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.
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.
Scapholunate dissociation usually presents following a fall with minimal swelling and pain localised over the dorsal scapholunate region. Presentation is often delayed in the absence of an associated fracture. Pain is increased by dorsiflexion.
The scapholunate ligament is a U-shape ligament which is arbitrarily divided into three anatomic compartments: dorsal, intermediate and volar. The dorsal compartment is 3 mm in thickness and composed of short, transversely oriented collagen fibres which plays more important role by resisting volar-dorsal translation than other compartments. The volar compartment measures 1mm in thickness. The proximal or intermediate compartment is mainly made of fibrocartilage, like a meniscus.5
Injury of the scapholunate ligament (complete tear of the dorsal component) 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.
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).
- 1. Wrist Dislocation by Kadeer M Halimi from emedicine.com. Wrist Dislocation
- 2. Mayfield JK, Johnson RP, Kilcoyne RK. Carpal dislocations: pathomechanics and progressive perilunar instability. J Hand Surg Am. 1980;5 (3): 226-41. Pubmed citation
- 3. Minami A, Kato H, Iwasaki N. Treatment of scapholunate dissociation: ligamentous repair associated with modified dorsal capsulodesis. Hand Surg. 2003;8 (1): 1-6. Hand Surg (link) - Pubmed citation
- 4. Scapholunate Advanced Collapse by Dimitrios Danikas from emedicine.com. Scapholunate Advanced Collapse
- 5. Berger RA. The gross and histologic anatomy of the scapholunate interosseous ligament. J Hand Surg Am. 1996;21 (2): 170-8. doi:10.1016/S0363-5023(96)80096-7 - Pubmed citation