Motorcycle crash

A stage IV injury, or lunate dislocation, results in a characteristic triangular appearance of the lunate on the posteroanterior view that is due to the rotation of the lunate in a volar direction. This triangular appearance is known as the “piece of pie sign.” This rotation also is visible on the lateral view of the wrist, in which the lunate looks like a cup tipped forward and spilling its contents into the palm. This latter appearance is called the “spilled teacup sign.” On the lateral view, the capitate is seen to lie posterior to the lunate and often has migrated proximally to contact the distal radius (Fig. 48-15).


Figure 48-15.  Lunate dislocation. A, This posteroanterior view shows the characteristic triangular shape of the lunate bone (7, arrow). B, Volar displacement of the lunate resembles a spilled teacup on the lateral view (arrow), with dorsal displacement of the capitate.
(From Propp DA, Chin H: Forearm and wrist radiology—Part I. J Emerg Med 7:393, 1989.)

Patients with these carpal dislocation injuries typically have a history of a fall on the outstretched hand. They complain of pain and swelling over the dorsum of the wrist, with limited range of motion. On physical examination, tenderness to palpation is noted over the dorsum of the wrist, particularly in the region of the scapholunate ligament. With perilunate and lunate dislocations, visible deformity of the wrist also is apparent, and two-point sensation in the median nerve distribution often is diminished. A provocative maneuver such as Watson's scaphoid shift test often will increase pain and produce a clunk or snap.[30] Complications of carpal dislocation injuries include median nerve injury and chronic carpal instability with resultant degenerative arthritis.