Contribution of Flexor Pollicis Longus to Pinch Strength: An In Vivo Study

Purpose To estimate the contribution of the flexor pollicis longus (FPL) to key pinch strength. Secondary outcomes include tip pinch, 3-point chuck pinch, and grip strength. Methods Eleven healthy volunteers consented to participate in the study. We recorded baseline measures for key, 3-point chuck, and tip pinch and for grip strength. In order to control for instability of the interphalangeal (IP) joint after FPL paralysis, pinch measurements were repeated after immobilizing the thumb IP joint. Measures were repeated after subjects underwent electromyography-guided lidocaine blockade of the FPL muscle. Nerve conduction studies and clinical examinations were used to confirm FPL blockade and to rule out median nerve blockade. Paired t-tests were used to compare pre- and postblock means for both unsplinted and splinted measures. The difference in means was used to estimate the contribution of FPL to pinch strength. Results All 3 types of pinch strength showed a significant decrease between pre- and postblock measurements. The relative contribution of FPL for each pinch type was 56%, 44%, and 43% for key, chuck, and tip pinch, respectively. Mean grip strength did not decrease significantly. Splinting of the IP joint had no significant effect on pinch measurements. Conclusions FPL paralysis resulted in a statistically significant decrease in pinch strength. IP joint immobilization to simulate IP joint fusion did not affect results.