Multiple, not single, recipient muscle tendon transfers produce well-directed thumb-tip forces in lateral pinch grasp: A simulation study with application to restoration of improved grasp after tetraplegia

Thumb tendon transfer surgical procedures in patients with cervical spinal injury engage the paralyzed flexor pollicis longus (FPL) muscle to enable lateral pinch grasp. However, functional outcomes are mixed, in part because the FPL cannot consistently produce force at the thumb-tip to promote a stable grasp. We used simulation to investigate whether a multiple recipient muscle tendon transfer, targeting sets of paralyzed muscles driven by a single donor muscle, could outperform a single recipient muscle tendon transfer with the FPL alone and restore lateral pinch. We formed 36 groups of 2 muscles, 84 groups of 3 muscles, and 126 groups of 4 muscles. We used nonlinear optimization and in-situ measurements of muscle endpoint forces in 3 lateral pinch postures. In each posture, we primarily compared the orientation of the endpoint force of each muscle group to that produced by the FPL alone. We found that 116 of the 246 muscle groups produced endpoint forces that were more closely aligned with the direction perpendicular to the thumb (palmar direction) than the FPL was for wide and narrow lateral pinch postures and a posture in between. When averaged across the three postures, muscle force ranged from 0.1 to 90.4 N, muscle group endpoint force magnitudes ranged from 2.5 to 14.9 N, and muscle group endpoint force directions ranged from 17° to 49° relative to the palmar direction. The muscle group endpoint force directional range represented a mean improvement of 8° to 40° over the FPL's mean endpoint force direction across all three postures. Our findings highlight the possibility of using multiple recipient muscle tendon transfers to restore grasp following cervical spinal cord injury.