Unraveling the Mysteries of the TMJ
ACVIM 2008
Jennifer Rawlinson, DVM, DAVDC
Ithaca, NY, USA

Introduction

Problems associated with the canine and feline temporomandibular joint (TMJ) are uncommon; therefore, this joint may sometimes hold the mystique of a "black box" when an abnormality arises. Although traumatic dislocations with or without associated condylar fractures are the most common TMJ abnormality, there are a small number of additional pathologic conditions that affect this joint. Armed with an anatomical understanding of the TMJ and surrounding structures, the clinician can start to develop a short list of differential diagnoses. Clinical presentation and diagnostic imaging are the keys to a final diagnosis.

Anatomy

The TMJ is formed by the transversely elongated condylar process of the mandible and the mandibular fossa of the temporal bone.1 Bony support for the mandibular condyle within the mandibular fossa is provided caudoventrally by the retroarticular process of the temporal bone and rostrodorsally by a process of the temporal bone in humans termed the articular eminence (this process remains unnamed in veterinary anatomy).2 The articular surfaces of the mandibular condyle and mandibular fossa are enclosed in a joint capsule. A thin fibrocartilagenous disc, meniscus, lies between the hyaline cartilage-covered articular surfaces. The joint capsule attaches to the entire edge of the meniscus as it passes between the two bones. Thus, the joint cavity is completely divided into a dorsal compartment, the meniscotemporal compartment between the disc and the temporal bone, and a ventral compartment, the meniscomandibular compartment between the disc and mandible.3,4 These two compartments do not communicate. A thick band of fibrous tissue on the lateral aspect of the joint capsule forms the lateral ligament which tightens when the jaw opens.

The TMJ is a condylar joint that can move in flexion, extension, and translation. Translation refers to the mandible's ability to move rostrally and laterally.3 The degree of translational ability is related to the dietary habits of the animal. A strictly carnivorous animal (cat) will have less movement laterally and rostrally than an omnivore (primate).2 This difference in TMJ mobility is the result of the fit of the mandibular condyle within the mandibular fossa of the temporal bone. The more snug the fit the less translational mobility. The cat has greater congruity of its mandibular condyle and fossa due to a more prominent retroarticular process and articular eminence; this results in decreased translational movement. Mandibular movements are mostly in unison but the synchondrosis at the symphysis of the mandible allows independent movements of the mandibular rami; this independent movement permits luxations of the TMJ to occur without fracture.5 The masseter, temporal, and medial pterygoid muscles are responsible for closing the mouth; the digastricus muscle with the aid of gravity opens the mouth. The lateral pterygoid muscle and the zygomaticomandibularis muscle (suspected deep part of the masseter muscle) are responsible for lateral movement of the mandible.6

Clinical Presentation

The most important question to be answered is: "Does this condition prevent the animal from opening or closing its mouth?" The vast majority of TMJ pathology and differentials can be placed in one or the other category.

Unable to Close the Mouth

 TMJ dysplasia with coronoid displacement

 TMJ luxation with or without associated fracture

Additional differentials:

 Mandibular neurapraxia

 Fracture of the zygomatic arch

 Neurogenic atrophy of muscles of mastication

 Neoplasia

Unable to Open the Mouth

 Craniomandibular osteopathy

 TMJ ankylosis

 Fracture of the zygomatic arch

 Osteoarthritis (too painful to open)

Additional differentials:

 Masticatory muscle myositis

 Tetanus

 Retrobulbar abscess or cyst

 Neoplasia

Additional associated presenting clinical signs could be: pain on mastication resulting in a reluctance to chew, audible "click" with jaw movement, pain on palpation of the TMJ or surrounding soft tissues, swelling of the TMJ area, malocclusion of teeth, decreased range of motion, and/or enophthalmos/exophthalmos. A good otic exam (and possibly imaging of the inner ear) should be included upon initial work-up to rule out ear disease as the cause for pain in the region of the TMJ.

Imaging

Obtaining detailed images of the TMJ usually expedites the search for a diagnosis. Extraoral radiographic technique is used to image the TMJ. Multiple views are required to assess the joint. Dorsoventral (or ventrodorsal) and right and left lateral oblique views are standard.7 Some clinicians find it useful to add an open-mouth view to a series. Positioning for the oblique views has been described in many ways. Two commonly accepted oblique views are the lateral rostrodorsal and the laterodorsal oblique. By convention, the joint imaged is always the TMJ closest to the film/table. To obtain the first view, the animal is positioned in lateral recumbency with the nose raised 20 degrees off the table and supported by a foam wedge or sandbag (different angulations from 15 to 35 degrees have been suggested for various skull shapes), projecting the TMJ through the masseteric fossa of the contralateral mandible;7,8 To obtain the second view, the animal is positioned in lateral recumbency with the nose raised slightly (7-10 degrees) AND the head is rotated 20 degrees along its median axis, projecting the TMJ ventral to other boney structures.7,8,9 The second view is most commonly advocated in the cat. The oblique views are challenging to position and reproduce. Practice with a cadaver head is recommended. Interpretation of TMJ radiographs can be difficult due to the complexity of surrounding structures and superimposition.

