Scapholunate dissociation, also known as rotary subluxation of the scaphoid, refers to an abnormal orientation of the scaphoid relative to the lunate, and implies severe injury to the scapholunate interosseous ligament and other stabilizing ligaments.
Carpal dissociation implies carpal instability, which has important clinical implications; thus it is essential to identify this finding on imaging. Note that absence of dissociation does not exclude ligamentous injury, as lower grade injuries which result in dynamic instability may present with normal radiographic carpal alignment.
The typical pattern of scapholunate dissociation consists of:
- relative flexion (volar rotation) of the scaphoid
- relative extension (dorsal rotation) of the lunate
Although "scapholunate dissociation" is a descriptive term, it is used synonymously with its primary pathophysiologic correlate, scapholunate interosseous ligament (SLIL) injury. Although injury to additional ligaments is present in scapholunate dissociation, the SLIL is the major target of surgical reconstruction.
Dorsal intercalated segment instability (DISI) is a related carpal malalignment pattern which also results from rupture of the scapholunate interosseous ligament 6.
Scapholunate dissociation most commonly results from trauma. It is the leading cause of SLAC (scapholunate advanced collapse) 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 interosseous ligament (SLIL) is a U-shaped ligament which is arbitrarily divided into three anatomic components: dorsal, intermediate and volar 7:
- strongest, most important in resisting volar-dorsal translation
- 3 mm in thickness and composed of short, transversely-oriented collagen fibres
- primarily composed of fibrocartilage, homologous to the meniscus of the knee 5
- 1 mm in thickness
Major injury of the SLIL (complete tear of dorsal component) and radiolunate ligament may result in scapholunate dissociation. However, even complete SLIL transection does not result in permanent dissociation due to the presence of the secondary scaphoid stabilizers, e.g. palmar radioscaphoid-capitate, scaphoid capitate, and anterolateral scapho-trapezio-trapezoid ligaments. Long-term wear on these secondary ligamentous stabilizers may lead to permanent deformity / scapholunate advanced collapse 6.
Mayfield et al. have proposed a four-stage process to describe perilunar wrist instability, in which 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 a widened scapholunate space greater than 4 mm, known as the Terry Thomas sign. This is exacerbated by 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
Acute non-displaced and chronic asymptomatic SLIL injuries may be treated conservatively with non-steroidal anti-inflammatories (NSAIDs) and immobilisation 8.
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
- 6. Lee DJ, Elfar JC. Carpal Ligament Injuries, Pathomechanics, and Classification. (2015) Hand clinics. 31 (3): 389-98. doi:10.1016/j.hcl.2015.04.011 - Pubmed
- 7. Cyrus P. Bateni, Roger J. Bartolotta, Michael L. Richardson, Hyojeong Mulcahy, Christopher H. Allan. Imaging Key Wrist Ligaments: What the Surgeon Needs the Radiologist to Know. (2013) American Journal of Roentgenology. 200 (5): 1089-95. doi:10.2214/AJR.12.9738 - Pubmed
- 8. Evan Watts. Scapholunate Ligament Injury & DISI. OrthoBullets