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By: Dr. Lydia Gray
The temporomandibular joint (“TMJ” for short) is where the lower jaw or mandible connects with the upper jaw or maxilla through the temporal bone of the skull. On the horse, the TMJ can be found just about midway on a straight line drawn between the outside corner of the eye to the base of the ear on the same side and is fairly easy to see when the horse is chewing. Although it is technically a joint – like the shoulder joint, elbow joint, and fetlock joint – the ends of the bones (the articular cartilage) are covered with fibrocartilage instead of hyaline cartilage, which can make it react a little differently at times than other joints.
There are a number of issues that can cause pain or dysfunction in and around the temporomandibular joint. This is because it is a complex area consisting of many different and connected parts as well as a wide variety of tissue types such as bone, articular cartilage, tendons, ligaments, muscle, nerves, blood vessels, salivary glands, and more. It also has the challenging tasks of chewing plus communicating with the rider through the bit. TMJ conditions include trauma to the area such as a kick or accident that results in a subluxation (partial dislocation), fracture, or other damage; arthritis, chronic “wear and tear,” inflammation, infection, muscle and soft tissue problems, and possibly more.
Although a horse with a temporomandibular joint condition may have heat, pain, and swelling in the area, many of the signs and symptoms of a TMJ disorder are general and could be caused by other problems. These include dental disease, gastric ulcers, neck arthritis, headshaking disease, behavioral and training problems, a variety of musculoskeletal issues such as lameness, and others. Discomfort in the temporomandidular joint may appear as:
A horse showing any of these signs should be examined by a veterinarian in order to identify the root cause of the problem, provide the horse immediate comfort, and correct the underlying issue. Expect the vet to perform a complete physical examination since this list of problems is not specific just to temporomandibular joint conditions. However, if there is heat, pain, or swelling in the TMJ area upon palpation, jaw muscle atrophy, a bony enlargement or protuberance, or restricted range of motion of the TMJ itself, then the vet may concentrate on this area with further diagnostic tests. For example, just like in joints of the limbs, a numbing agent or local anesthetic can be injected into the TMJ region to see if blocking pain makes a difference. Joint fluid can be collected for testing if an infection is suspected. And sometimes imaging tools such as X-ray, ultrasound or even a CT (which must be performed under general anesthesia) can be helpful in confirming the diagnosis or establishing a prognosis.
About Dr. Lydia Gray