When extraoral radiographs do not provide enough detail of the TMJ, computed tomography (CT) and magnetic resonance imaging (MRI) may be necessary. CT is indicated when more information is needed about the three-dimensional shape and internal structure of the osseous components of the joint. This modality may be considered for evaluating ankylosis, osteoarthritis, dysplasia, neoplasms, fractures, luxations and subluxations. Intravenous iodinated contrast medium may be useful with CT to investigate neoplasms or mass lesions, masticatory myositis, neighboring middle ear disease, and possible cerebral involvement of any lesion.9 MRI is only used when information about the soft tissue structures of the joint (meniscus, joint capsule, articular cartilage, and lateral ligament) are necessary. The use of MRI though common in man has yet to be reported for use in dogs and cats for the investigation of TMJ disorders.

Disorders of the TMJ

Dysplasia

Reported in Bassett hounds, Dachshunds, Irish Setters, and a handful of additional dog and cat breeds, dysplasia is a rare congenital or developmental condition. Dysplasia generally consists of malformation of the boney structures of the TMJ, and it results in brief to extended periods of intermittent open-mouth jaw locking. Yawning usually precipitates an event. The mandible will be laterally shifted, and an ipsilateral protuberance on the lateral, ventral aspect of the zygomatic arch may be noted. The protuberance is the displaced coronoid process of the mandible.5 The shift and the protuberance are located on the opposite side of the dysplastic joint. Both joints can be affected; therefore, dogs may present with alternating signs. A shallow mandibular fossa, underdeveloped retroarticular process and/or articular eminence, abnormally angled and flattened mandibular condyle, slack lateral ligament, and excessively mobile mandibular symphysis are required to create open-mouth jaw locking due to TMJ luxation or subluxation with or without lateral dislocation of the coronoid process over the zygomatic arch.5,9 Radiographic features of dysplasia consist of flattening of the mandibular condyle and fossa with a hypoplastic or misshapen retroarticular process, a widened irregular joint space with periarticular osteophytosis and an increased obliquity of the articular surface of the mandibular condyle.9 Immediate treatment for coronoid displacement consists of manually opening the jaw even further to release the coronoid process from the lateral aspect of the zygomatic arch; sedation may be necessary. Long-term recommended treatment consists of surgical reduction of the coronoid process, surgical resection of portions of the zygomatic arch, or a combination of both techniques.1

**Open-mouth jaw locking with coronoid displacement can occur as sequelae to traumatic or developmental flattening of the zygomatic arch, malunion fracture of the mandibular body, mandibular symphyseal laxity, and brachycephalic skull conformation without TMJ dysplasia.9

Luxation

TMJ luxations occur in both the dog and cat as a result of trauma or dysplasia. Malocclusion will be present resulting in the inability of the animal to close its mouth fully. With rostrodorsal luxation the mandible will shift rostrally and laterally to the contralateral side. This shift may go unnoticed if the mandibular body is fractured. The cat has a higher incidence of TMJ luxation due to decreased mandibular symphyseal movement and shorter jaw length.2 A very well developed retroarticular process and articular eminence cradles the feline condyle and assumes most of the responsibility in resisting displacement of the joint.2 Consequently, fracture of the mandibular condyle, articular eminence, or surrounding boney structures may be obligatory for TMJ luxation to occur in the cat. Luxation and fracture of the canine TMJ is less common due to greater mandibular symphyseal mobility, stronger lateral ligamentous support preventing rostral condylar displacement, and a smaller articular eminence.2 The retroarticular process in both species naturally prevents caudal luxation of the mandibular condyle. Therefore, luxation of the mandibular condyle usually occurs in a rostrodorsal direction. A dorsoventral radiograph best demonstrates a TMJ luxation. Radiographic findings include increased width of the TMJ space and a rostral shift of the mandibular condyle on the affected side.5

Reduction of a rostrodorsal TMJ luxation is obtained by placing a wood dowel (pencil in smaller animals) between the carnassial teeth (dowel acts as a fulcrum) and closing the jaw against the dowel while simultaneously easing the jaw caudally.1,5 The reduction is usually unstable, and a tape muzzle for 1-2 weeks is indicated.10 Fractures of the mandibular condyle, mandibular fossa or temporal bone are usually treated conservatively as they may heal by boney union or as a pain-free and functional nonunion.10 Comminuted and intra-articular fractures which can lead to joint arthrosis and ankylosis may be treated with a condylectomy though it is controversial whether to perform this surgery at the time of trauma or later if healing complications occur.10 Tape muzzles may be contraindicated in these situations as immobility of the joint may promote ankylosis.

