A joint is a connection between two or more bones, allowing for motion in a specific direction. Joints are stabilized by muscles and ligaments. When a joint becomes sore, through overuse, trauma or age-related degeneration, it stops functioning appropriately.
The temporomandibular joint (TMJ) is responsible for creating motion of the mandible (lower jaw) around the cranium - together, these two structures form the skull. The TMJ moves the mandible, whilst the cranium is attached to the atlas (first bone of the spine), and therefore remains stable. By moving the lower jaw, the TMJ can open and close the mouth and provide the motion required to chew. It therefore works in multiple planes - up and down to open the mouth, and side to side whilst chewing. There are 2 TMJs, one on either side of the head.
How does a horse’s jaw move when chewing? (1)
3 phases of motion are present:
Opening of the mouth for food to get into the right place to be chewed.
Closing of the mouth to actually be able to chew the food.
The power stroke - grinding of the teeth to break down the food.
As a horse opens its mouth, its mandible moves downwards, towards the side the food is going to go to and slightly backwards. As it starts to close, the mandible moves upwards, slightly away from the food and forward again. As the upper and lower teeth come together, the horse performs the power stroke, sliding the mandible upwards and away from the side where the food has collected.
When chewing pellets instead of hay, horses chew more frequently with less force (2). This may lead to less grinding of the teeth and the requirement of more frequent dental work.
Basic TMJ anatomy
The TMJ is composed out of two joint compartments separated by an intra-articular disc (a disc within a joint, kind of like a spacer). The two joints are the:
Discotemporal joint - between the disc and the temporal bone of the cranium - mainly responsible for the sliding of the joint (side to side).
Discomandibular joint - between the disc and the condylar part of the mandible - mainly responsible for the hinging of the joint (up and down).
The discotemporal joint is larger than the discomandibular joint. Generally, no communication portals are present between the two compartments (3). The TMJ does, however, communicate with parts of the ear (4) - apart from forming part of the discotemporal joint, the temporal bone also houses the ear canal.
The muscles involved in mastication are:
Masseter muscle.
Pterygoid muscles (lateral and medial).
Temporal muscle.
The muscles involved in lowering the jaw are:
Digastric muscle.
Geniohyoid muscle.
Mylohyoid muscle.
Out of all of these muscles, only the masseter and temporal muscles can be palpated/felt - the other muscles are either on the inside of the mandible or covered by other structures.
TMJ pain
The main causes of TMJ pain are from trauma or degenerative changes (such as arthritis). Degenerative changes can occur from overuse or poor biomechanical function/ motion.
Another cause of TMJ, in my opinion, is bridle fit and buckle position.
Note the position of the buckle, rubbing on the TMJ. This will increase the sensitivity of the joint.
What signs will my horse have if it has TMJ pain?
Sudden inflammation of one TMJ results in a change in the chewing pattern, decreased opening of the mandible and increased occurrence of quidding (dropping food) (5).
In a study of over 1000 horses (6), whilst performing head CT for reasons other than TMJ pain, more than 35% of horses over 12 months of age had degenerative changes within at least one of their TMJs. Changes within the TMJ have been linked with:
Poor performance (7)
Quidding (dropping food whilst eating) (8)
Intermittent colic (8)
Dangerous behavior changes under saddle (9)
Hindlimb lameness (9)
Note that with the above mentioned issues, each scientific paper was based on a separate but single case - meaning that each of those papers was written about a single horse (different horse in each study). It is therefore difficult to know how TMJ pain may affect your horse. The horses in points 2-5 also exhibited a clicking sound whilst chewing (8, 9), and both the clicking and issues stopped when the TMJ was treated. Unfortunately, the dangerous behavior eventually returned and the horse had to be euthanised (9).
Unfortunately, there is no single indicator of TMJ pain, making it extremely difficult to know exactly how TMJ pain will affect your horse.
So why does TMJ pain cause hindlimb issues and poor performance?
The TMJ is directly linked to the vestibular system due to the temporal bone also housing the ear canal. The vestibular system is responsible for balance. If the temporal bone has arthritic changes and is sore, it may result in discomfort within the vestibular system, therefore causing imbalance which will affect performance.
A number of myofascial lines (you can read more about them here: https://www.ebequine.org/post/myofascial-release) end on the TMJ and within the temporalis and masseter muscles. These include the superficial dorsal and ventral, lateral and spiral lines. If one of these myofascial lines is dysfunctional and contracted, it may cause TMJ tightness.
