The effect of percutaneous pin fixation of the interphalangeal joint on the thumb-tip force produced by the flexor pollicis longus: A cadaver study

Interphalangeal joint stabilization often is performed concomitantly with tendon transfers that restore key pinch (lateral pinch) to the paralyzed thumb. The goal of this study was to measure the effect of interphalangeal joint stabilization via percutaneous pin fixation on the thumb-tip force produced by the flexor pollicis longus (FPL). We applied 10 N of force to the tendon of the FPL in 7 cadaveric specimens and measured the resulting thumb-tip force in the intact thumb and after stabilization of the interphalangeal joint. The nominal thumb-tip force was approximately 6 times less than the applied force and was directed primarily in the thumb’s plane of flexion-extension at an oblique angle of 44° relative to the palmar direction (the direction that is perpendicular to the thumb tip in the plane). Joint stabilization increased significantly the nominal force and oriented the force more toward the palmar direction (ie, decreased the obliqueness of the force). After paralysis and a tendon transfer to the paralyzed FPL the FPL is often the only muscle actuating the thumb. We conclude that the oblique nominal force direction is prone to cause the thumb to slip during pinch. Joint stabilization, however, has the capacity to reduce the tendency for slippage because it rotates the force toward the palmar direction.