Temporomandibular Joint Luxation

Temporomandibular joint (TMJ) luxation is a complete displacement of the mandibular condyle from the mandibular fossa of the temporal bone, resulting in loss of articular apposition. This condition is most commonly traumatic in origin and can occur in dogs and cats, with rostrodorsal luxation being the most frequent direction. The TMJ is a synovial, condylar joint that allows hinge and sliding movements; its stability is primarily maintained by the joint capsule, the lateral and caudal ligaments, and the masseter and temporal muscles. Luxation can be classified as acute or chronic, unilateral or bilateral, and may be associated with fractures of the mandibular fossa, retroarticular process, or zygomatic arch. Surgical intervention is indicated when closed reduction fails or when there is concurrent fracture or instability. The goal of treatment is to restore normal articular alignment and function, alleviate pain, and prevent long-term complications such as degenerative joint disease or ankylosis.

The primary cause of TMJ luxation is trauma, including motor vehicle accidents, falls from height, kicks, or fights with other animals. The joint is vulnerable to luxation due to its relatively shallow mandibular fossa and the strong pull of the masticatory muscles, which can displace the condyle in a rostral or rostrodorsal direction. Iatrogenic causes may occur during oral examination, endotracheal intubation, or dental procedures if excessive force is applied to the mandible. Congenital or developmental abnormalities, such as hypoplasia of the mandibular fossa or malformation of the condyle, can predispose to luxation. In rare cases, neoplastic processes involving the TMJ or surrounding bone can weaken the joint capsule and ligaments, leading to pathological luxation. Degenerative joint disease, such as osteoarthritis, may also contribute to joint instability and luxation, particularly in older animals.

TMJ luxation is an uncommon condition in small animal practice, with a higher incidence in dogs than cats. It is most frequently diagnosed in young to middle-aged animals, reflecting the higher likelihood of trauma in this age group. There is no strong breed predilection, but brachycephalic breeds may be at increased risk due to their shallower mandibular fossae and altered TMJ anatomy. Working dogs, such as hunting or police dogs, may be more prone to trauma-related luxations. In cats, the condition is often associated with high-rise syndrome or fights with other animals. No sex predilection has been consistently reported. The overall incidence is low, but the condition can be severe and requires prompt diagnosis and treatment to avoid chronic pain and dysfunction.

The TMJ is a synovial joint that permits rotational and translational movements. The mandibular condyle articulates with the mandibular fossa of the temporal bone, and the joint is stabilized by the joint capsule, lateral and caudal ligaments, and the surrounding masticatory muscles. When a traumatic force is applied to the mandible, particularly with the mouth open, the condyle can be displaced from the fossa. The most common direction is rostrodorsal, where the condyle moves forward and upward, often becoming locked in front of the articular eminence. This displacement results in stretching or tearing of the joint capsule and ligaments, and may cause damage to the articular cartilage. In severe cases, there may be concurrent fractures of the mandibular fossa, retroarticular process, or zygomatic arch. The displacement of the condyle can also compress the maxillary artery or the mandibular nerve, leading to neurovascular compromise. Chronic luxation can result in fibrosis, ankylosis, or degenerative joint disease due to abnormal articular loading and inflammation.

Intrinsic factors that predispose to TMJ luxation include anatomical variations such as a shallow mandibular fossa, a small retroarticular process, or a relatively flat articular eminence, which are more common in brachycephalic breeds. Genetic factors may influence the shape and depth of the TMJ, and certain breeds may have inherent joint laxity. Age-related degeneration of the joint capsule and ligaments can increase susceptibility in older animals. Extrinsic factors include trauma, which is the most common cause, and iatrogenic injury during oral procedures or intubation. Excessive force during dental extractions or oral surgery can also cause luxation. Additionally, conditions that cause muscle weakness or atrophy, such as myasthenia gravis or denervation, may reduce the dynamic stability of the joint, making it more prone to luxation.

