Applied Radiology

^† Dr. Tello died on March 8, 2005. At the time of his death, he was a Professor of Radiology, Epidemiology and Biostatistics, Boston University School of Medicine, Boston Medical Center, Boston, MA.

At the time this article was written, all of the remaining authors were affiliated with Boston University School of Medicine as medical students, residents, or fellows; they have now moved to other facilities. Dr. Roth is Director of Breast MRI, Department of Radiology, Lakey Clinic, Burlington, MA. Dr. Ward is a radiologist with Sullivan's Island Imaging, LLC, Sullivan's Island, SC. Dr. Tsai is Staff Radiologist, Department of Radiology, Caritas St. Elizabeth's Medical Center, Boston, MA. At the time this paper was written, Dr. Zolotor was a medical student at the Boston University School of Medicine.

This article is adapted from a presentation that was given at the 2002 meeting of the American Roentgen Ray Society, Atlanta, GA.

Magnetic resonance imaging (MRI) has replaced computed tomography (CT) and arthrography as the primary modality in the evaluation of the temporomandibular joint (TMJ). Direct visualization of the disk afforded by MRI is a distinct advantage over arthrography. ¹ Despite the superior resolution of CT and limited visualization of cortical bone by MRI, most osseous pathology is accurately depicted. ² Intra-articular abnormalities are readily visible on MRI images, providing further information not available with other imaging modalities. ³

The TMJ is a ginglymoarthrodial (hinge and glide) articulation with some degree of diathrosis (free motion) formed by the mandibular condyle and glenoid fossa of the temporal bone ⁴ (Figure 1). Within this synovial joint is a fibrous disk or meniscus that divides the joint into superior and inferior compartments that do not communicate unless disk integrity is compromised. The biconcave disk has 3 functional segments: A thick posterior band that is separated from the anterior band by the thin intermediate zone. Multiple ligamentous attachments provide disk stability. Posteriorly, the bilaminar zone attaches the disk and capsule to the condyle and temporal bone. Laterally, the disk is continuous with an unnamed ligament attaching to the neck of the condyle. The superior belly of the lateral pterygoid muscle inserts into the anterior portion of the disk.

The surrounding supporting ligamentous structures are the temporomandibular, sphenomandibular, and stylomandibular ligaments. The temporomandibular ligament provides lateral support extending from the zygomatic process of the temporal bone to the condylar neck. The sphenomandibular and stylomandibular ligaments provide medial support coursing from the spine of the sphenoid bone to the lingual of the mandibular foramen and from the styloid process to the mandibular ramus, respectively. No discrete ligaments are normally observed anteriorly or posteriorly around the joint.

The relationships of the structures of the TMJ change with mouth position. When the mouth is closed, the disk is positioned at approximately 11 o'clock with respect to the mandibular condyle; the posterior segment of the disk caps the apex of the mandibular condyle. As the jaw opens, the disk remains between the condyle and the articular eminence as the condyle translates anteriorly. The thin intermediate zone of the disk is interposed between the condyle and the articular eminence in the fully open mouth position (Figure 2).

Imaging the TMJ

A small surface coil is placed over the TMJ; a bilateral examination can be performed with coupled coils. Images are obtained in the open-and closed-mouth positions to assess the position and reducibility (or recapture) of the articular disk. This is facilitated by placing a specialized device in the patient's mouth to keep it open and by instructing the patient to bite down on it for the closed-mouth views.

From axial localizing images, sagittal and coronal planes are prescribed. Imaging is most commonly performed in these planes in order to document the position of the disk. Oblique sagittal and coronal images can be oriented to the condyle, but are unnecessary to demonstrate internal derrangements.

T1-weighted sagittal images are the cornerstone of the TMJ examination; the anatomy is clearly depicted, and the imaging plane is optimal for assessing articular disk position. T2-weighted images are useful for detecting degenerative periarticular changes and the presence of a joint effusion. ⁴ Fat saturation or inversion recovery renders these findings more conspicuous.

Gradient-echo techniques have been implemented to obtain cine-loop motion studies. Three-dimensional volume acquisitions allow a volume of tissue to be imaged rapidly and subsequently viewed in any plane. The use of intra-articular and intravenous gadolinium may provide utility in certain clinical instances-for instance, the inflamed synovium or an inflamed arthropathy will avidly enhance after the administration of intravenous gadolinium. ^5-7

Internal derangement

An abnormal position of the disk constitutes internal derangement. The cause is usually not elicited; postulated causes include trauma, malocclusion, bruxism, stress, and primary osseous abnormalities. ³ The disorder is 3 to 5 times more common in females and commonly manifests by the fourth decade. Initially, there is anterior displacement of the disk that reduces with jaw opening. As the fibers of the posterior bilaminar zone loosen, the disk no longer reduces. Disk deformity ultimately results and secondary osseous and articular abnormalities ensue.

Abnormal disk morphology and position are the earliest and most sensitive signs of internal derangement. The earliest finding is often T2 hyperintensity in the bilaminar zone. Disk degeneration is reflected as desiccation or loss of signal that is typically intermediate on T1weighted images and hyperintense on T2-weighted images. ⁹ Disk deformity may also develop; the disk may become biconvex, thickened, or folded among other previously described morphologic abnormalities. Late in the course of the disease, the disk or bilaminar zone may perforate.

