Perilunate Dislocation

A perilunate dislocation is the second stage of ligamentous injury around the lunate. It occurs at the midcarpal row, with dorsal dislocation of the capitate from the lunate and concordant dorsal translation of the rest of the carpus. The lunate stays where it is, articulating with radius.

Perilunate dislocations are the most frequent carpal dislocation and represent the least severe of the ligamentous injuries around the lunate. When purely ligamentous, they are referred to as lesser arc injuries. When associated with a fracture of one or more bones around the lunate, they are referred to as greater arc injuries (twice as common as lesser arc injuries).

Greater arc injuries have a special naming convention. The bone with the fracture gets a "trans-" prefix, followed by "perilunate dislocation." For example, a perilunate dislocation associated with a scaphoid fracture (the most common associated fracture) is referred to as a transscaphoid perilunate dislocation.

Fractures of the other perilunate carpal bones (trapezium, capitate, hamate, or triquetrum) are less common. Combination fractures can also occur. One such combination fracture is the scaphocapitate fracture, a transverse fracture through the scaphoid waist and capitate, where the proximal capitate fragment ends up perpendicular to the distal fragment and the long axis of the radius with its articular surface towards the palm.

On radiographs, look for disruption of the carpal arcs; triangular or pie-slice appearance of the lunate (indicating volar rotation of the lunate); fractures of the carpal bones around the lunate; and the David Letterman sign, indicating widening of the scapholunate interval.

The lateral projection is the most helpful when positive and may show dorsal translocation of the capitate and the rest of the carpus in relation to the lunate. The lunate should keep its articulation with the radius.

The images show lateral, oblique, and frontal radiographs of a perilunate dislocation. The green arrow shows the David Letterman sign. A small avulsion fragment seen on CT in the lunotriquetral space (blue arrow) was not appreciated on the radiographs.

References

• Kaewlai R, Avery LL, Asrani AV, Abujudeh HH, Sacknoff R, Novelline RA. Multidetector CT of carpal injuries: anatomy, fractures, and fracture-dislocations. Radiographics. 2008 Oct;28(6):1771-84.
• Monahan PR, Galasko CS. The scapho-capitate fracture syndrome. A mechanism of injury. J Bone Joint Surg Br. 1972 Feb;54(1):122-4.