Lunate and Perilunate Dislocations

Introduction

Lunate and perilunate dislocations following trauma to the wrist are uncommon but not rare. These injuries are unfortunately missed too often and present considerable management problems when diagnosis is delayed.

Anatomy

Normal Variants

This is a type I lunate. Note that the lunate does not articulate directly with the hamate.	This is a type II lunate- the hamate articulates directly with the lunate.

The Distal Radius, Scaphoid, Capitate, Lunate and Metacarpal Relationship

When you review your wrist radiography in a trauma setting, it is important to consider whether you can see a normal relationship between the distal radius, lunate, scaphoid and capitate. It is fortunate that the lunate, scaphoid and capitate are often the easiest to visualise- all three have a smooth proximal convex cortical margin and they almost always occur in the same order.

The relationship between the distal radius, lunate, capitate and 3rd metacarpal has been likened to a spoon (3rd metacarpal) cracking an an egg (capitate) sitting in an eggcup (lunate) on a table(radius)	The capitate, scaphoid and lunate can usually be identified by their 3 inferior cortical margins which usually appear as 3 convexities in order as shown above	The scapholunate angle should fall within the range of 30 60 degrees

Lunate Dislocation or Perilunate Dislocation?

Lunate Dislocation	Perilunate Dislocation
In cases of lunate dislocation, The normal alignment of the distal radius, carpus and metacarpals should remain normal with the exception of the lunate which will be displaced and rotated in a palmar direction	In cases of perilunate dislocation, the alignment of the distal radius, carpus and metacarpals will be the opposite of that seen in lunate dislocations- the lunate will demonstrate normal (or near normal) alignment, and the remainder of the carpus and metacarpals will be displaced in a dorsal direction.

Lunate Dislocation	Perilunate Dislocation
The distinction between perilunate and lunate dislocation on the PA projection image is likely to be very subtle. In cases of lunate dislocation, the lunate appearance is likely to be neither normal nor will it have the triangular (piece of pie) appearance. The position and outline of the lunate will show considerable derangement.	In cases of perilunate dislocation, the lunate will have a triangular appearance (piece of pie sign). This appearance is caused by palmar rotation of the lunate which is being rotated by the pressure applied by the carpus positioned dorsally. Given the significant damage to the carpal ligaments, the other carpal bone (and in particular the scaphoid) may be demonstrated with abnormal alignment.

Perilunate Dislocation Radiological Appearance

- 4 types of perilunate dislocations

1 transscaphoid-perilunate
2 perilunate
3 transradial-styloid
4 transscaphoid-trans-capitate-perilunar

1 Transscaphoid-perilunate Dislocation	2 Perilunate Dislocation	3 transradial-styloid Perilunate Dislocation	4 Transscaphoid-trans-capitate-perilunate Dislocation
Fractured scaphoid associated with perilunate dislocation	Perilunate dislocation with concomittant bony trauma	Perilunate dislocation with fractured radial styloid.

Case 1

This 32 year old man presented to the Emergency Department after falling from a ladder. He was referred for an extensive list of radiographic examinations including left wrist. This is the first image from this particular trauma series. (in cases of serious trauma, the wrist imaging would not be a high priority)

There is loss of parallelism of the carpal bones. The ulnar styloid suggests a normal variant or old injury.	There is loss of the normal proximal and distal carpal arc lines

The loss of normal carpal arcs was noted by the radiographer who proceeded to perform a lateral wrist view.

There lunate is normally aligned within the limitations of the previous wrist fracture. The remainder of the carpus are dislocated in a dorsal direction. There is a perilunate dislocation of the carpus. The radiographer noted that there was also a negative palmar tilt of the radius (the relationship between the distal radial articular surface and the lunate is not normal). On questioning the patient, it was revealed that the patient had sustained a wrist fracture in his youth. This would explain the negative palmar tilt and the appearance of the ulnar styloid on the PA image.

There is a small dorsal radius fracture which is likely to have been caused by direct impact with the scaphoid.

The radiographer paid particular attention to the integrity of the scaphoid.

