From the Guidelines
The posterior fat pad sign on elbow X-ray is pathognomonic for fracture. When a fracture occurs around the elbow, bleeding into the joint causes displacement of the fat pads that normally reside in the joint recesses. While the anterior fat pad may be visible on a normal lateral elbow X-ray (appearing as a radiolucent triangle anterior to the distal humerus), the posterior fat pad is not normally visible. Visualization of the posterior fat pad indicates joint effusion, which in the setting of trauma strongly suggests an occult fracture, even if the fracture line itself is not visible on the X-ray. This is particularly important in adults, where the most frequent fracture involves the radial head or neck and accounts for 50% of cases 1. Some key points to consider include:
- The presence of a joint effusion can imply an occult elbow fracture, especially in combination with the clinical context of acute trauma 1.
- Avulsion fractures can also be identified at the attachment sites of tendons and ligaments, and triceps tendon tears may result in avulsion fractures of the olecranon or an olecranon enthesophyte 1.
- Injuries to the coronoid process are sequela of prior elbow dislocation, which is typically associated with soft tissue injury, and should prompt the referring provider to assess for associated tendon injuries 1. The pathophysiology involves hemorrhage into the joint space displacing the fat pad from its normal position in the olecranon fossa, making it visible on lateral radiographs. Key considerations for diagnosis and management include:
- Conventional radiographs are often the first-imaging modality used to exclude a fracture or dislocation 1.
- The presence of a posterior fat pad sign should prompt treatment as having a fracture with appropriate immobilization and orthopedic follow-up, even if no fracture line is identified.
From the Research
Fat Pad Sign on X-ray
The fat pad sign on X-ray is a significant indicator of elbow fractures.
- The posterior fat pad sign (PFPS) is particularly useful in diagnosing suspected nondisplaced fractures about the elbow after a subtle injury 2.
- A study found that 76% of children with an elevated posterior fat pad sign had evidence of a fracture, making it a reliable predictor of occult fractures in children 3.
- In adults, the fat pad sign is also associated with occult fractures, with 75% of patients with an effusion but no visible fracture on radiographs having radiographically occult fractures identified on MR imaging 4.
Pathognomonic Sign for Fracture
The posterior fat pad sign is considered pathognomonic for fracture, especially in children.
- A retrospective study of 45 consecutive children with an elevated posterior fat pad sign found that 34 (76%) had evidence of a fracture 3.
- Another study found that the presence of a PFPS may predict the successful nonoperative treatment of a Gartland III-type fracture 2.
- The fat pad sign is also useful in adults, with a study finding that 75% of patients with an effusion but no visible fracture on radiographs had radiographically occult fractures identified on MR imaging 4.
Diagnostic Accuracy
The diagnostic accuracy of the fat pad sign, X-ray, and computed tomography in elbow trauma has been evaluated in several studies.
- A retrospective study found that considering the fat pad sign in X-ray images improved the diagnostic accuracy of X-ray in detecting elbow fractures, with a sensitivity of 60.2% and specificity of 81.2% 5.
- Another study found that X-ray alone was insufficient for detecting elbow fractures and determining the need for surgical treatment, but incorporating the fat pad sign improved the diagnostic accuracy of X-ray 5.
- The fat pad sign is a useful indicator of elbow fractures, and its presence should be considered when evaluating X-ray images, especially in cases where suspicion of fracture is high 2, 3, 4, 5.