The following right wrist x-rays are from two different patients who each experienced a fall on their outstretched hand. Each show two different abnormalities so what can you deduce from the x-rays?

 

 

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The first set of wrist x-rays show a perilunate dislocation whereas the second set exhibits a lunate dislocation.

Lunate and Perilunate dislocations:

• Common mechanism is a fall on the outstretched hand.
• Disruption of the ligament between the capitate and the lunate leads to dislocation of the capitate from the cup-shaped distal surface of the lunate.
• Normally in a lateral wrist view, the distal radius, the lunate and the capitate lie in a straight line. In a perilunate dislocation, the capitate and the metacarpals lie posterior to the line drawn through the distal radius and the lunate. However, in a lunate dislocation, the line drawn through the distal radius goes through the capitate but the lunate is volarly displaced.
• A lunate or perilunate dislocation can be diagnosed on an AP wrist view by noting a triangular lunate. Ordinarily, the lunate has a rhomboid shape on the AP view with parallel upper and lower borders.
• Risk with a lunate dislocation is acute carpal tunnel syndrome due to the compression of the median nerve by the volarly displaced lunate (incidence is approximately 25%)
• A perilunate dislocation is usually associated with other carpal bone fractures but most commonly with a transscaphoid fracture.
• Treatment for both types of dislocations is emergent closed reduction, splinting, followed by open reduction, ligament repair and internal fixation.

Normal lateral wrist alignment:

Perilunate dislocation:

Lunate dislocation:

My mnemonic is 3 P’s : Perilunate – Capitate – Posterior

 References:

• Fundamentals of Diagnostic Radiology by Brant and Helms
• http://www.orthobullets.com/hand/6045/lunate-dislocation-perilunate-dissociation

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