Whilst the appearances suggested a simple perilunate dislocation of the carpus, associated fractures of the scaphoid are frequently missed. The distinction between a simple peri-lunate dislocation and a trans-scaphoid perilunate dislocation is important in terms of surgical treatment. The radiographer performed a dedicated scaphoid view (above left) which did not demonstrate a scaphoid fracture. This view has considerable limitations because of the abnormal siting of the scaphoid associated with the peri-lunate dislocation. The reverse of this view (reversing the tube angulation) would project the scaphoid clear of the radius but would almost be an axial view of the scaphoid (extreme foreshortening) The linear artifacts are caused by the type of cardboard splint fitted by the ambulance officers in the field.

This is the patient's lateral wrist image. The conventional scaphoid view (1) has limitations given that the scaphoid is largely projected over the radius. A reverse angled view (2)will project the scaphoid clear of the radius but is of limited utility because the scaphoid will appear foreshortened (in fact it would almost be an axial view).

The patient proceeded to CT followed by the operating theatre for reduction of the carpal bone dislocation.

This PA CT wrist image demonstrates the position that the lunate would normally occupy (dotted line)	There is dorsal dislocation of the capitate with respect to the lunate. There is a also a bony fragment demonstrated

This is the post-reduction image from the mobile image intensifier.

Comment

This case demonstrates a thoughtful approach to the radiographic demonstration of this patient's carpal injury. The radiographer recognised the carpal dislocation from the PA image. The radiographer was mindful of the importance of obtaining a good lateral projection. Equally, the radiographer paid careful attention to the imaging of the scaphoid (given the association between this carpal dislocation and scaphoid fracture). Importantly, the surgical approach in cases where a scaphoid fracture is demonstrated is different to a simple closed reduction of a perilunate dislocation.

The radiographer also noted the negative palmar tilt and considered the possibility of an old wrist fracture. This was confirmed by the patient and this information was relayed in person by the radiographer to the reporting radiologist.

This case supports the contention that trauma radiography is a specialised area of plain film radiography. In addition to all of the above, the radiographer relayed the findings in person to the referring doctor in the ED. This facilitated a timely undertaking of orthopaedic review and subsequently CT imaging of the wrist and surgical reduction. This type of personal communication helps to engender a co-operative relationship, sense of common purpose, and camaraderie between the radiographer and the referring doctor.

Finally, I would suggest that the satisfaction experienced by the radiographer in successfully completing this examination would be vastly different to a radiographer who simply undertook AP and lateral views of the patient's wrist and was unaware that the patient had sustained a significant wrist injury.

Case 2

This 28 year old male presented to the Emergency Department following unknown trauma with a painful left wrist which appeared grossly abnormal. He was referred for left wrist radiography.

The PA wrist projection image demonstrates an abnormally shaped and aligned lunate. The scaphoid is abnormally aligned and overlaps the distal radius. The triquetrum shows abnormal alignment with respect to the distal radius (overlap) The remainder of the carpal bones demonstrate normal alignment There is a minimally displaced fracture of the ulnar styloid.

There is a posterior perilunate dislocation The lunate appears to be normally aligned

Case 3

	This 25 year old male presented to the Emergency Department following a fall. He was examined and found to have a very painful right wrist. He was referred for right wrist radiography. There is a scaphoid cortical irregularity and fracture line suggesting a scaphoid fracture. The lunate is flexed (volar rotation)- this is referred to as a piece-of-pie sign. There is discontinuity of Gilula's second arc at the lunate. The distance between the proximal cortical border of the capitate and the corresponding articular surface of the lunate is abnormally large (parallelism) The appearances suggest a transcaphoid perilunate dislocation Old fracture of the ulnar styloid is noted.
	The lunate is outlined. The distance between the lunate and capitate is abnormally large (parallelism)
	The lunate is displaced in a volar direction. There is a bony fragment volar to the lunate which is a fragment of the scaphoid.
	The proximal capitate is positioned dorsal to the lunate. The lunate is outlined in white The proximal capitate is outlined in red.
	The two fragments of the fractured scaphoid are outlined in white.
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