DDx
Media Gallery
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Lunate dislocation. Posteroanterior projection of the wrist showing the pie shape of the lunate.
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Perilunate dislocation. On the posteroanterior radiograph, crowding is evident between the proximal and distal carpal bones.
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Perilunate dislocation. The lunate is in a normal anatomic position with respect to the radius. The rest of the carpal bones are displaced dorsally.
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Scapholunate dislocation. The scapholunate space is usually greater than 4 mm, a scenario also known as the Terry-Thomas sign. Rotation of the scaphoid causes the scaphoid to be viewed end-on, producing the classic signet-ring sign.
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Progressive perilunar instability pattern as reported by Mayfield et al. Stage I involves scaphoid instability; stage II, scaphoid and capitate instability; stage III, scaphoid, capitate, and triquetrum instability; and stage IV, lunate dislocation.
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On a normal lateral radiograph, the 4 Cs should be easily visualized. The 4 Cs are the convexity of the distal radius, the convexity and the concavity of the lunate, and the convexity of the capitate. A longitudinal axis aligns the radius, the lunate, the capitate, and the third metacarpal bone. The scapholunate angle is normally 30-60 degrees.
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