What causes the crowded carpal sign on the PA wrist radiograph?
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Answer:
The crowded carpal sign is foreshortening and overlap of the carpus on a posteroanterior wrist radiograph, with the distal carpal row superimposed on the proximal row, so that the carpal height is reduced and the normal spaces between the bones are lost. It indicates a volar perilunate dislocation, in which the capitate and distal row displace volarly and proximally relative to the lunate, projecting over the proximal row on the frontal view. It is not Madelung deformity, which is a chronic developmental abnormality of the distal radius with volar-ulnar tilt and a triangular carpus. Perilunate injuries are frequently missed on the PA film alone, so the sign should always trigger scrutiny of the lateral view, where loss of collinearity of the radius, lunate and capitate is the definitive finding. Other frontal-view clues are disruption of Gilula’s arcs, the piece-of-pie sign (the lunate appearing triangular), and the signet ring sign of scaphoid rotatory subluxation.
Why is it called so?
The carpal bones appear crowded together, overlapping instead of being spread out in two neat rows.
Pathophysiology
In a perilunate injury the lunate remains attached to the radius through the strong radiolunate ligaments while the rest of the carpus dislocates around it, tearing the scapholunate and lunotriquetral ligaments or fracturing the scaphoid on the way. When the displacement is volar, the capitate and the whole distal row move both palmarly and proximally. Because the PA radiograph is a projection along the axis of the forearm, that proximal displacement shortens the apparent carpal height and superimposes the two rows on one another, obliterating the joint spaces and producing the crowded appearance.
Alternative names: None
Other associated named signs: Gilula’s arcs disruption, piece-of-pie sign, signet ring sign of scaphoid rotation, spilled teacup sign of lunate dislocation, and the Terry Thomas sign
References
Access all radiology signs posted so far: https://radiogyan.com/radiology-signs/
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