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E2084. Thumb MRI: There’s a Lot More to See Than Just the Ulnar Collateral Ligament
Authors
  1. Robert MacGregor; San Antonio Military Medical Center
  2. Christopher Bailey; San Antonio Military Medical Center
  3. Stephanie Bernard; San Antonio Military Medical Center; Uniformed Services University of the Health Sciences
  4. Douglas Byerly; San Antonio Military Medical Center; Uniformed Services University of the Health Sciences
  5. David Oettel; San Antonio Military Medical Center
  6. Nathan Cecava; San Antonio Military Medical Center; Uniformed Services University of the Health Sciences
Background
Primary care and subspecialty physicians often utilize imaging to assist in cases of thumb pain and dysfunction. Overall, radiologists are comfortable evaluating thumb radiographs, but interpreting thumb MRI can be daunting if there are knowledge gaps in anatomy and typical pathological conditions. Most radiologist have MRI experience with metacarpophalangeal (MCP) joint ulnar collateral ligament (UCL) tears, but other thumb injuries can be a blind spot. Moreover, many chronic inflammatory conditions or mass lesions of the thumb can present on MRI. The purpose of this exhibit is to review the thumb MRI anatomy and use a case-based review to illustrate a wide range of thumb MRI pathology.

Educational Goals / Teaching Points
Understand the anatomy of the thumb with emphasis on the joints, sesamoids, ligaments, pulleys tendons, and thenar muscles. Identify a range of thumb injuries on MRI and comprehend typical mechanisms of injury. Recognize typical MRI features of various thumb masses and inflammatory diseases and comprehend situations when other imaging modalities enhance characterization. Understand clinical disposition for thumb injuries, inflammatory conditions, and masses.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
The thumb is the unique opposable digit and has a greater range of motion than the fingers. It is subject to alternate forces and injuries and therefore requires the radiologist to understand its distinctive anatomy and the requisites for high quality MR imaging. Understanding thumb anatomy is key to identifying ligament, sesamoid, tendon and muscle injuries. MCP UCL injuries are more frequent than other ligamentous injuries, however radial collateral and accessory collateral ligament injuries occur with modest frequency. Additional overlooked injuries include sesamoid fractures, entrapment, and intersesamoidal ligament tears. Thumb MCP volar plate injuries often occur without bony injury and may be a subtle finding on MRI. Other sites of injury include the carpometacarpal and interphalangeal joints, thumb tendons, and thenar muscles. Numerous benign soft tissue masses occur in the thumb including ganglia, epidermal inclusion cysts, tenosynovial giant cell tumor, hemangioma and glomus tumors. Malignant soft tissue masses are rare but may not be distinguishable from bengin masses. Osseous and parosteal lesions include geode, enchondroma, osteoid osteoma, bizarre parosteal osteochondromatous proliferation, and giant cell tumor. Overuse inflammatory conditions such as DeQuarvain’s tenosynovitis and systemic inflammatory conditions are occasionally encountered.

Conclusion
Understanding the normal anatomy and MRI appearance of thumb structures will aid the radiologist in identifying a multitude of injuries, inflammatory conditions, and neoplasms on thumb MRI. Armed with case experience, radiologists can be confident in their ability to identify pathologic conditions of thumb and understand how the imaging report impacts the clinical disposition of the patient.