False-Negative and False Positive Fat Pad Signs

It is tempting to take the fat pad sign as sine qua non for elbow fractures, but here are some numbers: The negative predictive value for the anterior fat pad sign is close to 90% for radial head and neck fractures and about 80% for other elbow fractures. The anterior fat pad sign is also pretty sensitive (85%) for detecting radial head fractures, but not as sensitive (35%) in detecting other elbow fractures.

A false-negative fat pad sign of the elbow may occur if there is poor positioning of the elbow (true lateral is best), extracapsular abnormality (e.g., massive soft-tissue swelling around the joint), or capsular rupture.

A false-positive fat pad sign of the elbow can be seen with elbow extension. Non-traumatic joint pathologies can also result in a positive fat pad sign, including effusion, purulent exudate, intra-articular bodies, and synovial proliferation.

References

• Goswami GK. The fat pad sign. Radiology. 2002 Feb;222(2):419-20.
• Irshad F, Shaw NJ, Gregory RJ. Reliability of fat-pad sign in radial head/neck fractures of the elbow. Injury. 1997 Sep;28(7):433-5.
• Yu JS. Soft-tissue signs of the upper extremity revisited: radiologic and magnetic resonance imaging correlation. Emergency Radiology (1999) 6: 282-289