Scapholunate dissociation or rotatory subluxation of the scaphoid may occur alone or in association with wrist fractures.
The injury ranges from grade I sprains of scapholunate interosseous ligament to complete scaphoid dislocation
Injuries to following ligaments may be associated with scapholunate dissociation
- Radioscapholunate ligament
- Radioscaphocapitate ligament
- Scaphotrapezial ligament complex
- Dorsal radiocarpal ligament
- Dorsal intercarpal ligament
Disruption of the scapholunate interosseous ligament results in separation of the motion between the scaphoid and lunate in the acute phase and the development of persisting widening of the scapholunate joint as a late clinical consequence .
Presentation of Scaphoid Dissociation
There is a history of injury with wrist in dorsiflexion and ulnar deviation. The patient would present with swelling and tenderness over the scapholunate area of wrist.
Provocative stress test and Watson test may be positive. Watson test if present is is highly suggestive of scpholunate instability.
Imaging of Scaphoid Dissociation
AP, lateral and oblique views are basic investigation.
Because scapholunate injuries may be associated with fractures of the radius [occurs in 10% of the cases], the xrays of patients with fractures of the distal radius should be evaluated for ligamentous injury too.
A scapholunate gap >3 mm suggests scapholunate dissociation. If the gap is more than 5 mm it is considered a confirmatory sign. [Terry Thomas Sign]
If scapholunate angle is more than 60 degrees or capitolunate angle is >15 degrees, it suggests scapholunate instability . If scpholunate angle is >80 degrees or capitolunate angle is greater than 20 degrees, it confirms scpholunate disability instability.
If routine xrays do not show any abnormality, clenched-fist views or radioulnar stress x-rays should be done.
MRI is helpful in discriminating the extent of ligament injury and should be performed in cases with normal xrays and clinical suspicion.
Classification of Scaphoid Dissociation
The injury can be divided by duration
Acute – < 4 weeks
Subacute – 4-24 weeks
Chronic – >24 weeks
Another way to classify is whether injury is static or dynamic.
Static : Injury can be identified on plain posteroanterior and lateral xrays.
Dynamic : instability not visible on plain xrays but evident on stress views.
Treatment of Scaphoid Dissociation
Acute Scapholunate Dissociation
If there is no instability, cast immobilization is preferred.
If instability is present then percutaneous reduction and K-wires after supplemented cast immobilization for 8 weeks. Few authors recommend ligament repair as well.
Open reduction and ligament repair is done in case not amenable to closed reduction.
Subacute Ligament Tear
Repair of the broken ligaments , internal fixation supplemented by plaster cast.
Chronic Scapholunate Instability
Treatment of chronic scapholunate instability depends on
- Whether dislocations are reducible
- Whether the ligaments are repairable
- Whether there is a secondary arthritis.
Whenever possible, normal carpal anatomy should be reduced, repair/reconstruction of the ligaments should be done.
In presence of secondary arthroses or heavy physical demands, partial or complete fusion of the wrist should be done.
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