Lunate and Perilunate Dislocation 

• Carpal dislocations described by extent of ligamentous injury (Mayfield)
  • Stage I
    • Isolated rotatory subluxation of scaphoid
      • Mechanism: acute dorsiflexion of wrist
      • May be associated with rheumatoid arthritis
      • Characterized by increased distance between scaphoid and lunate > than 2 mm (Terry Thomas sign)
      • Scaphoid ring sign – ring-shaped shadow of cortex of distal pole of scaphoid seen on end
      • Associated more than 50% of the time with distal radial fractures
  • Stage II
    • Dislocation or subluxation of capitate
      • Exceedingly rare
  • Stage III
    • Perilunate dislocation
      • Triquetral and scaphoid malrotation
      • Result of a fall on an outstretched, hyperextended hand
      • Relatively rare
      • Occurs when the lunate maintains normal position with respect to the distal radius while all other carpal bones are dislocated posteriorly
      • Very commonly associated with a scaphoid waist fracture
        • Sometimes ulnar styloid as well
      • Lunate appears triangular in shape on frontal view
      • Lunate rotates forward slightly on lateral view
      • In lateral view, all other carpal bones are dislocated posterior with respect to lunate
  • Stage IV
    • Lunate Dislocation
      • Most severe of carpal instabilities
      • Most commonly associated with a trans-scaphoid fracture
      • Involves all the intercarpal joints and disruption of most of  the major carpal ligaments
      • Produces volar dislocation and forward rotation of lunate
        • Concave distal surface of lunate comes to face anteriorly
      • Capitate drops into space vacated by lunate
      • Capitate and all other carpal bones lie posterior to lunate on lateral radiograph
      • Triangular appearance of lunate on frontal projection