Craniomandibular Osteopathy

This disease is seen primarily in 3 to 7 month old West Highland White, Scottish, and Cairn Terriers (rarely in other breeds), and it is of unknown etiology associated with development.5 Puppies may present with swelling of the jaws, inappetence, lethargy, and fever, and they will be resentful on palpation of the cranium and mandible.5 These dogs are reluctant to open their mouths. Its distinctive radiographic feature is an extensive, bilateral, irregular, periosteal reaction of the mandible which can extend to the TMJ, tympanic bullae, and calvarium.9 Radiographs reveal the extent of the TMJ involvement. Long-term treatment with non-steroidal anti-inflammatory drugs or corticosteroids is recommended. The condition tends to run an undulant course, and symptoms may regress at about one year of age (in puppies that survive).5

Ankylosis

Temporomandibular joint ankylosis usually occurs as a complication following trauma to the region of the TMJ. Ankylosis has also been reported as a sequela to extensive new bone formation associated with otitis media and craniomandibular osteopathy.5 Ankylosis results in a progressive inability to open the mouth. The restriction in mouth opening may result in malnutrition, weight loss, dehydration, atrophy of the muscles of mastication, and respiratory obstruction.11 Two types have been recognized: 1) true or intracapsular ankylosis; and 2) false or extracapsular ankylosis.10 An example of extracapsular would be ankylosis between the zygomatic arch and coronoid process without true TMJ involvement. Key radiographic features of true ankylosis are the loss of a regular TMJ space and mandibular condyle contour and associated irregular new bone formation.9 Treatment for intracapsular ankylosis consists of condylectomy and excision of all associated osteophytes. Prognosis is guarded due to high rates of reankylosis of cut boney surfaces. Treatment for extracapsular ankylosis is completely dependent on the nature and location of the lesion.

Fracture of the Zygomatic Arch

Fractures of the zygomatic arch towards the cranium or fractures that have been allowed to heal in a depressed position can inhibit rostral movement of the coronoid process. A decreased range of motion will be noted on physical exam as well as an irregular zygomatic arch. Range of motion is contingent on the severity and angulation of the fracture and/or callus formation. Cats are less prone to this type of injury because of the wide curvature of the zygomatic arch.5 Treatment entails removal of the offending section of arch or rigid fixation of a fresh fracture.

Osteoarthritis

Though rare, osteoarthritis of the canine and feline TMJ has been reported. It can be a sequela to TMJ dysplasia or contralateral mandibulectomy.9 Sometimes proliferative changes of an adjacent tympanic bulla may extend to include the TMJ. Osteophyte formation can be severe, and it may surround the articular surfaces of the temporal bone and mandibular condyle.5 These animals will have proper alignment of dentition, but they will be extremely reluctant to open their mouths. Treatment is limited to medical therapy.

Neoplasia affecting the TMJ, mandibular neurapraxia, masticatory muscle myositis, and neurogenic atrophy of the muscles of mastication will briefly be covered in this lecture.

Reference

1.  Harvey CE, et al. Small Animal Dentistry. 1993.

2.  Capron TM. Veterinary and Comparative Orthopedics and Traumatology 1995; 8: 58-60.

3.  Miller ME. Miller's Anatomy of the Dog. 1993.

4.  Bojrab MJ, Tholen M. Small Animal Oral Medicine and Surgery. 1990.

5.  Lang JG. Vet Annual 1982; 22: 175-187.

6.  Scapino RP. J Morph 1965; 116: 23-50.

7.  Eisner ER. Vet Clin North Am Small Anim Pract 28(5): 1073-1075, 1998.

8.  Ticer JW, Spencer CP. J Am Vet Rad Soc 1978; 19: 146-156.

9.  Schwarz T, Weller R, Dickie AM, et al. Vet Radiol Ultrasound 43(2): 85-97, 2002.

10. Slatter D. Textbook of Small Animal Surgery. 2003.

11. Lantz GC. Compendium on Continuing Education for the Practicing Veterinarian 13(10): 1570-1576, 1991.

Speaker Information
(click the speaker's name to view other papers and abstracts submitted by this speaker)

Jennifer Rawlinson, DVM, DAVDC
Cornell University
Ithaca, NY


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