Midcarpal Instability

 

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Definition/Description

MCI is characterized by a lack of support of the proximal carpal row and the midcarpal joint and loss of normal joint forces between proximal and distal carpal rows. Midcarpal instability can be palmar less common dorsal and extrinsic.  [1]

Clinically Relevant Anatomy

The arcuat ligament complex is the most important  stabilizer of the midcarpal joint and exist of an ulnar arm, the triquetrohamatecapitate ligament and a radial arm who extends distal of the radioscaphocapitate ligament. The dorsal radiolunotriquetral ligament is also an important stabilizer of the proximal carpal row. 

Epidemiology /Etiology

Palmar midcarpal instability (PMC) is due to the laxity of the ulnar arm of the arcuat ligament and an increased laxity of the dorsal radiolunotriquetral ligament. A dynamic flexion deformity occurs in the proximal row as the distal row translates volarly due the ligament laxity.There is no longer the coupled rotation of the carpus with the gradual transition of the proximal row from volar to dorsal intercalated segment instability. Instead the proximal row remains flexed and the distal row remains volarly subluxed for a prolonged period, Until extreme ulnar deviaton when the distal row abruptly reduces on the proximal row and the proximal row jumps into a DISI configuration and we hear  a painful  catch–up clunk.[2]
Dorsal midcarpal instability is due to a laxity of the palmar radioscapocapitate ligament. There is a dorsal subluxation of the midcarpal joint.You hear also a clunk, but more a subluxation clunk
Extrinsic midcarpal instability is mostly caused by malalignment of a distal radius malunion. There is a dorsal displacement and angulation of the distal radius and an adaptive z-deformity of the carpus. [3]

Characteristics/Clinical Presentation

The patients often have no history of traumatic events, The patients have complaints in the form of loss of strength, laxity, impaired prehension, uncomfortable feeling and clicking and / or snapping sounds with an ulnar deviation.

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