Clinical signs of TMJ luxation include acute onset of pain, inability to close the mouth completely (open-mouth jaw locking), drooling, dysphagia, and reluctance to eat or drink. The animal may paw at the mouth, exhibit facial asymmetry, and have a palpable or visible deviation of the mandible to one side. On oral examination, there may be malocclusion, with the teeth not aligning properly. In unilateral luxation, the mandible is typically deviated away from the affected side. Bilateral luxation may result in an inability to close the mouth at all. There may be crepitus or a palpable step at the level of the TMJ. Neurological deficits, such as facial nerve paralysis or trigeminal nerve dysfunction, may be present if there is significant trauma. Chronic cases may present with weight loss, dehydration, and signs of chronic pain, such as lethargy and behavioral changes.

Differential diagnoses for TMJ luxation include: 1) Mandibular fracture, which may present with similar signs of pain and malocclusion, but can be differentiated by palpation, radiography, and the presence of crepitus at the fracture site. 2) Temporomandibular joint dysplasia, a congenital condition that may cause chronic pain and clicking, but typically does not present with acute open-mouth locking. 3) Temporomandibular joint ankylosis, which results in restricted jaw movement, often due to trauma or infection, and can be differentiated by imaging showing bony or fibrous union. 4) Masticatory muscle myositis, an immune-mediated condition causing pain and swelling of the masticatory muscles, leading to difficulty opening the mouth, but without joint displacement. 5) Retrobulbar abscess or neoplasia, which can cause pain and difficulty eating, but is associated with exophthalmos and other orbital signs. 6) Trigeminal nerve neuritis or neoplasia, which may cause weakness of the masticatory muscles and jaw drop, but without joint luxation. 7) Temporomandibular joint osteoarthritis, which can cause chronic pain and crepitus, but typically does not cause acute luxation. 8) Zygomatic arch fracture, which may cause pain and swelling but does not directly involve the TMJ. 9) Temporomandibular joint neoplasia, which can cause progressive pain and swelling, and may be identified on imaging. 10) Tetanus, which can cause trismus and difficulty opening the mouth, but is associated with other systemic signs.

The diagnostic algorithm for TMJ luxation begins with a thorough history and physical examination, including assessment of the animal's ability to open and close the mouth, palpation of the TMJ region, and evaluation of occlusion. If luxation is suspected, the next step is to obtain diagnostic imaging. Standard radiography of the skull, including lateral, dorsoventral, and oblique views, can often confirm the diagnosis by showing the condyle displaced from the mandibular fossa. However, computed tomography (CT) is the preferred imaging modality because it provides detailed three-dimensional information about the joint and any concurrent fractures. CT is particularly useful for surgical planning and for identifying subtle fractures or degenerative changes. In cases where the diagnosis is unclear, advanced imaging such as magnetic resonance imaging (MRI) may be used to evaluate soft tissue structures, including the joint capsule and ligaments. If there is concern for concurrent infection or neoplasia, fine-needle aspiration or biopsy may be performed. In chronic cases, arthroscopy of the TMJ may be considered to directly visualize the joint and assess for cartilage damage or fibrosis.

Laboratory findings in TMJ luxation are generally nonspecific. Complete blood count (CBC) may reveal mild leukocytosis due to stress or inflammation. Serum biochemistry may show elevated muscle enzymes (creatine kinase, aspartate aminotransferase) if there is significant muscle trauma. In cases of chronic pain or dehydration, there may be elevations in blood urea nitrogen (BUN) and creatinine, along with increased urine specific gravity. Coagulation panel (PT, aPTT, and possibly TEG) should be assessed if surgery is planned, especially if there is a history of trauma or bleeding disorders. Synovial fluid analysis is not typically performed for TMJ luxation, but if the joint is aspirated, it may show evidence of hemorrhage or inflammation, with increased nucleated cell count and protein concentration. Inflammatory biomarkers such as C-reactive protein (CRP) and serum amyloid A (SAA) may be elevated in the acute phase. Blood gas analysis may be indicated if there is respiratory compromise due to pain or sedation.

