Scapholunate ligament

X-ray images indicate scapholunate ligament instability when the scapholunate distance is more than 3 mm, which is called scapholunate dissociation.[7] A static scapholunate instability is generally readily visible, but a dynamic scapholunate instability can only be seen radiographically in certain wrist positions or under certain loading conditions, such as when clenching the wrist, or loading the wrist in ulnar deviation.[6]

In order to diagnose a SLAC wrist you need a posterior anterior (PA) view X-ray, a lateral view X-ray and a fist view X-ray.[8] The fist X-ray is often made if there is no convincing Terry Thomas sign. A fist X-ray of a scapholunate ligament rupture will show a descending capitate bone. Making a fist will give pressure at the capitate, which will descend if there is a rupture in the scapholunate ligament.

The Watson's test may be used in diagnosis.

• 
  Static scapholunate dissociation
• 
  Dynamic instability: Increased scapholunate distance (between yellow lines) upon ulnar deviation of the wrist, but not otherwise.
• 
  Dynamic scapholunate instability visible upon clenching the wrist
• 
  Scapholunate ligament disruption associated with a Colles' fracture

Treatment will vary depending upon the degree of injury and can range from observation and direct ligament repair through to reconstruction.

Eventually, untreated scapholunate instability generally causes a predictable pattern of wrist osteoarthritis called scapholunate advanced collapse (SLAC).[2]