How do I check if my horse’s TMJ is sore?
When a structure is sore, it is painful when it is palpated (imagine if somebody presses down on a bruise!). Learning how to gently apply pressure to the TMJ can help to determine whether your horse is struggling with TMJ pain.
A simple way to check is to stand just behind your horse’s head, facing the same direction. Put your index fingers on the left and right TMJ at the same time, so that your left index finger is on the left TMJ and right index finger is on the right TMJ. If you are standing on the left side of your horse, pass your right hand under the jaw to get to the right TMJ, being cautious not to accidentally poke the right eye. Apply gentle pressure to each side at separate times and assess your horse’s response. If the horse repeatedly pulls away from the pressure, it is likely that the TMJ you are touching is sore.
Rarely, the cheek muscles on the side of discomfort appear to be smaller (atrophied) compared to those on the opposite side.
How can we reduce TMJ pain?
Routine dental work can help to ensure correct biomechanical motion and glide of the mouth and teeth during chewing, meaning that the horse’s TMJ is achieving a full range of motion.
Reducing the amount of hay that the horse gets with sloppy mashes and pellets can reduce the amount of force through the TMJ (2).
As the joint gets sore and inflamed, anti-inflammatories can be used to reduce acute inflammation within the joint - a list of options can be found here: https://www.ebequine.org/post/systemic-joint-therapy.
It can also be treated directly with intrasynovial medication (injecting drugs directly into the joint), and this too can be done after a joint block and further imaging. A list of options can be found here: https://www.ebequine.org/post/joint-medication-part-3-commonly-used-joint-medications.
During my assessment, I use a combination of laser acupuncture, acupuncture and chiropractic to immediately decrease TMJ pain. A one off treatment generally results in long term pain relief.
As an owner, you can apply gentle massage of the masseter and temporalis muscles to help reduce some discomfort as well.
Conclusion
TMJ pain is non-specific and frustrating. If you think that your horse has TMJ pain, organizing an appointment with a chiropractor or vet (or chirovet) that understands the biomechanics of the TMJ can help to improve long term dental health and performance.
References
Bonin, S., Clayton, H., Lanovaz, J. and Johnson, T., 2006. Kinematics of the equine temporomandibular joint. American Journal of Veterinary Research, 67(3), pp.423-8.
Bonin, S., Clayton, H., Lanovaz, J. and Johnston, T., 2007. Comparison of mandibular motion in horses chewing hay and pellets. Equine Veterinary Journal, 39(3), pp.258-262.
Pimentel, K. and Carmalt, J., 2021. The Frequency of Communication Between the Synovial Compartments of the Equine Temporomandibular Joint: A Contrast-Enhanced Computed Tomographic Assessment. Frontiers in Veterinary Science, 8.
Rodríguez, M., Agut, A., Gil, F. and Latorre, R., 2010. Anatomy of the equine temporomandibular joint: study by gross dissection, vascular injection and section. Equine Veterinary Journal, 38(2), pp.143-147.
Smyth, T., Carmalt, J., Treen, T. and Lanovaz, J., 2016. The effect of acute unilateral inflammation of the equine temporomandibular joint on the kinematics of mastication. Equine Veterinary Journal, 48(4), pp.523-527.
Carmalt, J., Kneissl, S., Rawlinson, J., Zwick, T., Zekas, L., Ohlerth, S. and Bienert-Zeit, A., 2016. Computed tomographic appearance of the temporomandibular joint in 1018 asymptomatic horses: a multi-institution study. Veterinary Radiology & Ultrasound, 57(3), pp.237-245.
Jørgensen, E., Christophersen, M., Kristoffersen, M., Puchalski, S. and Verwilghen, D., 2014. Does temporomandibular joint pathology affect performance in an equine athlete?. Equine Veterinary Education, 27(3), pp.126-130.
Smyth, T., Allen, A. and Carmalt, J., 2015. Clinically significant, nontraumatic, degenerative joint disease of the temporomandibular joints in a horse. Equine Veterinary Education, 29(2), pp.72-77.
Carmalt, J. and Reisbig, N., 2021. Arthroscopic treatment of bilateral mandibular condylar cysts and associated osteoarthritis of the temporomandibular joints in a horse. Equine Veterinary Education,.