TEMPOROMANDIBULAR JOINT (TMJ) DISEASE
The temporomandibular joint (TMJ) of the horse resides just below and in front of the base of the horse's ear on either side of the head. The left TMJ is highlighted in the picture above. Movement within the joint is easily visualized when the horse is chewing. As with any joint, the TMJ can become unstable, inflamed and therefore a source of pain. TMJ pain can result in decreased performance, especially during periods of increased rein tension.
WHAT IS THE TEMPOROMANDIBULAR JOINT (TMJ)?
The TMJ represents the articulation between the upper jaw (maxilla) and lower jaw (mandible). There are two TMJ's, one on the right side and one of the left side of the head. These joints are used to open and close the mouth (such as during chewing) and are under considerable challenge when biting down on firm objects.
The joint can also experience stress when pressure is applied to the bit (via rein tension). This is especially true when pressure is applied in a downward direction, thereby applying downward pressure on the mandible.
The joint consists of two pouches (dorsal and ventral) which are separated by a meniscus residing at the rostral margin (towards the front) of the joint. The pouches usually do not communicate with one another except in cases of excessive joint effusion (i.e. a lot of extra joint fluid). Both pouches are easily palpable in the horse.
WHAT ARE THE CLINICAL SIGNS OF TMJ DISEASE?
Pain in the TMJ generally manifests as quick and involuntary tossing of the head. In many cases, the horse's tongue will also be visible to one side of the mouth during work. The response is often intermittent, but seems to be most commonly triggered when rein tension is applied in a downward direction on the lower jaw (mandible). For example, horses tend to react most violently to this tension during certain gaits such as the collected canter. In some cases, the horse will stick the head straight up in the air, refuse to move forward, and even back up. Horses with more severe pain might rear, jump, and/or buck.
As you might guess, TMJ disease is often confused with other musculoskeletal problems that manifest as behavioral resistance or head shaking. Unlike most other problems, however, TMJ pain can be readily elicited by applying
rein tension in the standing (non-moving) horse. If the problem only becomes evident when the horse is in motion then TMJ disease is not a likely cause.
To review other causes of head shaking in the horse, click HERE.
Although both TMJ's are typically involved, one side is worse in almost every case. Considerable difference in resistance is observed depending on which direction the horse is moving (and therefore on which rein is under more tension).
WHAT CAUSES TMJ DISEASE?
Although it is usually difficult to identify a primary cause, subluxation (i.e. partial dislocation) of the TMJ is most commonly implicated at the initial "trigger" of instability and pain within the joint(s). Subluxation of the joint may occur when an oral speculum is placed (such as during oral examination or teeth floating) or a firm object (such as a carrot or hard treat) is chewed.
TMJ is also considered by many to be genetically-induced, although this has not yet been clinically demonstrable.
Click HERE to learn more about joint disease in the horse.
HOW IS TMJ DISEASE DIAGNOSED?
History is usually very specific for this problem and is one of the primary tools used in diagnosis. The rider will complain of head tossing, refusal to move, and overt resistance to move in one or both directions.
Recent history of oral examination and/or teeth floating can be another important piece of the diagnostic puzzle. Many horses resist the oral speculum used to keep the mouth open by applying counter pressure (biting against it), thereby subluxating (partially dislocating) one or both TMJ's. Approximately 35% of the horses diagnosed with TMJ disease by The Atlanta Equine Clinic were fitted with an oral speculum within the 6-month period prior to initial examination.
In more severe cases horses may struggle to chew hard objects such as an alfalfa cube, carrot or sugar cube (even though they may have an excellent appetite).
In approximately one-half of the cases seen by The Atlanta Equine Clinic, gross enlargement of one of the TMJ's is visible during clinical examination. Excessive protrusion of the joint may be evident when viewing the head straight-on at eye level. In severe cases, joint effusion (i.e. extra joint fluid) and/or associated soft tissue inflammation (edema) may also be present. Usually one joint (the more affected one) is visibly larger than the other.
In our experience, a positive response to digital palpation of the ventral (bottom) joint pouch is almost always observed in horses with TMJ inflammation/ pain. The degree of diagnostic confidence is dramatically increased if one joint consistently elicits more reaction than the other, and the more painful side correlates with a history of directional resistance.
Although diagnostic imaging (such as radiographic examination) can be useful during evaluation of TMJ disease, this modality alone cannot confirm or deny diagnosis in our opinion. Most horses with (even severe) cases of arthritis will have a normal radiographic appearance to the TMJ's. Therefore, this strategy is used more to determine prognosis for future performance rather than confirm a diagnosis of TMJ disease.
HOW IS TMJ DISEASE TREATED?
We implement the same principles to treatment of the TMJ's as we do with other joints in the horse.
If you would like to learn more about treatment strategies for DEGENERATIVE JOINT DISEASE, click HERE.
Fortunately, the TMJ's are easily accessed with a needle and respond well to intraarticular therapy (local injection). The dorsal and ventral joint pouches are usually treated separately to insure adequate dispersement of the medication within the entire joint.
In more than 50% of the cases we evaluate, single (one-time) treatment of the TMJ's permanently resolves the problem. In order to prevent recurrence, we recommend that the client refrain from feeding any hard treats (such as alfalfa cubes, sugar cubes or carrots) and that oral examination/ teeth floating is only performed when absolutely necessary. When performing oral procedures on horses with TMJ disease, we strongly encourage the use of ample sedation to reduce the risk of speculum resistance and secondary joint subluxation.
In cases of long-term and/or severe TMJ instability, multiple (serial) treatments may be necessary to maintain optimum performance. Fortunately, we have observed minimal deleterious effects within the TMJs as a result of performing multiple injections. These joints appear to be very forgiving with regard to this form of therapy, which makes this technique one of our "front-line" approaches to successfully addressing this problem in the horse.