The temporomandibular joint (TMJ) connects the lower jaw, or mandible, to the temporal bone of the skull, allowing for complex movements necessary for speech, chewing, and yawning. TMJ ankylosis is a severe condition defined by the fusion or restriction of movement within this joint. This immobility profoundly impairs basic functions, making it difficult to eat, speak, and maintain proper oral hygiene. The resulting loss of mobility requires specialized medical intervention to restore function.
Defining TMJ Ankylosis
TMJ ankylosis describes a partial or total loss of mobility in the joint, caused by the formation of abnormal tissue between the mandibular condyle and the temporal fossa of the skull. This condition is classified based on the type of tissue that forms within the joint space, primarily fibrous or bony ankylosis, which can also present as a mixed fibro-osseous type.
Fibrous ankylosis involves the proliferation of dense scar tissue within the joint, severely limiting the gliding and rotational movements of the condyle. Although this type does not involve true bone fusion, the scar tissue prevents normal jaw opening. Bony ankylosis is the most severe form, characterized by the complete fusion of the mandibular bone directly to the base of the skull, eliminating joint space and movement.
This fusion mechanism results in trismus, the inability to open the mouth adequately. The functional impairment is often measured by the maximum interincisal opening, which can be restricted to less than 3 centimeters in severe cases.
Factors Leading to the Condition
The most frequent cause of TMJ ankylosis is severe trauma to the jaw area, accounting for a majority of cases. This often involves a fracture of the mandibular condyle, particularly in childhood, where an intra-articular hematoma forms within the joint space after the injury.
The body’s healing response to this trauma can become exaggerated, leading to scarring and excessive bone formation in the joint space. Prolonged immobilization of the jaw after a condylar injury, especially in children under ten, further increases the risk of fusion. This post-traumatic process stimulates bone-forming cells to differentiate, eventually leading to ossification and joint fusion.
Infection is the second most common cause of TMJ ankylosis, though its incidence has decreased due to antibiotic therapy. Local infections, such as those spreading from the middle ear (otitis media), can inflame and damage the joint structures, triggering the fusion process. Less frequently, systemic inflammatory conditions, such as rheumatoid arthritis or ankylosing spondylitis, can cause chronic inflammation leading to destructive changes and ankylosis.
Identifying Symptoms and Diagnosis
The primary clinical presentation of TMJ ankylosis is a progressive and chronic limitation of mouth opening. While the condition is associated with severely restricted movement, the joint itself is often painless, which can delay diagnosis. Patients may report a slow decrease in their ability to open their mouth over months or years.
When ankylosis develops during the active growth phase of childhood, it can profoundly affect the development of the jaw and face. The restricted growth of the mandible on the affected side leads to facial asymmetry, causing the chin to deviate toward the fused joint. This developmental impairment also results in mandibular hypoplasia, which causes a recessed lower jaw and associated malocclusion.
Diagnosing TMJ ankylosis relies heavily on advanced medical imaging to confirm the extent and type of joint fusion. Initial evaluation often includes panoramic X-rays, which may show the loss of normal bony architecture. Computed Tomography (CT) scans are considered the definitive tool for diagnosing bony ankylosis, as they clearly reveal the fusion mass and the complete obliteration of the joint space.
CT imaging provides detailed three-dimensional information essential for surgical planning, particularly for visualizing the relationship between the fused bone and nearby vital structures. Conversely, Magnetic Resonance Imaging (MRI) is the preferred modality for assessing fibrous ankylosis, as it provides superior visualization of soft tissues, distinguishing scar tissue from true bone fusion. Diagnosis is made by correlating the physical examination with the findings from these specific imaging studies.
Treatment and Post-Operative Care
The treatment of TMJ ankylosis is complex and almost always requires surgical intervention to restore mobility. The main objectives of surgery are to achieve adequate mouth opening, re-establish joint function, and prevent recurrence. Surgical procedures focus on removing the abnormal fused tissue, often requiring a radical resection of the ankylotic mass.
One common surgical approach is gap arthroplasty, which involves creating a sufficient space between the cut surfaces of the bone. A gap of at least 1.5 to 2.5 centimeters is typically created to ensure the bones cannot fuse again. However, gap arthroplasty is often discouraged because it can lead to re-ankylosis and failure to maintain vertical facial height.
A more advanced technique is interpositional arthroplasty, where the resected joint space is immediately reconstructed. This involves placing biological materials, such as a patient’s own tissue (like a temporalis muscle flap or costochondral graft), into the gap to act as a barrier and a new articulating surface. For severe cases or revisions, total joint reconstruction using alloplastic materials, or synthetic joint prostheses, may be necessary.
Regardless of the surgical technique used, the long-term success of TMJ ankylosis treatment hinges on post-operative physical therapy. Patients must begin immediate and sustained jaw opening exercises, often using specialized mechanical devices. Noncompliance with this physiotherapy regimen is the primary cause of re-ankylosis, as the initial bone formation process can easily be triggered again.