The temporomandibular joint (TMJ) is a synovial condylarthrosis between the condyloid process of the mandible and the mandibular fossa of the temporal bone. The fossa is delimited rostrodorsally by a shallow convexity, which corresponds to the articular eminence in humans, and caudoventrally by the retroarticular process, which is particularly well developed in cats. A connective capsule encompasses the joint, with a lateral thickening in dogs and a true lateral ligament in cats. The joint cavity is divided by a fibrocartilaginous meniscus into dorsal and ventral compartments.

In feline patients, the condyles are elongated transversally, with a common axis of rotation. Therefore, the joint only allows minimal latero-lateral mandibular motion.

In dogs, the long axis of each condyle is slightly angled as compared to the common axis of rotation. Consequently, the lateral mobility of the mandible is more pronounced than in cats.

Some independent mobility of right and left mandibles is allowed by the fibrocartilagineus symphysis, even though in some patients a bony union develops with time.

Mandibular flexion is permitted by the action the temporal, masseter and pterygoideus medialis muscles. Gravity and the digastricus muscle act during extension, and adduction (and flexion) is mainly favored by the action of the pterygoideus lateralis muscle. Many conditions (congenital, traumatic infectious, degenerative or neoplastic) may affect the joint, causing intra-articular pain, mechanical hindrance or both. Therefore, TMJ disorders often result in difficulties opening or closing the mouth. A number of extra-articular conditions may also affect mandibular mobility, mimicking TMJ diseases (Tables 1 and 2).

Table 1. Common conditions causing difficulties opening the mouth

Table 2. Common conditions causing difficulties closing the mouth

Diagnostic imaging (i.e., radiography and particularly computed tomography) is often necessary to properly diagnose TMJ disorders.

Congenital Agenesia

This is a rare disorder, which has only been reported in a cat, with unilateral mandibular ramus agenesia and flattening of the mandibular fossa of the temporal bone.

Dysplasia

TMJ dysplasia is relatively common in certain canine breeds such as Basset hound, Irish Setter, Gordon Setter, and Cavalier Kind Charles. However, it has also been reported in other breeds (e.g., Weimaraner, Dalmatian, Boxer, Golden Retriever, Labrador Retriever, Spaniels, Pekingese, Dachshund). Its incidence in cats is unknown.

The mandibular fossa and articular eminence may appear flat, the retroarticular process thickened and shortened, the mandibular condyle flat and abnormally angled, and the articular space wider than normal.

Dysplasia is often asymptomatic and may be diagnosed as an incidental finding during imaging of the skull. In some cases, though, the deviated condyloid process stretches the articular capsule and lateral ligament during mandibular extension, which may lead to joint instability and subluxation.

Open mouth jaw locking may then develop. Due to the increased lateral range of motion of the dysplastic mandible, following yawning or intense mandibular activity the coronoid process of the unaffected side may get locked lateral to the zygomatic arch. In these cases, the animal presents in great distress, unable to swallow, with the mouth wide open and the mandible deviated ventrally towards the locked side. A bulge (the tip of the coronoid process) may be palpated lateral to the zygomatic arch. If the animal tries to close the mouth, the coronoid process is locked in position even further.

It is very important to obtain a proper diagnosis (following clinical examination and diagnostic imaging procedures), because if rostrocaudal TMJ luxation is suspected instead and the mouth is forced closed trying to bring the condyle back into position, the mandible is locked even further.

No treatment is usually necessary for asymptomatic dysplasia.

Manual replacement following jaw locking is performed by maximally extending the mandible and rotating the displaced mandible medially, to bring the coronoid process medial to the zygomatic arch. A tape muzzle should be recommended for a few weeks. In case of recurrence, ostectomy of the coronoid process and/or the zygomatic arch of the displaced side should be performed.

Osteomyelitis

Osteomyelitis of the TMJ is often secondary to middle ear disease.

