The Modern Equine Vet
August 2023
Vol 13 Issue 8 2023
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Cover Story | Lameness

Maybe That Lameness is All in the Horse’s Head

By Marie Rosenthal MS

It might seem a bit out there, but inflammation of the equine temporomandibular joint (TMJ) may play a role in lameness and poor performance of the horse, admitted James L. Carmalt, MA, VetMB, MVetSc, PhD, FRCVS, DABVP(Eq), DAVDC(Eq), DACVSMR(Eq), DACVS, a professor of Equine Surgery, Dentistry, Sports Medicine & Rehabilitation, in the Department of Large Animal Clinical Sciences, Western College of Veterinary Medicine, University of Saskatchewan.

“This is not your classic lameness with the head ‘bobbiness’,” he said, “and we may end up opening a can of worms as we get into this.”

However, the work of he and his colleagues point to a link between TMJ inflammation and lameness.

Until 10 or 15 years ago, no one paid much attention to the TMJ, but they came to realize that the joint is clearly dynamic, and just as other joints do, there are changes associated with aging and use. Additionally, some young horses have developmental abnormalities in the joint, which can also affect mastication if inflamed.

Dr. Carmalt did come across a horse, which kept throwing its rider. The team noted a mild left hindlimb lameness, which was confirmed by using the Lameness Locator by Equinosis. Interestingly, they noticed that the lameness worsened when the side-reins (attached to a surcingle) were shortened. When they blocked the TMJ, the baseline lameness remained, but it did not worsen when the reins were shortened.

“We had not touched this horse in any way, other than to block the temporomandibular joints,” Dr. Carmalt said, “and it fundamentally changed the movement of the horse. Not only subjectively—but because we were thinking about publishing it—we got objective data, and the objective data supported that subjective assessment.”

They treated the horse successfully, and it was better for about 9 months before the behavior became a problem again.

Prior to this first clinical case, they had been pretty sure that TMJ inflammation could result in a performance problem but were not sure how, or to what degree. At first glance, the joint is not under a heavy load, but consider it a moment, and it’s easy to see that the bit is in the mouth and under the control of the rider, so there could be a link between pain or abnormalities of the joint and the horse’s movement, he said.

Taking the Reins
Part of the problem of rein lameness is its definition, he explained. “The definition of rein lameness is difficult because it’s a lameness that can be appreciated [felt or seen] only when the horse is being exercised under saddle or wearing a bridle and being exercised.

“And we’ve all had these cases where the client comes and says, ‘I feel my horse is off.’

“These riders are feeling something. You look at it as a veterinarian without any person on the horse. You trot it, you lunge it, you can’t see it. I’m thinking, if I can’t see it, how the heck, do you expect me to work this out?

“Now you put some tack on the horse, with or without the rider, and start moving the horse. That changes everything, and now, you can begin to see what you’re talking about,” Dr. Carmalt told the Modern Equine Vet.

Dr. Carmalt and his colleagues set out to see if there was a measurable link between inflammation of the TMJ, lameness and performance. The objective of the study was to determine the effect of temporomandibular joint inflammation on rein tension with the hypothesis being TMJ inflammation would alter the response to rein input on horses moving on a treadmill.

They trained 5 horses to walk and trot on a treadmill wearing long-reining equipment with rein-tension devices and 28 reflective optical tracking markers. They assessed the horse’s movement during a free walk and trot without rein-tension and again at the same gaits with rein-tension. Data were collected continually from both sides of the horse using the rein-tension devices and by using 12 kinematic cameras focused on the optical tracking markers. After collecting the initial data, one TMJ was randomly assigned to receive an injection of a substance that causes mild irritation but was self-limiting. Six hours later the study was repeated. The horses got 10 days off after the injection, before the study was repeated, and this time the opposite TMJ was injected.

Throughout the study, the highly experienced horse handler and research collaborator, Nathalie Reisbig, MedVet MS, PhD, had no idea which joint was injected.

“Despite that, she could feel it in her hand. If we injected the dominant side, the horse switched, and the dominance went to the originally non-dominant side. And if we injected the non-dominant side, the horse became very heavy in the hand on the previous dominant side.

“If, for example, we injected the left side and then she needed a little bit of left rein to keep the horse in the middle of the treadmill, this horse would overcompensate and go really left,” he explained.

“This makes sense,” Dr. Carmalt said, “The horse is trying to avoid the pain.”

One interesting finding, he said, was that they did not see large changes in horse movement when they looked at the kinematic findings. The one significant finding was that after injection, horses had a more vertical head position at trot. “Again, this makes sense. The horse has pain that is being exacerbated by rein tension. If the horse stays “behind the bit” the pressure exerted on the TMJs by the handler is reduced.

Overall, the lack of other significant differences may have been because the “horse handler was working substantially harder to keep that horse straight and moving correctly. So, the handler themselves probably had an important effect on the negative result of the kinematics,” he said.

Listen to the Riders
They have blocked the temporomandibular joints on several other client horses, and the riders tell him that the horses are moving substantially better.

He said the moral of the story is just because a veterinarian cannot see the problem, when a rider or client comes in saying that they are “feeling” something is different when they ride the horse, consider blocking the TMJ.

“If you’ve got TMJ inflammation, horses can have dramatic responses to rein input without being classically lame,” he said.

“The riders are not wrong. They are feeling something that is different, and our job is therefore to go after what is different.”

Despite the current interest in TMJ disease, it is important to rule out the more common causes of lameness, such as foot pain, or bone spavin before looking at TMJ problems, said Dr. Carmalt, who spoke about the study and the issue during the 68th Annual AAEP Convention, held in San Antonio. MeV

For more information:

Reisbig NA, Pifko J, Lanovaz JL, et al. The effect of acute equine temporomandibular joint inflammation on response to rein-tension and kinematics. Front Vet Sci. 2023;19(10):1213423.

Also check out Dr. Carmalt’s facebook page: