ARRS 2022 Abstracts

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E1922. Post-Traumatic Extramedullary Migration of Fat: Imaging Findings and Clinical Significance
Authors
  1. Ryan Graham; Temple University - - Philadelphia, PA
  2. Jee Moon; Temple University - - Philadelphia, PA
  3. Daniel Kushner; Temple University - - Philadelphia, PA
  4. Stephen Ling; Temple University - - Philadelphia, PA
  5. Sayed Ali; Temple University - - Philadelphia, PA
  6. Padmaja Jonnalagadda; Temple University - - Philadelphia, PA
Background
Following an intra-articular fracture, migration of fat into the joint leading to lipohemarthrosis is a common and well known entity. Other less common sites of fat migration following fracture include the tendon sheath, bursa, subperiosteum, and adjacent soft tissues. Free floating fat within the tendon sheath may alert the radiologist to the presence of an occult intra-articular fracture, a fracture at the osseous attachment of the tendon or laceration of tendon sheath by a fracture fragment. Awareness of these tendon sheath lipohematomas is important because they can lead to tendon ischemia, infarction, and subsequent rupture. Lipohematoma of the subacromial-subdeltoid bursa in patients with proximal humerus fractures indicates associated full thickness rotator cuff tear. Extrusion and proximal migration of fat into a subperiosteal location or into soft tissues can aid in occult fracture diagnosis. Finally, Morel-Lavallee lesion with fat-fluid level will be described.

Educational Goals / Teaching Points
Following injury, extra-osseous soft tissue migration of fat is frequently encountered on imaging In this exhibit, we will demonstrate free-floating fat within the tendon sheath, bursa, subperiosteum, and soft tissues. Their imaging characteristics, clinical significance, and management will be described. Failure to recognize these findings may alter management and affects prognosis

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
Tendon sheath lipohematomas are a consequence of damage to the tendon sheath following trauma, often secondary to disruption by adjacent bone fragments. The low-density fat migrates away from the fracture site within the tendon sheath, which can indicate both direct injury to the tendon sheath itself and can alert the radiologist to the presence of an underlying nearby occult fracture. Bursal lipohematomas can occasionally be identified on CT in patients with proximal humerus fractures. Its presence in the subacromial-subdeltoid bursa is an important indicator of a concomitant rotator cuff tear secondary to fat and blood products migrating from the bone marrow into the shoulder bursa. This can aid orthopedic surgeons in surgical management and potentially lead to improved functional outcomes. Extrusion of medullary fat into the subperiosteum has been well-documented following trauma. Subperiosteal floating fat can migrate proximally, and its presence may indicate a fracture located outside the FOV, prompting the radiologist to extend imaging more distally. Soft tissue fat extravasation could also be a harbinger for subsequent fat embolism and may affect prognosis. Morel-Lavallee lesions are closed degloving injuries resulting in collections of hemolymphatic fluid and necrotic fat between the hypodermis and underlying fascia. MRI can help determine severity and often demonstrates a well-defined fluid collection with septations and may have fluid-fluid or fat-fluid levels. Chronic lesions often demonstrate a hypointense peripheral rim reflecting a fibrous pseudocapsule.

Conclusion
Post-traumatic extramedullary fat migration is a useful radiologic sign that can indicate additional injury and aid management.