Scapholunate instability is spectrum of wrist instbilities that have occult scapholunate interosseous ligaments sprains on one side and scpaholunate advanced collapse on the other side.
Often the term is used interchangeably with scapholunate dissociation but in strict sense the scapholunate dissociation is one of the type of scapholunate instability.
Scapholunate instability is of following types. Each type also represents a higher stage of severity.
- Scapholunate Dissociation
- Dorsal intercalated Segment instability
- Scapholunate Advanced Collapse
Occult Scapholunate Instability
It is the mildest form of scapholunate instability.
It usually is initiated by a fall on the outstretched hand. The condition results from a tear or attenuation of a portion of the scapholunate interosseous ligament.
The xrays are normal in these patients and most of them also have normal stress xrays. The instability may be noted on flouroscopy.
Occult scapholunate instability may benefit from conservative treatment such as casting, splinting and symptomatic medications.
In selected cases, arthroscopic debridement followed by pinning or capsule repair has been said to provide satisfactory outcome.
Dynamic Scapholunate Instability
Patients with dynamic instability have a complete scapholunate ligament tear. The condition results from an injury of greater magnitude than that causes occult scapholunate instability. The xrays might be absolutely normal like occult instability. But instability is noted in both the plains on stress xrays.
Repair of the ligament and the capsule is necessary for the treatment of dynamic scapholunate instability.
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.
Dorsal Intercalated Segment Instability
Lunate extends when there is loss of radial ligamentous stability
Scapholunate Advanced Collapse
Scapholunate advanced collapsed wrist is the final stage of scapholunate instability spectrum, where there is a specific pattern of osteoarthritis and subluxation which results from untreated chronic scapholunate dissociation.
The condition however can also occur in cases of from chronic scaphoid non-union and degenerative changes abnormal loading.
SLAC wrist or Scapholunate advanced collapsed wrist is the final stage of scapholunate instability spectrum, where there is a specific pattern of osteoarthritis and subluxation which results from untreated chronic scapholunate dissociation.
The condition however can also occur in cases of from chronic scaphoid nonunion and degenerative changes abnormal loading.
Scapholunate dissociation is said to occur when a complete tear of the scapholunate ligament and one or more secondary ligaments allows the scaphoid to rotate into flexion with increase in the scapholunate interval. The rotation of the lunate becomes independent of the scaphoid.
This leads to static deformity which can be visualized on static xrays.
- Flexion of the scaphoid
- Extension of the lunate and triquetrum
- Dorsal and proximal translation of the capitate and distal carpal row
The postural changes become irreversible with time and the changed kinematics [the way forces would be handled] lead to abnormal articular loading and progressive degenerative changes known as Scapholunate advanced collapse (SLAC).
Types of SLAC Wrist
Depending on the extent of arthritis, the SLAC wrist can be divided into
SLAC wrist I – Arthritis along the scaphoid facet of the distal radius. It is the first stage
SLAC wrist II – In addition to above, the arthritis develops along the proximal radioscaphoid joint.It is second stage.
SLAC wrist III – The arthritis also develops in radial midcarpal joint. This is third stage.
SLAC wrist IV – The arthritis involves radiolunate joint and entire carpus. This is final stage and occurs after many years of initial injury.
The xrays would reveal the deformities and arthritis depending on the stage of SLAC.
These patients usually require surgical treatment.
Surgical options for SLAC wrist vary according to the joints that are involved.
It means removal of radial styloid. It is a good option for patient with stage I SLAC wrist. It will change the progression of the degenerative process, but is at most short- to midterm treatment to avoid intercarpal arthrodesis.
It can be considerd in patients with SLAC II disease with preservation of the midcarpal joint. The procedure is typically combined with distal scaphoid excision.
This option can be exercised in patients with a relatively well-preserved midcarpal joint. This is a motion sparing surgery.
This is done in patients with extensive degenerative changes at the midcarpal joint, with preservation of the radiolunate joint.
The four corner arthrodesis involves arthrodesis of capitate–lunate–hamate–triquetral arthrodesis.
Four-corner arthrodesis can be performed for SLAC I, II, and III disease as well.
Advantage of wrist arthroplasty is that it preserves motion and should be done in patients with low demands.
Should be done in patients with high demands.
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