Quantitative assessment of the midcarpal shift test.

The subjective, clinical midcarpal shift test was compared with a quantitative measurement of carpal volar/dorsal translation versus ulnar deviation using a mechanical testing system. Testing was performed on 19 healthy volunteers (mean age, 33 years) and 3 patients (four wrists; mean age, 23 years) who had been diagnosed with ulnar midcarpal instability, a nondissociative form of carpal instability. During physical examination, each subject's wrist was graded I to V using the previously described classification of the degree of laxity and clunk observed with the midcarpal shift test. Each subject was also evaluated using a quantitative mechanical testing system that simulates the subjective clinical test. The testing system measures displacement of the distal carpal row, more specifically, the capitate, as the wrist is moved from neutral to ulnar deviation under a constant axial load of 44 N directed volarly at the head of the capitate. Reflective markers were attached to the skin above the proximal and distal ends of the third metacarpal and at the point where the 44-N load was applied to the carpus. Motion of the markers was used to calculate ulnar deviation and dorsal/volar translation of the carpus. The maximum slope of the carpal translation versus ulnar deviation curve was measured for each subject and compared with the results of the clinical midcarpal shift test. Higher maximum slopes were seen in subjects with the higher grades of carpal laxity. There were also differences with regard to the point at which the clunk occurred; the higher the clinical grade of laxity, the greater the ulnar deviation of the wrist at the point at which the clunk was observed. These differences were not significant, however. These data confirm the validity of the clinical test and establish its usefulness as a diagnostic indicator of midcarpal nondissociative carpal instability. The mechanized test also may be useful as a biomechanical marker, enabling the results of ligament sectioning to be effectively compared with defined clinical laxity.