Toward an understanding of the small groups of thumb muscles that produce lateral pinch movement: application to surgical restoration of grasp following neurologic impairment

PURPOSE. Tendon transfer surgeries that engage the flexor pollicis longus muscle (FPL) are commonly performed to enable lateral pinch grasp in persons with tetraplegia. Functional outcomes, however, have been mixed. This might be the case, in part, because FPL produces hyperflexion at the interphalangeal (IP) joint and radial deviation at the carpometacarpal (CMC) joint. Therefore, the goal of this simulation study was to investigate whether small groups of muscles could produce movement with less IP joint hyperflexion and CMC ab/adduction than FPL produces during lateral pinch movement. METHODS. We adapted a published, open-source computational musculoskeletal model of the hand to implement lateral pinch grasp. A forward-dynamics simulation approach was used to drive the thumb for 27 muscle groups being considered from an extended posture to a flexed posture to make contact with the side of the index finger. We calculated CMC ab/adduction deviation from the flexion-extension plane and IP joint flexion in the plane that all muscle groups produced, and compared those joint angle movements to those of FPL when it alone drove the thumb. RESULTS. Of the 27 simulations, three muscle groups, each consisting of three or four muscles, generated lower IP joint flexion and CMC ab/adduction than those of FPL. CONCLUSIONS. This simulation work points to the potential of novel, multi-insertion site tendon transfer surgeries to out-perform the current standard of care to restore lateral pinch grasp following tetraplegia.