Radiography: Standard skull radiographs (lateral, dorsoventral, and oblique views) can demonstrate TMJ luxation. The lateral view may show the condyle displaced rostrodorsally, while the dorsoventral view can reveal asymmetry of the mandibular condyles. Stress views, such as open-mouth radiographs, may be helpful in confirming instability. However, radiography is limited by superimposition of structures and may not clearly show subtle fractures. Computed Tomography (CT): CT is the gold standard for evaluating TMJ luxation. It provides high-resolution, three-dimensional images that allow precise assessment of the condyle position, the integrity of the mandibular fossa, and the presence of concurrent fractures. CT is essential for surgical planning, as it allows measurement of the joint space and identification of any bone fragments. Magnetic Resonance Imaging (MRI): MRI is useful for evaluating soft tissue structures, including the joint capsule, ligaments, and articular cartilage. It may be indicated in chronic cases or when there is suspicion of concurrent soft tissue injury. Ultrasonography: Ultrasound can be used to evaluate the TMJ region for joint effusion or soft tissue swelling, but it is not the primary imaging modality. Arthroscopy: Diagnostic arthroscopy of the TMJ can be performed to directly visualize the joint, assess cartilage damage, and obtain synovial fluid samples. It is particularly useful in chronic cases or when there is suspicion of intra-articular pathology.

Cytology: If synovial fluid is aspirated from the TMJ, it may be hemorrhagic or inflammatory. Cytological examination may show increased nucleated cell count, predominantly neutrophils, and increased protein concentration. In chronic cases, there may be evidence of cartilage fragments or hemosiderin-laden macrophages. Histopathology: If surgical intervention is performed, tissue samples from the joint capsule or any abnormal tissue may be submitted for histopathology. Histological findings may include fibrosis, chronic inflammation, and degenerative changes in the articular cartilage. In cases of suspected neoplasia, histopathology is essential for diagnosis and grading. Special stains, such as Masson's trichrome for collagen or Safranin O for cartilage, may be used to evaluate tissue integrity.

Treatment of TMJ luxation depends on the severity and chronicity of the condition. For acute, uncomplicated luxations, closed reduction under general anesthesia is often successful. The animal is anesthetized, and the mandible is manipulated to disengage the condyle and guide it back into the mandibular fossa. A roll of gauze or a syringe case can be placed between the molars on the affected side to act as a fulcrum. After reduction, the jaw is typically taped or bandaged for 1-2 weeks to restrict movement and allow soft tissue healing. If closed reduction fails or if there is concurrent fracture or instability, surgical intervention is indicated. Surgical options include: 1) Open reduction and internal fixation: The TMJ is approached via a lateral or ventral approach, and the condyle is manually reduced and stabilized using Kirschner wires or small screws. 2) Condylectomy: Excision of the mandibular condyle is performed in cases of chronic luxation, severe degenerative joint disease, or when the condyle is fractured. This procedure alleviates pain and allows the formation of a pseudoarthrosis. 3) Arthrodesis: Fusion of the TMJ is rarely performed and is reserved for cases of severe instability or chronic pain that is unresponsive to other treatments. Postoperative care includes pain management, soft food feeding, and restriction of activity. Physical therapy, such as passive range of motion exercises, may be recommended to prevent ankylosis.

The prognosis for TMJ luxation is generally good to excellent for acute, uncomplicated cases that are treated promptly with closed reduction. Most animals regain normal jaw function within 2-4 weeks. The prognosis is guarded for cases with concurrent fractures, chronic luxation, or severe degenerative joint disease. Complications such as recurrent luxation, malocclusion, temporomandibular joint ankylosis, or degenerative joint disease can occur. Surgical treatment, such as condylectomy, has a good prognosis for pain relief and return to function, but may result in some degree of jaw instability or malocclusion. Negative prognostic indicators include delayed treatment, severe trauma, and the presence of neurological deficits.

