Midcarpal instability is characterized by a loss of normal alignment between proximal and distal carpal row bones following ligament injuries of the wrist, when placed under load. It is a type of non dissociative carpal instability.
There are four types of MCI
Midcarpal Instability Type I
It is also palmar midcarpal instability and occurs secondary to an injury to the palmar midcarpal ligaments
- Scaphotrapeziotrapezoid ligaments
- Triquetrohamate and triquetrocapitate ligaments
The injury may be to either or both the ligaments.
The direction of subluxation is palmar.
Midcarpal Instability Type II
Also called dorsal midcarpal instability. It is a type I midcarpal instability in combination with an injury to the radioscaphocapitate ligament.
The direction of subluxation is dorsal.
Midcarpal Instability Type III
Midcarpal Instability Type IV
It is also called as extrinsic midcarpal instability . Mostly it is due to a malunited radius with dorsal angulation resulting in adaptive deformity of the carpus.
This leads to progressive stretching of the radiocapitate ligaments.
The direction of subluxation is mostly dorsal.
Most of the patients present with painful clunking on the ulnar side of the wrist during activities that involve active ulnar deviation.
There would be a history of asymptomatic wrist clunking for many years. Further probe may There could be a history of hyperextensive injury or there could be absence of any significant trauma.
It is frequently associated with congenital ligamentous laxity and a hypermobile wrist.
Being subtle the deformity patterns may be difficult to detect in early stages.
The diagnosis is aided bye midcarpal stress test which is performed by applying an axial load to a pronated and slightly flexed wrist, which then is brought into ulnar deviation. A painful clunk is a sign of instability [ 50% of patients with MCI may also have a clunk in the normal contralateral wrist and should always be examined]
Plain x-rays are m ostlynormal. VISI (in type I MCI)or DISI deformity (in type II MCI) may be noted in some cases.
Malunion with adaptive deformity of the carpus may be noted in type IV mid carpal instability.
Fluoroscopy may reveal the dynamic instability. Comparison with normal opposite normal wrist is often useful
Arthroscopy gives an opportunity to have a direct look at instability. Inflammatory synovitis and clear ligament laxity are the diagnostic signs.
Milder forms of midcarpal instabilities respond well to nonoperative management.
The measures that can be taken are
- External support
- Musculotendinous exercises
- Steroid injections
- Change or modification of profession/employment
Surgey can be considered for the cases where conservative treatment has failed.
Aim of surgery is to stabilize the proximal carpal row and prevent abnormal motion.
Reconstruction of the damaged ligament and percuateous percutaneous fixation with kwires would help in dynamic instabilites.
Those patients with VISI instability lunate-capitate-triquetrum-hamate fusion [four quadrant fusion] is the gold standard.
In type II static deformities or failed reconstruction of type II deformities, fusion of the midcarpal joint is the best treatment.
In type III midcarpal instability, radiocarpal fusion or proximal row carpectomy can be considered.
In type IV midcarpal instability, a corrective open-wedge osteotomy and bone grafting in combination with plate fixation of the radius is the treatment of choice.
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