Can Coordinated Flexor Muscles in the Extended Thumb Produce Well-Directed Grasp Forces? Application to Grasp-Reconstructive Surgery Following Tetraplegia
Associate Professor Swarthmore College Swarthmore, Pennsylvania, United States
Introduction:: The quality of life of up to 135,000 persons around the world, who annually suffer a cervical spinal cord injury [1, 2], is severely affected due to loss of hand function. Tendon and nerve transfer surgeries that engage the flexor pollicis longus muscle (FPL) are commonly performed to enable lateral pinch grasp [3]. Functional outcomes, however, have been mixed. That is, maximum, post-surgical pinch strength has differed by 10-fold among people and has been as low as tenths of a pound [4, 5]. We believe this is the case, in part, because transfer surgeries historically have not been designed with a biomechanics-based understanding of the endpoint forces that muscles generate when the thumb executes a grasping task. We believe that the outcomes of transfer surgeries could be improved by the following: (1) accounting for muscle endpoint forces in different thumb postures [6, 7]; and (2) expanding the number of recipient muscles, that are targeted for reinnervation or actuation by a donor muscle, from one to several. An understudied aspect of grasp is a wide-grip grasp in which the thumb is in an extended posture and thus enables grasp of large objects. Consequently, the goal of this study was to determine whether it was possible for small groups of thumb muscles to each produce an endpoint force directed more in the “pinch” direction than FPL while the thumb is in the extended posture.
Materials and Methods:: Three-dimensional (3D), in-situ/cadaveric muscle endpoint forces, collected in the extended thumb [6], and linear optimization were used to investigate the potential of small groups of muscles to produce endpoint forces that are better directed, for improved pinch strength, than FPL’s endpoint force. Each muscle group represented an alternative tendon or nerve transfer surgery in which a donor muscle or multiple innervations would differentially drive each paralyzed or recipient muscle. Linear optimization was used to predict maximal endpoint force production in the pinch or palmar direction (see palmar direction in Fig. 1) that each muscle group produced when endpoint forces were allowed to deviate by at most 10 N in other directions. As a first step, muscle combinations included only the six thumb flexor muscles [8] from which 15 muscle pairs and 20 muscle triplets were formed. The 3D orientation of the predicted force that each muscle group produced was computed and compared to FPL’s using the 95% confidence-interval (CI) of FPL’s endpoint force direction. If a muscle group produced an endpoint force with a direction less than the lower bound of the 95% CI of FPL’s endpoint force direction (95% CI: 75 to 78 deg; lower bound: 75 deg), then that group’s force was directed more in the pinch direction than FPL at the alpha < 0.05 level.
Results, Conclusions, and Discussions::
Of the 35 muscle groups considered, 34 produced endpoint forces that were more directed in the pinch or palmar direction than FPL. Specifically, 14 of 15 muscle pairs produced an endpoint force that was directed, on average, at 59.4 (standard deviation = 6.2) deg, with respect to the palmar direction. One such muscle pair is illustrated in Fig. 1. For the muscle triplets, all 20 muscle groups produced an endpoint force whose mean direction was 55.2 (3.2) degrees relative to the palmar direction. As a group, the ability of muscle triplets to produce better directed endpoint forces than FPL was greater than that of the muscle pairs (cf. 95% CI for each: 57.0 to 61.9 deg [muscle pairs] and 53.7 to 56.8 deg [muscle triplets]).
This simulation study shows that it is possible for many small groups of thumb muscles to produce more favorably directed endpoint forces during wide-grip grasp than FPL can. Therefore, it is possible for such muscle groups to produce greater pinch forces than FPL. It is unclear if the difference between the performances of muscle pairs and muscle triplets is functionally relevant. In either case, however, the mean endpoint force direction was still quite large when considering the requirements to maintain slip-free contact during a grasping task. Buccholz and colleagues [9] showed that, in the extreme case of an object made of sand-paper, endpoint force would need to be directed within 33 deg (static frictional coefficient = 0.65) of the palmar direction. The mean endpoint force directions found in this study far exceeded the requirement for slip-free grasp even for a rough surface like sandpaper.
While findings in this simulation study are promising in the sense that they show that it is possible to redirect endpoint force production favorably through the involvement of small muscle combinations in grasp-reconstructive surgery, further improvements in endpoint force production may require the actions of other muscles or thumb postural changes [6]. Notwithstanding, this work is a first step toward offering muscle endpoint mechanics-based guidance on the design of grasp-restorative surgeries following tetraplegia.
Acknowledgements (Optional): : The author thanks Fiona O’Donnell (Swarthmore College) for her feedback.
References (Optional): : [1] Lee BB et al (2014), Spinal Cord, 52: 110-116. [2] National Spinal Cord Injury Statistical Center, Facts and Figures at a Glance (2015). Available at https://msktc.org/sites/default/files/lib/docs/Data_Sheets_/MSKTC_SCIMS_Fact_Fig_2015.pdf and accessed May 4, 2023. [3] Fox KI et al (2018), Topic Spinal Cord Inj Rehabil, 24(3): 275-287. [4] Johanson ME et al. (2016), Arch of Phys Med Rehab, 97:S144-53. [5] Waters R et al (1985), J Hand Surg Am, 10A: 385-391. [6] Towles JD (in press), Proceedings of Engineering in Medicine and Biology Conference. [7] Towles JD et al (2008), Clin Biomech, 387-394. [8] Smutz P (1998), J Biomech, 31: 565-570. [9] Buccholz B et al (1987), Ergonom, 31(3): 317-325.