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INTRODUCTION: Palmar midcarpal instability is poorly understood, with most treatments based on pathomechanical assumptions. The purpose of this study was to create a biomechanical model of palmar midcarpal instability by selective ligament sectioning, and to analyze treatment by simulated partial wrist arthrodesis.

MATERIALS & METHODS: Nine fresh-frozen cadaver arms were moved through three servohydraulic actuated motions and two passive wrist mobilizations. The dorsal radiocarpal, triquetrohamate, scaphocapitate, and scaphotrapeziotrapezoid ligaments were sectioned to replicate palmar midcarpal instability. Kinematic data for the scaphoid, lunate, and triquetrum were recorded before and after ligament sectioning, and again after simulated triquetrohamate and radiolunate arthrodesis.

RESULTS: Following ligament sectioning, the model we created for palmar midcarpal instability was characterized by significant increases in a) lunate angular velocity b) lunate flexion-extension and c) dorsal/volar motion of the capitate during dorsal/volar mobilizations. Simulated triquetrohamate arthrodesis caused significantly more scaphoid flexion and less extension during the wrist radioulnar deviation motion. It also increased the amount of lunate and triquetral extension during wrist flexion-extension. Simulated radiolunate arthrodesis significantly reduced scaphoid flexion during both wrist radioulnar deviation and flexion-extension.

CONCLUSIONS: Both simulated arthrodeses eliminate wrist clunking and may be of value in treating palmar midcarpal instability. However, simulated radiolunate arthrodesis reduces proximal row motion while simulated triquetrohamate arthrodesis alters how the proximal row moves. Long term clinical studies are needed to determine if these changes are detrimental. This study provides valuable information for the clinician to design better treatment strategies for this unsolved condition.