Normal anatomy and MRI technique
Normally, with the mouth closed, the mandibular condyle (processus condylaris) is located in the mandibular fossa (fossa mandibularis). Anteriorly, the joint is bounded by the articular tubercle (tuberculum articulare), and posteriorly by the external acoustic meatus (meatus acusticus externus). The articular disc (discus articularis) acts as a shock absorber. Posteriorly, it is attached by fibroelastic tissue to the skull base and the fibrous capsule, while anteriorly, fibers of the lateral pterygoid muscle (m. pterygoideus lateralis) insert into it.
To evaluate the TMJ, MRI is used:
- Sequences: T1-weighted images (T1WI) or proton density (PD) are used for anatomy, while T2WI are used to detect effusion and inflammation.
- Slices: Parasagittal slices of 3 mm thickness are performed, taking into account the medial angulation of the condylar head.
- Coronal plane: Useful for visualizing medial or lateral disc displacement and assessing degenerative changes.
- Normal MRI appearance: The disc appears as a biconcave, bow-tie-shaped structure with low MR signal. It is located strictly between the anterior part of the condyle and the mandibular fossa.
TMJ dysfunction and internal derangements
Dysfunction often includes myofascial pain and disc displacement.
Myofascial pain is more common in young women and is associated with stress and teeth clenching. Since this is a type of fibromyalgia, the bone tissues are not affected, and there are no specific radiological changes. The diagnosis is made clinically (muscle tenderness, headaches).
Internal derangements are associated with displacement of the articular disc (most commonly in anteromedial, medial, or lateral directions). There are two main types:
- Anterior disc displacement with reduction. With the mouth closed, the disc is displaced anteriorly. Upon opening the mouth, it returns ("reduces") to its normal position over the condyle. This is accompanied by a characteristic click.
- Anterior disc displacement without reduction. The disc is permanently displaced anteriorly, and the condyle cannot move beneath it upon opening the mouth. This causes jaw locking, limited mouth opening, severe discomfort, and trismus. In late stages, it is often accompanied by osteoarthritis and effusion (on T2WI).
Condylar process developmental anomalies
Developmental pathologies can lead to impaired mandibular growth and facial asymmetry.
- Hemifacial microsomia: Underdevelopment of the ramus and condyle on one side. CBCT (cone-beam CT) reveals flattening of one half of the face, and the mandibular fossa is shallower compared to the healthy side.
- Condylar hyperplasia: Excessive cartilage growth after puberty. Clinically manifested by a posterior open bite or jaw deviation. Radionuclide imaging may be required prior to treatment.
- Hurler syndrome (Gargoylism): A specific radiological sign — the articular surface of the condyle has a concave shape instead of the normal convex one.
- Bifid condyle: Appears as an indentation on the condylar head. Most often asymptomatic and is the result of vascular supply impairment or early trauma.
Osteoarthritis and juvenile idiopathic arthritis
Degenerative disease (Osteoarthritis) occurs when the TMJ loses its ability to adapt to loads. Clinically, it is often manifested by crepitus. Radiographs (OPG) reveal:
- Flattening and irregularity predominantly of the anterosuperior surface of the condyle.
- Bone tissue sclerosis.
- Osteophyte formation.
Juvenile idiopathic arthritis (JIA) affects the TMJ in 17–87% of pediatric patients. The disease is latent, but without treatment leads to micrognathia and joint destruction. MRI is the method of choice. Disease activity is confirmed by contrast enhancement of the thickened synovium. This helps distinguish active inflammation from ordinary joint effusion.
TMJ trauma and fractures
In cases of traumatic facial injuries, it is important to remember the rule of associated fractures. Mandibular fractures occur in typical "weak" spots: the coronoid process, condylar neck, angle, body, and symphysis.
Important rule: A fracture of the mandibular body is accompanied by a contralateral (opposite) fracture in approximately 50% of cases — most commonly, the condylar neck breaks. Orthopantomography (OPG) or CT in severe combined injuries is optimal for diagnosis.
The normal shape of the disc on MRI resembles a bow tie, which should "sit" strictly between the condyle and the fossa. If the bow tie has "flown away" anteriorly and does not return upon opening the mouth, this is displacement without reduction.
Class and exam questions
Which MRI sequence is best to look for TMJ effusion?
In T2-weighted imaging (T2WI). The presence of effusion often indicates synovitis, especially in late stages with disc displacement without reduction.
Why does a click occur when opening the mouth?
Clicking is characteristic of anterior disc displacement with reduction. It occurs at the moment when the anteriorly displaced disc returns to its normal position over the condylar head.
What are the radiological signs of TMJ dysfunction in myofascial pain?
Specific radiological and MRI changes are absent. This is a muscular pathology in which bone structures and cartilage are not affected.
What else to study on this topic
- Radiodiagnostics of upper and lower jaw fractures
- MRI features in juvenile idiopathic arthritis in children
- CBCT and 3D modeling in post-traumatic deformities
- Differential diagnosis of hemifacial microsomia
- Avascular necrosis of TMJ bone structures
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