The temperomandibular joint (TMJ) has a critical function in both jaw function for mastication as well as the neurologic input for balance. [excerpted from 2010 AHVMA Proceedings)
ANATOMY OF THE TMJ
The TMJ consists of two joints, an upper sliding joint and a lower hinge joint. An articular disc separates the two joints. The joints are encapsulated and synovial. They are also highly innervated. The area of the TMJ contains many important structures, including cranial nerves (Facial, Glossopharyngeal, Vagus, Accessory, and Hypoglossal) vasculature (Carotid artery), and muscles (Masseter). The lower hinge joint is the main joint used for mastication in the canine and feline, while the lateral motion of the upper joint allows for grinding in the equine.
FUNCTION OF THE TMJ
The TMJ is part of the Stomatognathic system, which consists of structures needed for balance and equilibrium. The components of the system all lie cranial to the shoulder and include the eyes, hyoid apparatus, proprioceptors along the cervical spine, and the labyrinth apparatus. Dural connections from the cervical region extend caudally to the sacrum and pelvis. Mechanically, proper function of the TMJ is needed for proper mastication as well.
PATHOLOGY OF THE TMJ
The TMJ is greatly affected by malocclusion. This can certainly result from congenital mandible or maxillary malformations, trauma, vertebral subluxation, dental malocclusion, or bite misalignment secondary to poor bitting in the equine or abnormal stress in the canine (eg, carrying a ball on one side of the mouth). Changes in dental occlusion lead to stimulation of the periodontal ligaments, which sense premature tooth strike and stimulate afferent input through the trigeminal nerve. Changes in muscle tone in the head and cervical region increase dural tension all the way to the sacrum. Reciprocal effects can be seen in the TMJ area secondary to sacral and pelvic areas via transmitted dural tension.
The normal cranial respiratory function of the cranial bones can be inhibited secondary to muscle spasm and dural tension from TMJ dysfunction. Cranial bones are normally mobile and flex and extend with the hydraulic action of the pulsation of cerebrospinal fluid throughout the craniosacral system. Altered cerebrospinal fluid rhythm not only affects dural tension, but can alter endocrine function as well. Due to changes in the cranial shape from altered cranial suture function, vision can be altered as well.
Pain in the TMJ also reduced mastication from joint pain as well as from local muscle spasm. Normal hinge motion of the jaw is needed for balance and coordinated movement. Muscle tension around the hyoid apparatus also alters the body’s sense of orientation in space.
DIAGNOSIS OF TMJ DYSFUNCTION
The most common indicator of TMJ dysfunction is pain. Pain can be assessed through GENTLE palpation of both the right and left joints in all species. Additionally, the health of the acupuncture point ST 5 (located in the center of the masseter muscle) can be assessed through palpation. Motion in the joint can be determined by observing mastication or by taking the joint manually through a range of motion. In cases where arthritis is suspected, radiographs and/or joint fluid analysis may be performed.
The diagnosis of TMJ dysfunction indicates the need for a full oral examination. In the equine, because of continually eruption of teeth, malocclusion can be common due to lack of proper dental care. Sedation of the horse with examination after placement of a full mouth speculum is essential. Many problems in the rear molars of the horse may be missed without the use of a full mouth speculum.
TREATMENT OF TMJ DYSFUNCTION
The first priority in treating TMJ disorders is to correct any predisposing conditions. Conditions specific to the mouth (eg, dental malocclusions, poor bitting) need to be corrected in order to keep the TMJ from becoming chronically inflamed. For the horse, this means a proper dental from a qualified practitioner. Secondary conditions that are affecting the TMJ through dural, muscular, or fascial tension should be addressed. This means trimming nails/feet, correcting other tack problems (eg, saddle or harness fit), and other mechanical issues (vertebral subluxation and other traditional causes of lameness).
Traditional treatment of inflammation in the TMJ would include such things as systemic anti-inflammatories, cryotherapy, thermal treatment, and injection of the joint with corticosteroids and/or chondroprotective agents. Oral or intramuscular chondroprotective agents may also be used to affect the health of the joint.
Integrative therapies for TMJ dysfunction would certainly include chiropractic adjustment, acupuncture, herbal or homeopathic antiinflammatories, manipulation of cranial sutures, and craniosacral therapy.
The TMJ is an integral part of the Stomatognathic system, which governs balance and movement. It is also critical for normal mastication. TMJ pathology can occur primarily from poor dental occlusion, chiropractic subluxation, or poor management practices. TMJ dysfunction can also be secondary to lameness and subluxation in other areas through dural, muscular, and fascial tension. Rapid treatment before permanent damage is essential to maintain normal biomechanics in the patient.
Evans HE. Miller’s Anatomy of the Dog. W.B. Saunders Company. 1993.
Shoemaker JM. Equine Dentistry - A major influence on biomechanical and neurologic function. Proceedings of the Annual Conference of the American Holistic Veterinary Medical Association. 2000:110-111.