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Background
Temporomandibular joint dysfunction/disorder affects up to 28% of the population and presents as jaw pain or dysfunction, earache, or headache. The most common pathology seen is internal derangement, closely followed by osteoarthritis. The advent of dynamic MRI has allowed for diagnosis and characterization of TMJ disorders. This exhibit will provide a primer for the anatomy of the temporomandibular joint, which is a challenging subject for many trainees, as well as highlight common and uncommon TMJ pathologies.

Educational Goals / Teaching Points
Review the prevalence and clinical presentation of TMJ disorders. Discuss different modalities for assessing the joint. Review the anatomy of the TMJ space, including the expected translation of the disc and condyle upon mouth opening and closing. Showcase different TMJ pathologies including derangement, osteoarthritis, rheumatoid arthritis, juvenile rheumatoid arthritis, CPPD arthropathy, pigmented villonodular synovitis, psoriatic arthritis, trauma, and arthroplasty.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
The articular disc of the temporomandibular joint is a biconcave structure which lies between the mandibular condyle and the mandibular fossa of the temporal bone, and is divided into the anterior band, intermediate zone, and the posterior band. In the closed mouth position, the anterior band should lie immediately inferior the articular eminence. However, with mouth opening, the disc should translate anteriorly and the intermediate zone, the articular eminence and the mandibular condyle should all align vertically. Derangement occurs when there is an abnormal relationship between the disc, condyle, articular eminence, and fossa - usually presenting as an anterior disc dislocation. Chronic derangement or trauma can result in osteoarthritis of the TMJ, which is characterized by joint space narrowing, sclerotic changes, subchondral cysts, and osteophytes, much like osteoarthritis in other joints. Similarly, TMJ rheumatoid arthritis manifests as the typical osseous erosions, pannus formation, and synovial enhancement. CPPD arthropathy usually affects older patients. CPPD and PVNS may both appear as T2 hypointense masses expanding the joint space on MRI. CT can help delineate between the two as calcifications are uncommon with PVNS.

Conclusion
TMJ disorder is a prevalent disorder and evaluation of the joint space relies on understanding of the normal anatomy and physiology. Many TMJ pathologies manifest with the same imaging features as in other joints elsewhere in the body. However, differentiating some pathologies from others may be difficult given overlap of features between different disorders, so radiologists should be familiar with the major imaging findings of each condition.