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Scapholunate dissociation

Scapholunate dissociation represents a significant ligamentous wrist injury that is important to identify on imaging. There is disruption of the scapholunate ligament with resultant instability. The condition may also be known as rotary subluxation of the scaphoid.

Epidemiology

Scapholunate dissociation most commonly results from trauma. It is the leading cause of SLAC wrist which is the most common pattern of degenerative arthritis in the wrist 4.

Clinical presentation

Scapholunate dissociation usually presents following a fall with minimal swelling and pain localized over the dorsal scapholunate region. Presentation is often delayed in the absence of an associated fracture. Pain is increased by dorsiflexion.

Pathology

The scapholunate ligament is a U shape ligament which is arbitrarily divided into three anatomic compartments: dorsal, proximal (also known as central), and palmar. The dorsal compartment is 5 mm in thickness and composed of short, transversely oriented collagen fibers which plays more important role by resisting volar-dorsal translation than other compartments. The volar compartment measures 3mm in thickmess. The proximal compartment is mainly made of fibrocartilage. The proximal region may extend distally a few millimeters into the scapholunate joint space, thus resembling a knee meniscus.5 

Injury of the scapholunate interosseous ligament (complete tear of the dorsal compartment) and radiolunate ligament will result in scapholunate dissociation.  Mayfield et al. have proposed a 4 stage process to describe perilunar wrist instability where scapholunate dissociation represents stage 1 2.

Radiographic features

Plain film

The scapholunate interval should be measured at the midpoint of the adjacent parallel articular contours of the two bones (the proximal part is wider and the distal part is narrower).  AP radiographs may demonstrate widened scapholunate space greater than 4 mm, known as the Terry Thomas sign. This is exacerbated in clenched fist views and PA views with the wrist in ulnar deviation. The scaphoid rotates to a more transverse position which will often increase the scapholunate angle to greater than 60 degrees. When this rotation causes the scaphoid to be viewed end-on it may produce an appearance termed the 'signet-ring sign'.

Treatment and prognosis

Surgical repair or reconstruction of the scapholunate interosseous ligament is normally required to prevent long-term complications 3, namely proximal migration of the capitate between the scaphoid and lunate with resultant degenerative disease known as SLAC wrist ( scapholunate advanced collapse).

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