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
Lunate extends when there is loss of radial ligamentous stability
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.