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 produce force at the thumb tip to promote a stable grasp. We used simulation to investigate whether a multi-site tendon transfer, targeting sets of paralyzed muscles driven by a single donor muscle, could outperform a single-site 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 endpoint forces in 3 lateral pinch postures. We found that 116 of the 246 muscle groups outperformed the FPL for wide and narrow lateral pinch postures and a posture in between. On average, the 116 muscle groups produced endpoint forces 8{degrees} to 40{degrees} better directed than the FPL, and groups of four muscles statistically significantly outperformed the FPLs force production. Our findings highlight the possibility of using multi-insertion site tendon transfers to restore grasp following cervical spinal cord injury.