What causes palmar midcarpal instability (PMCI)?

Updated: Apr 26, 2021
  • Author: David M Lichtman, MD; Chief Editor: Harris Gellman, MD  more...
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Various authors have called palmar midcarpal instability (PMCI) [17, 25, 26] by many names, including ulnar midcarpal instability, capitolunate instability pattern, ulnocarpal instability, and midcarpal instability. The most likely etiology in patients who present with a painful midcarpal clunk is dysfunction of the key ligaments that causes a loss of normal joint reactive forces between the proximal and distal rows. These ligaments include the arcuate, triquetrohamate, and capitolunate ligaments volarly and/or the radiotriquetral ligament dorsally.

In a normal wrist with ulnar deviation, the distal row translates from volar to dorsal as the proximal row rotates from flexion to extension. With laxity, attenuation, or traumatic disruption of these ligaments, the coupled rotation of the carpus is no longer present. Instead, the proximal row stays flexed, and the distal row remains excessively volarly translated until the extreme of ulnar deviation is reached, causing the proximal row to abruptly snap back into extension and the distal row to reduce (translate dorsally). The diagnosis frequently is based on the history and radiographic studies, in addition to the results of physical examination and midcarpal shift testing.

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