Learning Radiology xray montage
 
 
 
 
 
Carpal Instabilities



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  

Perilunate Dislocation
 
  • 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
 

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
        • Triquetrial 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