Frontal radiograph of the wrist reveals overlap of the distal carpal row with the proximal carpal row. This overriding gives rise to the "crowded carpal" appearance and is a sign specific for volar perilunate dislocation. It is secondary to proximal migration of the distal row of carpal bones during subluxation.
Reference
Klein A, Webb LX: The crowded carpal sign in volar perilunar dislocation. J Trauma 1987 Jan;27(1):82-4.