From the Guidelines
A fat pad sign on an X-ray is indicative of an elbow joint effusion, which can imply an occult elbow fracture, particularly in the context of acute trauma. This sign is characterized by the elevation of posterior and anterior fat pads, as noted in the study published in the Journal of the American College of Radiology in 2024 1. The presence of a joint effusion, as suggested by the fat pad sign, is often associated with fractures, especially of the radial head or neck, which account for 50% of cases in adults 1.
The fat pad sign appears as displacement of the normal fat pads around the elbow joint, with the anterior fat pad becoming more visible and the posterior fat pad becoming visible when it's normally hidden. The mechanism behind this sign involves joint capsule distension from fluid accumulation (blood or synovial fluid) that pushes the fat pads away from their normal positions.
Key points to consider when interpreting a fat pad sign on an X-ray include:
- The clinical context of acute trauma, which increases the likelihood of an underlying fracture
- The need for further investigation, typically with additional imaging such as CT or MRI, especially in patients with elbow pain and limited range of motion following trauma
- The importance of this finding in pediatric patients and adults with elbow injuries where fractures may be subtle or non-displaced, as approximately 85% of patients with a positive fat pad sign will have an underlying fracture, most commonly of the radial head.
From the Research
Fat Pad Sign Indications
The fat pad sign on an X-ray is indicative of several conditions, including:
- Elbow joint effusion, which may be associated with occult fractures 2, 3
- Nondisplaced fractures about the elbow after a subtle injury 4
- Displaced extension type supracondylar fractures of the pediatric elbow 4
- Bone bruises and other soft tissue injuries 3
Diagnostic Accuracy
The diagnostic accuracy of the fat pad sign, X-ray, and computed tomography in elbow trauma has been studied, with results showing:
- X-ray alone has limited sensitivity (46.9%) and specificity (85.9%) for fracture detection 5
- Incorporating the fat pad sign improves the diagnostic accuracy of X-ray, with a sensitivity of 60.2% and specificity of 81.2% 5
- Computed tomography is more accurate than X-ray for detecting elbow fractures, and is crucial for guiding treatment decisions 5
Clinical Implications
The presence of a fat pad sign on an X-ray has several clinical implications, including:
- The need for further imaging, such as computed tomography or MRI, to confirm the presence of a fracture 2, 3, 5
- The potential for nonoperative treatment of displaced extension type supracondylar fractures using the Blount method 4
- The importance of careful clinical evaluation and follow-up to ensure proper treatment and prevent complications 6, 3