Poster P19 - Can small groups of muscles produce lateral pinch movement patterns important for reconstruction of grasp in the tetraplegic hand?

Introduction: The quality of life of up to 135,000 persons worldwide, who have sustained a cervical spinal cord injury, has been severely affected by the loss of hand function. Hand function is often restored through tendon transfer surgery. Specifically, tendon transfer surgeries engage the flexor pollicis longus (FPL) muscle, a thumb flexor, to enable lateral pinch grasp. The FPL is the most convenient muscle to use for this procedure as it is the only thumb muscle that causes flexion at all three joints and displays the least amount of radial-ulnar deviation, however functional outcomes of the surgery have been mixed. Maximum, post-surgical pinch forces have differed by up to 10-fold among those who have received the surgery. These varied outcomes may be due to FPL’s tendency to cause hyperflexion of the IP joint and its tendency to produce some radial-ulnar deviation of the thumb during thumb flexion. Both characteristics can adversely affect FPL’s ability to produce well-directed thumb-tip force and therefore affect FPL’s ability to promote stable or slip-free pinch contact. We believe that surgical outcomes can be improved with a biomechanics-based understanding of the endpoint trajectories that muscles generate during grasping tasks. We further believe that small groups of muscles have the potential to create more desirable movement patterns throughout the flexion-extension plane during lateral pinch than FPL alone and thus improve functional outcomes following tendon transfer surgeries. The goal of this study was then to identify muscle combinations that produce better movement patterns than FPL in a musculoskeletal model.

Materials and

Methods: In our previous study, we showed that four muscle pairs, ten muscle triads (groups of 3 muscles), and 13 muscle tetrads (groups of 4 muscles) produced well-directed endpoint forces during lateral pinch while in the flexed position. In this current study, we then looked to determine which of those muscle combinations could produce endpoint trajectories that moved the thumb from an extended posture to a flexed one primarily in the flexion-extension plane of lateral pinch. A previously developed musculoskeletal model of the hand and wrist in OpenSim was adapted for this purpose. In the model, the thumb was positioned in a fully extended and neutrally abducted posture at the start. Twenty-eight simulations of thumb flexion were performed, one for each of the 27 muscle combinations being tested and one for the FPL. For each simulation, only the muscles of interest were maximally and simultaneously activated to contract and each simulation was run for over 150 ms. Data from simulation trials were truncated to end at the time of first contact between the thumb and index finger, and max IP joint flexion and total CMC joint ab/adduction were quantified. Simulation results were ranked to determine which muscle groups minimized IP joint flexion, CMC range of motion, and both characteristics. Muscle combinations that outperformed FPL in both metrics were reported as combinations that could produce more favorable thumb-tip movement than FPL throughout the plane of lateral pinch movement.

Results, Conclusions, and Discussions: Results and

Discussion: All 28 simulations achieved successful lateral pinch with 20 of 27 (74%) muscle combinations producing less IP joint flexion and four of 27 (15%) muscle combinations displaying decreased CMC ab/adduction joint range of motion as compared to FPL alone. Three muscle groups--one muscle triad and two muscle tetrads--produced more favorable thumb-tip movement both in terms of IP joint flexion and CMC ab/adduction than FPL alone (Figs. 1, 2). Reduced hyperflexion of the IP joint reduces the risk of callus formation on the thumb-tip, creates greater thumb surface contact during grasping, and thus improves the quality of grasp. Reduced CMC ab/adduction movement or out-of-plane movement has a greater potential for ideal contact between the thumb and the object being grasped than otherwise. By ideal contact, we mean that the thumb lands on the central portion of the grasped object and thus improves the quality of grasp.

Conclusion: The primary goal of this work was to use a musculoskeletal model to identify small groups of muscles that produce improved endpoint trajectories in comparison to FPL alone as potential targets to reimagine lateral pinch reconstruction surgery. This study identified three possible muscle groups of 27 that minimize both IP joint flexion and out-of-plane movement compared to FPL. As far as we know, this simulation study is the first to apply an endpoint trajectory-based biomechanics approach to the design of tendon transfers aimed at restoring lateral pinch grasp following cervical spinal cord injury throughout the flexion-extension plane and highlights the possibility of employing “multi-insertion site tendon transfers.” Investigating the surgical feasibility of engaging multiple recipient paralyzed muscles, including intrinsic muscles, in a tendon transfer surgery and determining how to implement the appropriate coordination of donor muscle force across recipient muscles are next steps to consider.

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