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Background
Stability of the thumb metacarpophalangeal (MCP) joint relies on numerous supporting structures. Without treatment, injury to these structures leads to profound functional impairment, progressive joint degeneration, and reduced quality of life. The purpose of this exhibit is to review thumb MCP joint anatomy and common injuries while highlighting the strengths, weaknesses, and pitfalls of the two primary advanced imaging modalities, MRI and ultrasound (US).

Educational Goals / Teaching Points
Review the relevant thumb MCP joint stabilizing structures. Understand the key imaging findings and clinical implications of injuries to the volar plate, ulnar collateral ligament (UCL), radial collateral ligament (RCL), pulley system, and sesamoid bones. Provide a wide spectrum of examples of MCP joint injuries as depicted by US and MRI while emphasizing the benefits of each modality. Demonstrate pitfalls and weaknesses of each modality. Many capsular and ligamentous injuries do not require surgical intervention. Advanced imaging identifies key surgical indicators in the setting of difficult clinical assessment due to swelling, pain, and injury complexity. Knowledge of injury patterns and anatomic relationships guides careful evaluation of these intimately associated structures. US provides excellent focused assessment of individual structures and offers dynamic evaluation. MRI offers a comprehensive evaluation of the thumb MCP joint while readily identifying nondisplaced fracture and bone marrow edema.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
Volar plate: the striated appearance of the volar plate and normal synovial recess may mimic a tear at MRI. Dynamic US allows identification of intermittently impinging structures, such as a disrupted volar plate or scar. Sesamoid bone: Intimately associated with accessory collateral ligaments and volar plate. The ulnar sesamoid accounts for the majority of sesamoid fractures, and a bipartite sesamoid may be difficult to distinguish from fracture on both MRI and US. UCL: majority of tears occur at the distal insertion on the proximal phalangeal base (90%). Stener lesions and full-thickness tears with >3 mm of displacement require surgical repair. RCL: tears occur much less commonly than UCL tears, and typically involve the proximal phalangeal attachment. Overlying abductor pollicis brevis prevents a Stener-like injury, but surgery is indicated with avulsion fracture fragment displacement >2 mm, palmar joint subluxation > 3 mm, dynamic joint instability. Pulley system: Acute traumatic injuries are rare, and commonly associated with multiligamentous injury. Trigger thumb, a chronic stenosing tenosynovitis, is a more commonly identified pathology. Multiligamentous injury: often associated with concurrent volar plate and dorsal capsular injury. MRI is necessary for full characterization.

Conclusion
By understanding the complex anatomy of the thumb MCP joint and the diagnostic capabilities of MRI and US, radiologists can help guide timely and appropriate treatment of this functionally critical articulation.