Osteoarthritis (OA)

OA is one of the most common TMJ disorders, both in dogs and cats. It may be primary or secondary to TMJ dysplasia, trauma, unilateral mandibulectomy or chronic overload. It causes pain and decreased range of motion of the mandible. On radiographs or CT, the articular bone may appear eroded. Sclerosis of the mandibular fossa and condylar process, flattening of the condylar process, reduced articular space and marginal osteophytes are also often visible.

Dislocation

TMJ luxation is apparently more frequent in cats than dogs. It may happen in rostrodorsal direction, with the condyle positioned over the articular eminence, or in caudoventral direction, which is usually accompanied by fracture of the retroarticular process. In the first case, the mandible appears deviated towards the unaffected side and the mouth slightly forced open. Because of the rostrally dislocated coronoid process, the ipsilateral ocular retropulsion will result decreased. Occlusal contact between ipsilateral premolar and molar teeth and traumatic occlusion between the canine teeth and the tissues on the opposite jaw are often present. If rostrodorsal luxation is bilateral, mandibular prognathism may develop. Ecchymosis of the caudal oral mucosa is common. Manual mandibular repositioning results impossible unless a significant amount of force is applied using a caudal fulcrum within the oral cavity and the rostral portion of the mandible is pushed in dorsal direction, which brings the condyle back into position. A tape muzzle is recommended for a few days to limit mandibular extension and favor soft tissues healing.

If caudoventral dislocation develops instead, the mandible appears caudally deviated towards the affected side. The mandible appears more mobile than usual, and a crepitus may be felt at the affected joint during mandibular extension. Ecchymosis of the caudal oral mucosa is frequently present. If both joints are involved, distoclusion may be evident. Manual closure of the mouth is possible in these cases. However, dislocation and deviation relapses immediately, because of the fractured retroarticular process. Maxillomandibular fixation for a few days/weeks is recommended in these cases.

Fracture

Fracture of the condylar process is often caused by trauma and may develop in combination with other maxillofacial injuries. It may cause pain at mandibular extension as well as difficulties closing the mouth. Clinically, ecchymosis of the caudal oral mucosa is often present. A click noise may be detected during mandibular extension. Mandibular mobility may appear increased. Often malocclusion (with an abnormal occlusal contact between mandibular and maxillary premolar/molar teeth) and mandibular deviation towards the ipsilateral side are noted. Single fractures that do not cause malocclusion and do not interfere with joint motion may be addressed conservatively. However, frequent re-examinations should be performed and mandibular extension measured. If a decrease in measurement (likely indicating an exuberant bony callus formation and joint ankylosis) is detected, condylectomy should be performed immediately.

Ankylosis

True (intra-articular) TMJ ankylosis often develops following condylar fracture and fibrous or bony callus formation, particularly in cats. Other less common causes for intra-articular ankylosis include osteomyelitis, osteoarthritis and neoplasia. Even if only one joint is affected, mandibular extension may become progressively reduced or absent. Mandibular deviation and malocclusion, and lingual ptosis are often present. If the bony callus is large, the TMJ may appear deformed, and the retropulsion of the ipsilateral eye globe may be reduced. The affected patient may show ptyalism, inability to eat, weight loss and dehydration, masticatory muscle atrophy, poor coat quality. In severe cases, orthograde intubation may be impossible to perform and temporary tracheostomy may be necessary is anesthesia is to be performed.

Radiographically, the articular space may appear thin or absent, and the condyle profile irregular. Bone sclerosis and osteophytes are often present. CT scan may allow addressing the presence of a fibrous ankylosis. Treatment entails wide ostectomy of all involved structures, using with great care an oscillating saw or a dental bur, or utilizing piezosurgery equipment, which has the great advantage to spare soft tissues. The most common intra- and postoperative complications include injury to the maxillary artery (which runs just medial to the TMJ), auriculopalpebral nerve injury and paralysis (and temporary or permanent secondary lagophthalmos), lingual ptosis, jaw dropping and malocclusion, and ankylosis recurrence.

Neoplasia

The most common neoplastic diseases affecting the TMJ are osteosarcoma, multilobular osteochondrosarcoma, squamous cells carcinoma, fibrosarcoma, and osteoma. They often initially cause articular pain and with time TMJ deformity and ankylosis.