DEFINITION/OVERVIEW

• Disorders of the temporomandibular joint (TMJ) lead to an alteration of the normal function of the masticatory system as the mobility and function of the joint are compromised
  
• Genetic, traumatic, degenerative, or idiopathic causes may result in pain, occlusal dysfunction, joint laxity, chronic arthritis, or open-mouth locking

ETIOLOGY/PATHOPHYSIOLOGY

• Dogs/cats
  
  
  
  • Patients at a higher risk to experience injuries: young; free roaming
    
  • Trauma may cause fractures or a luxation resulting in immediate problems, as well as future degenerative problems
    
  • Mandibular neuropraxia: carrying heavy objects by mouth
    
  • Masticatory muscle myositis (MMM): adult; large breeds (e.g., German shepherds)

SIGNALMENT/HISTORY

• No breed, sex, or age predisposition in most TMJ disorders
  
• Open-mouth mandibular locking: basset hounds; Irish setters
  
• There may be a genetic predisposition in certain breeds (e.g., basset hounds) to develop TMJ disorders

CLINICAL FEATURES

• General
  
  
  
  • Difficulty opening and/or closing mouth
    
  • Laxity or excessive lateral movement of the mandible
    
  • Pain when masticating, yawning, and/or vocalizing
  
  
• Specific
  
  
  
  • TMJ luxation/subluxation: history of trauma or mouth locked open; radiographic evidence of luxation
    
  • Open-mouth mandibular locking: coronoid process of the mandible “slips” lateral to the ventral surface of the zygomatic arch and is locked in that position; large bulge palpated on affected side of face (Fig. 47-1)
    
  • Traumatic injury: evidence of trauma; mouth dropped open; mobility of mandible (may have multiple fractures); radiographs indicate fracture
    
  • Osteoarthritis/chronic posttraumatic changes: crepitation and pain when eating or if mandible is forced to move; radiographs may show osseous reaction indicative of arthritic changes; ankylosis

Figure 47-1 Skull simulating locking of the coronoid process of the mandible lateral to the zygomatic arch.

DIFFERENTIAL DIAGNOSIS

• Craniomandibular osteopathy (CMO) (see Chapter 13): dogs
  
• Primary or secondary hyperparathyroidism
  
• Mandibular neuropraxia: stretching of the nerve branches (motor) of the masticatory muscles; usually caused by carrying heavy objects in the mouth; mandible hangs open but can be easily closed manually (dogs)
  
• MMM: autoimmune disease of type 2M myofibers of masticatory muscles supplied by trigeminal nerve with necrosis, phagocytosis, and fibrosis; trismus progresses to total inability to open jaws (typically dogs; see Chapter 57)

DIAGNOSTICS

• Serum autoantibodies to type 2M myosin: to rule out MMM
  
• Muscle biopsy: to rule out MMM
  
• Cytology of fluid aspirated from TMJ: may be beneficial in diagnosis of a polyarthropathy in which the articular surfaces of the joint are inflamed
  
• Imaging
  
  
  
  • Skull radiography: essential to perform proper radiographic technique in order to visualize the TMJs
    
  • Magnetic resonance imaging (MRI): gold standard for imaging the TMJ
    
  • Computed tomography (CT): very helpful for proper assessment and planning

THERAPEUTICS

Drugs

• Analgesics: for painful disorders
  
• Anti-inflammatory drugs: for postoperative pain and chronic inflammation
  
• Muscle relaxants: help prevent increased muscle activity due to chronic pain response

Procedures

• Definitive treatment is aimed at eliminating or altering the etiologic factor responsible for the disorder, as well as correcting the problem
  
• TMJ luxation: traumatic; luxation often occurs in a rostral direction; place a “dowel” (pencil) across the mouth between the carnassial teeth (Fig. 47-2); gently close the rostral portion of the mouth with a gentle “push” to reduce the luxation (push caudally for a rostral luxation); chronic luxation may not reduce and may require surgery
  
• Open-mouth mandibular locking: immediate attention; sedate animal, open the mouth further, and apply gentle pressure on the bulging coronoid process to allow it to slip back under the zygomatic arch; surgical management—excise ventral portion of the zygomatic arch and/or a dorsal portion of the coronoid process to relieve future lockings
  
• Injury or fracture at TMJ: depends on extent of damage; fixation is difficult; condylectomy sometimes necessary
  
• Chronic osteoarthritis or ankylosis: if severe, condylectomy may be needed, possibly bilaterally
  
  
  
  • Referral to a specialist may be advised
    
  • Semi-emergent tracheostomy may be necessary
    
  • Remove zygoma, callus, condyle, angular process and caudal ramus: small rongeurs or piezosurgery unit
    
  • Caution with maxillary artery and facial nerve
    
  • Use muscle flap between cut surfaces to minimize reankylosis
    
  • Exercising full range of motion to inhibit callus formation
  
  
• “Dropped jaw” (mandibular neuropraxia): conservative treatment; rest, anti-inflammatory drugs
  
• MMM: immunosuppressant medications; possible forcible, gradual opening of mouth

Figure 47-2 With a luxation of the temporomandibular joint, a dowel can be placed in between the carnassial tooth with gentle pressure to close the rostral portion of the mouth to reduce the luxation.

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