The appearances demonstrated on the AP projection demonstrate a loss of the normal radiocarpal alignment. The scaphoid is fractured and overlaps the radius. The lunate is displaced to the ulnar side and looks “odd”. There is also loss of the normal alignment of the proximal and distal row of carpal bones that should normally be roughly parallel. In this case the capitate overlaps the proximal row. These findings indicate a dislocation of the lunate or a perilunate dislocation. The lateral clarifies the anatomy better. There is loss of the normal alignment of the radius and the lunate which normally sits on the radius with the concave side facing up. In this case the lunate has tipped forward in a volar location (curved arrow). This is sometimes referred to as the “tipped teacup” appearance. These appearances in combination are referred to as a trans-scaphoid lunate dislocation. Other appearances that may be seen on the AP projection – but not seen in this case - are the “piece of pie” appearance (instead of the typical semilunar appearance) with the apex of the pie projecting distally often overlapped by the capitate.
The key other consideration in these cases is that of a perilunate dislocation. Both occur due to similar mechanisms of high energy impact on an outstretched dorsiflexed hand. The appearances on the AP projection with overlap of the capitate and lunate can appear comparable. However, recall that the perilunate (“around the lunate”) dislocation is not actually a dislocation of the lunate. The difference is easily appreciated on the lateral film. In perilunate dislocation the radiolunate articulation on the lateral would be normal. The dislocation occurs above the lunate with the distal carpal bones angulated in the opposite dorsal direction. In lunate dislocation the capitate long axis remains aligned with the radius (see red line), but in perilunate dislocation the capitate angles backwards.
Perilunate and lunate dislocation can –as in this case- often be associated with fractures of the scaphoid, acquiring the trans-scaphoid term. Fracture of the scaphoid may therefore also somewhat alter the typical AP and lateral appearances.
With lunate dislocations it is important to consider and report the angle of rotation of the lunate. This is only 30-45° in this case but can be as much as 270°. Urgent surgical correction of dislocation and associated repair of ligaments is required although even with appropriate management premature arthritis and wrist instability is common.
The key other consideration in these cases is that of a perilunate dislocation. Both occur due to similar mechanisms of high energy impact on an outstretched dorsiflexed hand. The appearances on the AP projection with overlap of the capitate and lunate can appear comparable. However, recall that the perilunate (“around the lunate”) dislocation is not actually a dislocation of the lunate. The difference is easily appreciated on the lateral film. In perilunate dislocation the radiolunate articulation on the lateral would be normal. The dislocation occurs above the lunate with the distal carpal bones angulated in the opposite dorsal direction. In lunate dislocation the capitate long axis remains aligned with the radius (see red line), but in perilunate dislocation the capitate angles backwards.
Perilunate and lunate dislocation can –as in this case- often be associated with fractures of the scaphoid, acquiring the trans-scaphoid term. Fracture of the scaphoid may therefore also somewhat alter the typical AP and lateral appearances.
With lunate dislocations it is important to consider and report the angle of rotation of the lunate. This is only 30-45° in this case but can be as much as 270°. Urgent surgical correction of dislocation and associated repair of ligaments is required although even with appropriate management premature arthritis and wrist instability is common.
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