What causes piece-of-pie sign in wrist on PA radiograph?
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Answer:
Triangular fragment or appearance of the lunate bone due to volar dislocation and rotation in true lunate dislocation (Mayfield stage IV perilunate injury), a high-energy carpal disruption often from hyperextension trauma such as fall on outstretched hand, associated with scapholunate ligament tear, lunocapitate and lunotriquetral disruptions, radiolunate ligament failure, and potential median nerve injury or carpal instability. This produces the piece-of-pie sign on PA radiograph.
Why is it called so?
The sign is named for the abnormal triangular “piece-of-pie sign” shape of the lunate on posteroanterior (PA) wrist radiograph, resembling a wedge-shaped slice of pie instead of its normal quadrangular outline.
Pathophysiology
Progressive perilunate instability leads to dorsal radiocarpal ligament failure, allowing volar dislocation of the lunate into the space of Poirier with palmar rotation and overlap of the capitate, foreshortening the lunate’s projection on PA view to create the triangular appearance while disrupting Gilula’s carpal arcs and normal radius-lunate-capitate colinearity.
Alternative names: Piece of cheese sign
Other associated named signs: Spilled teacup sign, signet ring sign
Access all radiology signs posted so far: https://radiogyan.com/radiology-signs/
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