This small joint, located on each side of the jaw, is almost solely responsible for allowing mammals to open and close their mouths. Almost 40% of horses have anatomical anomalies in the joint attaching the jaw to the skull, said James Carmalt, MA, VetMB, MVetSc, PhD, FRCVS, Dipl. ABVP, AVDC, ACVSMR, ACVS, professor of equine dentistry, surgery, sport medicine & rehabilitation at the University of Saskatchewan’s Western College of Veterinary Medicine, in Saskatoon, Canada. Additionally, as horses age, they can accrue wear-and-tear changes characteristic of osteoarthritis (OA) in their TMJs—but that doesn’t mean those alterations are affecting them.
“We’ve been looking for clinical signs of TMJ disease (specifically OA) in horses for 15 years, and so far we’ve only confirmed two cases,” said Carmalt.
Confirmed TMJ Clinical Cases
An 18-year-old Tennessee Walking Horse mare was having frequent colic episodes, with as many as eight in a single year, Carmalt said. She was also “difficult to ride,” fighting the bit, and dropping food (“quidding”). She’d lost 60 kilograms (140 pounds) in a year and was sensitive to touch on one side of her face. When she chewed, she had a clicking noise coming from her jaws. Carmalt’s team injected her TMJs with corticosteroids, and the horse’s problems resolved for a few months, before she had to be injected again. When she was eventually euthanized for other reasons, Carmalt scanned the mare’s head with MRI and confirmed arthritic degeneration of the joint, which could have caused her pain, he said.
Meanwhile, an 8-year-old Thoroughbred-cross dressage mount was having unpredictable, explosive behavior under saddle “to the point that the owner didn’t want to ride him,” said Carmalt. His team blocked the horse’s TMJs with local anesthesia (to remove pain), and the behavior improved.
“A computed tomography scan (CT) showed bilateral (on both sides) cysts in the mandibular condyle, which were treated surgically,” Carmalt said. “He returned to work and has shown no sign of the aberrant behavior since then.”
40% of Horses Have TMJ Changes
Degenerative changes in the TMJ are fairly common in aging horses, Carmalt said. Having examined CT scans of the heads of more than 1,000 horses, he found that nearly 40% had TMJ changes, which became more likely with advancing age. Despite these changes, though, the horses’ owners said they never noticed signs of TMJ-related problems.
Whether this means the horses don’t have pain related to TMJ changes is uncertain, he said.
“As prey animals, horses are very good at hiding these kinds of things,” Carmalt explained. “You can also find horses with fractured teeth that are eating and drinking normally, and you’d only know the teeth were fractured by looking in their mouths. But if that happened to us as humans, we’d be rolling around in agony. So it’s hard to say. That said, I do not doubt that pain from the TMJ is significantly more common in the horse than we appreciate. It is possible the underlying cause of a number of ‘rein-lame’ horses is pain and inflammation within this joint.”
The ‘Catch-All’ Diagnosis
Because it’s so “hard to prove,” Carmalt said, it’s easy to give a diagnosis of TMJ disease when caregivers can’t find anything else. “It’s nebulous, so nobody can prove you wrong.”
What’s more, steroid injections into the joint could resolve clinical signs or performance issues even if the TMJ isn’t the source of the problem, he added.
“There are structures around that joint that will respond to a steroid injection into it—cranial nerves, the temporohyoid joint, the paracondylar process, soft tissues, and so on,” he said. “Steroids can even get distributed systemically (into the rest of the horse) by injecting them into the joint. So you can get good results out of sheer luck, and then you’re even more convinced that it’s the TMJ that’s causing the problem, when it’s possible that there’s no disease there at all.”
Blindly Injecting the TMJ: A Bad Idea
Injecting the TMJ in hopes it might fix other issues isn’t wise, due to the joint’s structure, Carmalt said. “The fibrocartilage in the TMJ is a fundamentally different cartilage compared to other joints, and we don’t really know how it responds to steroid injections,” he said. “Nobody has done the work on that yet, not even in humans.
“Vets sometimes inject joints because clients want a quick and easy fix,” he continued. “But it’s important to keep in mind that steroids can have an effect on the ‘normal housekeeping’ of articular cartilage. Nothing is entirely without side effects, and essentially if you don’t need to inject a joint, don’t do it. ”
Ideally, we should aim for a clear diagnosis rather than just “blaming” the TMJ, said Carmalt. “The TMJ is just one facet of the entire body, and that entire machine needs to be functional for optimum performance,” he said. “A horse needs to be worked up properly. Veterinarians need to rule out hock pain, arthritis in the back, neck issues, and so on. Objective lameness exams can help point to certain issues not related to the TMJ.
“The team evaluating a problem horse needs to be starting from ground zero, looking at his history, assessing the movement of the horse from the ground and also under saddle, or tacked and performing its normal work, and address all possibilities before just getting pushed into a corner of having to inject the TMJ for lack of a better diagnosis, when ultimately this isn’t in the best interest of the horse.”