Subterminal key pinch dynamometry: a new method to quantify strength deficit in ulnar nerve paralysis

Abstract

Precise pre- and postoperative assessments are fundamental to recording the quality of recovery after ulnar nerve repair. Because of its imprecision, manual muscle testing is being replaced by dynamometry to measure grasping and key-pinch strengths. However, both grasping and key pinch are dependent not only on the ulnar nerve but also the median and radial nerves. We propose to measure strength using a new sort of pinch, called the ‘subterminal key pinch’. Strength was measured using a commercially available pinch meter. Patients applied pressure on the dynamometer with the interphalangeal joint of the thumb, maintaining the joint in extension to avoid enhancement of strength by the flexor pollicis longus. We examined 17 patients before ulnar nerve repair. Preoperatively, grasping strength was 46% of normal, while key pinch was 58%, pinch-to-zoom strength was 26% and subterminal key pinch only 7%. Subterminal key pinch was the most affected pinch with a strength deficit of over 90%. Level of evidence: IV