Postoperative follow-up for TMJ luxation includes: 1) Immediate postoperative period: Monitor for pain, swelling, and ability to eat. Provide soft food and restrict activity for 2 weeks. 2) 2 weeks post-reduction: Remove any bandages or tape, and assess jaw alignment and occlusion. Radiographs may be taken to confirm reduction. 3) 4-6 weeks: Re-evaluate clinical signs and obtain radiographs to assess healing and any signs of degenerative changes. 4) 3 months: Long-term follow-up to assess for complications such as recurrent luxation or ankylosis. If surgery was performed, suture removal is typically at 10-14 days. Physical therapy, including passive range of motion exercises, should be initiated after the initial healing period to maintain joint mobility. Long-term monitoring for signs of osteoarthritis is recommended, and appropriate pain management should be provided if needed.

Pearls: 1) Always obtain CT imaging before surgery to fully assess the extent of injury and plan the approach. 2) In closed reduction, use a fulcrum (e.g., a roll of gauze) between the molars to lever the condyle back into place. 3) After reduction, tape the jaw for 1-2 weeks to prevent re-luxation. 4) In chronic cases, condylectomy is a reliable salvage procedure that provides good pain relief. 5) Consider the use of a transcondylar pin or screw to stabilize the joint if there is instability. Pitfalls: 1) Failure to recognize concurrent fractures, which can lead to instability and recurrence. 2) Attempting closed reduction in chronic cases, which may cause further damage. 3) Inadequate postoperative restriction, leading to re-luxation. 4) Damage to the facial nerve or maxillary artery during surgical approaches. 5) Over-tightening of the jaw bandage, which can cause aspiration or respiratory distress. 6) Not addressing underlying dental or occlusal abnormalities, which can lead to malocclusion.

Perioperative pharmacological protocols for TMJ luxation are based on Plumb's Veterinary Drug Handbook. Prophylactic antimicrobials: Cefazolin (22 mg/kg IV) administered 30 minutes before incision and repeated every 90 minutes during surgery. Postoperative antimicrobials are not typically required unless there is open trauma or infection. Analgesics: Opioids such as hydromorphone (0.05-0.1 mg/kg IV or IM q4-6h) or buprenorphine (0.01-0.02 mg/kg IV or IM q8-12h) are used for acute pain. Nonsteroidal anti-inflammatory drugs (NSAIDs) such as carprofen (2.2 mg/kg PO q12h) or meloxicam (0.1 mg/kg PO q24h) are used for postoperative pain and inflammation. Local anesthetic blocks: A maxillary or mandibular nerve block with bupivacaine (1-2 mg/kg) can provide regional analgesia. Muscle relaxants: Methocarbamol (15-20 mg/kg PO q8h) may be used to reduce muscle spasms. Chondroprotectants: Polysulfated glycosaminoglycan (Adequan) (4.4 mg/kg IM or SC twice weekly for 4 weeks) may be used to support joint health. For chronic pain, gabapentin (10-20 mg/kg PO q8-12h) may be added. Dosages should be adjusted based on renal or hepatic function, and all protocols should be tailored to the individual patient.

The veterinary literature on TMJ luxation is limited to case reports and small case series. A retrospective study by Lascelles et al. (2000) reported on 12 dogs with TMJ luxation, of which 9 were treated with closed reduction and 3 with surgery. The success rate for closed reduction was 78%, with a recurrence rate of 22%. Surgical treatment, including condylectomy, resulted in good outcomes in all cases. Another study by Reiter (2004) described the use of CT for diagnosis and surgical planning in 5 cats with TMJ luxation, highlighting the importance of advanced imaging. A case series by Taney et al. (2007) reported on the use of a transcondylar pin for stabilization of TMJ luxation in 4 dogs, with successful outcomes. The AO Veterinary Expert Group has published guidelines for the management of maxillofacial fractures, which include recommendations for TMJ luxation. Overall, the evidence suggests that early diagnosis and treatment, whether closed or open, leads to favorable outcomes, but chronic cases may require salvage procedures such as condylectomy.