Calculation of flexor pollicis longus moment arm for wrist motion in a cadaver model validates the tenodesis effect for therapy

Introduction: Synergies of fingers and wrist motion have been incorporated into therapies for finger flexor tendon injuries to improve repair outcomes. Similar synergistic therapy strategies have not been well documented for the thumb. Purpose of the Study: The purpose of this study was to investigate the extent to which wrist motion enables a synergistic effect at the thumb in a cadaveric model by measuring flexor pollicis longus excursion and calculating the moment arm of this tendon at the wrist joint. Study Design: This is a basic science research. Methods: Eight fresh-frozen cadaveric arms were obtained from our anatomical bequest program. The proximal arm was fixed in neutral pronation/supination position, and motion of the wrist was guided through either flexion/extension or radial/ulnar deviation. Fingers were fixed in extension, thumb interphalangeal and metacarpophalangeal joints were fixed in neutral extension, and the carpometacarpal joint was fixed at 30° palmar abduction. The flexor pollicis longus tendon was exposed proximal to the wrist crease and connected to a rotary potentiometer to measure tendon excursion. Optical markers were attached to the hand to capture kinematics. Wrists were moved from a neutral position over the range of flexion and extension and then from the neutral position through the range of radial to ulnar deviation. Moment arms were calculated. Results: Moment arm calculation indicated that the flexor pollicis longus acts as a wrist flexor over the entire motion range and as a weak radial deviator at ulnarly-deviated positions. Conclusions: This study provides a mechanistic rationale for passive interphalangeal joint motion in varying wrist positions when treating thumb flexor tendon injuries, with benefits seen primarily for wrist extension.