Carmalt says. “I’ve had a number of events where the two of us talked about the case and worked it up together,” Carmalt says. “That’s been really, really cool.”

If a horse owner or veterinarian suspects that a horse in their care might have TMJ, Carmalt encourages them to get in touch by either emailing him directly (james.carmalt@usask.ca) or contacting the researchers through the Facebook group.

The most important thing, Carmalt says, is “just being open to discussion.”