The position of the disk must be assessed first on sagittal closed-mouth images. The disk most commonly displaces anteriorly or anterolaterally. However, the disk can become displaced in any direction; medial and lateral disk displacements account for up to 30% of cases. ⁴ Early in the course of the disease, the disk may reduce in position (Figure 3). With progression, the disk does not reduce during mouth opening (Figure 4). Finally, an irreducible disk may become adherent and remain fixed anteriorly during both mouth opening and closing (Figure 5). Fluid may accumulate within the joint. Cortical erosions, followed by condylar head flattening and anterior osteophytosis, develop. Subchondral marrow edema followed by low-signal sclerosis is the natural progression.

Arthropathies

The TMJ is susceptible to the same varieties of arthritis that involve other joints in the body. Inflammatory arthritis and degenerative arthritis are the most common offenders; however, other types of arthritis known to involve the TMJ include: Infectious, posttraumatic, and metabolic arthritis. ²

Among the inflammatory arthritides affecting the TMJ, rheumatoid arthritis is the most common and has received the most coverage in the literature. The abnormalities described in the TMJ are similar to those in other synovial joints afflicted with rheumatoid arthritis (Figure 6). The advantage of MRI in evaluating involvement of the TMJ in rheumatoid arthritis is its ability to reveal the soft-tissue abnormalities. The inflammatory synovial pannus eventually destroys the disk and its supporting structures, resulting in abnormal disk position, abnormal morphology, and possibly complete destruction of the disk. The presence of a joint effusion can be depicted with T2-weighted images. Direct visualization of the synovial pannus with T1-weighted postgadolinium images has been reported with variable success. ^1,8 In any event, synovial enhancement is nonspecific and may occur in any inflammatory process, including osteoarthritis. ¹⁰

Despite the superiority of CT in depicting osseous anatomy, the ability of MRI to depict the bony abnormalities of rheumatoid arthritis has been shown to equal CT. ⁵ Destruction of the condyle and articular eminence are typical findings, and marrow signal abnormalities may reflect edema or, occasionally, subchondral sclerosis. Eventually, bony apposition ensues with destruction of the intervening soft-tissue structures. Because the bony and soft-tissue findings in rheumatoid arthritis are frequently bilaterial, both TMJs should be imaged.

In degenerative arthritis of the TMJ, MRI initially reveals subchondral changes, such as increased T2 hyperintensity and cysts. These findings are shared by primary and secondary osteoarthritis. Secondary osteoarthritis can be associated with previous trauma, surgery, internal derangement, or congenital malformation. The common end point is subchondral sclerosis, marginal osteophytes, condylar flattening, occasional subchondral cysts, and distortion of the disk due to adhesions. ^2,4 In advanced cases, fusion of the joint may occur (Figure 7).

Developmental anomalies

Innumerable conditions can affect the normal development of the TMJ, including hereditary and nonhereditary syndromes, postnatal trauma, infection, radiation, and endocrine and dietary disturbances. ² The uniting feature of these disparate entities is their effect on the growing condyle. The result is a spectrum of abnormalities, ranging from condylar agenesis to condylar hyperplasia.

Condylar agenesis is associated with congenital syndromes, such as otomandibular dysostosis, hemifacial microsomia, and mandibulofacial dysostosis. Depending on the extent of involvement, the condyle, glenoid fossa, coronoid process, ramus, and even the mandibular body may be absent. Deficiencies in the external ear, the auditory canal, and the middle and inner ear may also be present.

Condylar hypoplasia is more commonly the result of a postnatal insult, such as trauma, infection, or radiation. Altered condylar morphology is assocated with a shallow sigmoid notch, a short ramus and mandibular body, and underdevelopment of the glenoid fossa (Figure 8). Soft-tissue abnormalities, such as deficiencies in the external ear, are not a feature of condylar hypoplasia.

Although condylar hyperplasia can result from hereditary syndromes and endocrine disturbances, the most common cause is idiopathic unilateral condylar hyperplasia. The condyle may be morphologically normal, or elongation of the condylar process may be noted. The mandibular ramus and body may be elongated, which will result in chin deviation toward the unaffected side when unilaterial.

Dislocation

Dislocation of the TMJ usually occurs anteriorly. Displacement posteriorly, superiorly, or medially is prohibited by the contour of the ipsilateral glenoid fossa. Laterial displacement is prevented, as the medial wall of the contralateral glenoid fossa confines the contralateral condyle. A dislocation in any direction except anterior implies a fracture of either the articular fossa or the mandible.

An anteriorly dislocated mandibular condyle is visualized anterior and superior to the articular eminence (Figure 9). A patient with a dislocated mandible is unable to close his or her mouth. Conversely, in subluxation, the condyle reduces in the closed-mouth position. ² Other stigmata of traumatic injury can be readily appreciated on MR images of the TMJ, such as fractures, bone marrow edema, and hemarthroses.

Conclusion

The exquisite tissue contrast of MRI is optimal for visualizing the soft tissue and periarticular structures of the TMJ. Careful attention to technique with dedicated coils and high spatial resolution are essential. Dynamic maneuvers (opening and closing the mouth) are a necessary component of the examination to assess the position of the articular disk (which usually dislocates anteriorly). MRI is capable of demonstrating abnormalities of the disk, its supporting structures, synovium, and periarticular structures associated with internal derangement, degenerative and inflammatory arthropathies, and developmental and traumatic conditions.