Small groups of thumb muscles produce lateral pinch movement better than the flexor pollicis longus muscle does alone: A simulation study with application to surgical restoration of lateral pinch grasp following tetraplegia

Cole D Smith ¹ Joseph D Towles ^2*

• ¹ Virginia Tech - Wake Forest University, Blacksburg, Virginia, United States
• ² Swarthmore College, Swarthmore, Pennsylvania, United States

Introduction. Tendon transfer surgeries that engage the flexor pollicis longus (FPL) muscle are commonly performed to enable lateral pinch grasp in persons with tetraplegia. Functional outcomes, however, have been mixed. This may be the case, in part, because the 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 joint ab/adduction than the FPL produces during lateral pinch grasp. Methods: We adapted a published, open-source computational musculoskeletal model of the hand to simulate lateral pinch grasp movement. A forward dynamics simulation approach was used to drive the thumb, with 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 joint 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 the 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 joint ab/adduction compared with the FPL. Conclusions: This study points to the potential of novel, multi-insertion site tendon transfer surgeries to outperform the current standard of care to restore lateral pinch grasp following tetraplegia.