The thumb is required for a majority of tasks of daily living. Biomechanical modeling is a valuable tool, with the potential to help us bridge the gap between our understanding of the mechanical actions of individual thumb muscles, derived from anatomical cadaveric experiments, and our understanding of how force is produced by the coordination of all of the thumb muscles, derived from studies involving human subjects. However, current biomechanical models do not replicate muscle force production at the thumb-tip. We hypothesized that accurate representations of the axes of rotation of the thumb joints were necessary to simulate the magnitude of endpoint forces produced by human subjects. We augmented a musculoskeletal model with axes of rotation derived from experimental measurements (Holzbaur et al., 2005) by defining muscle–tendon paths and maximum isometric force-generating capacity for the five intrinsic muscles. We then evaluated if this augmented model replicated a broad range of experimental data from the literature and identified which parameters most influenced model performance. The simulated endpoint forces generated by the combined action of all thumb muscles in our model yielded comparable forces in magnitude to those produced by nonimpaired subjects. A series of 8 sets of Monte Carlo simulations demonstrated that the difference in the axes of rotation of the thumb joints between studies best explains the improved performance of our model relative to previous work. In addition, we demonstrate that the endpoint forces produced by individual muscles cannot be replicated with existing experimental data describing muscle moment arms.
The wrist is essential for hand function. Yet, due to the complexity of
the wrist and hand, studies often examine their biomechanical features in
isolation. This approach is insufficient for understanding links between
orthopaedic surgery at the wrist and concomitant functional impairments at the
hand. We hypothesize that clinical reports of reduced force production by the
hand following wrist surgeries can be explained by the surgically-induced,
biomechanical changes to the system, even when those changes are isolated to the
wrist. This study develops dynamic simulations of lateral pinch force following
two common surgeries for wrist osteoarthritis: scaphoid-excision four-corner
fusion (SE4CF) and proximal row carpectomy (PRC). Simulations of lateral pinch
force production in the nonimpaired, SE4CF, and PRC conditions were developed by
adapting published models of the nonimpaired wrist and thumb. Our simulations
and biomechanical analyses demonstrate how the increased torque-generating
requirements at the wrist imposed by the orthopaedic surgeries influence force
production to such an extent that changes in motor control strategy are required
to generate well-directed thumb-tip end-point forces. The novel implications of
our work include identifying the need for surgeries that optimize the
configuration of wrist axes of rotation, rehabilitation strategies that improve
post-operative wrist strength, and scientific evaluation of motor control
strategies following surgery. Our simulations of SE4CF and PRC replicate
surgically-imposed decreases in pinch strength, and also identify the
wrist's torque-generating capacity and the adaptability of muscle
coordination patterns as key research areas to improve post-operative hand
function.
Current biomechanical models of the thumb do not replicate either the maximum pinch forces produced via coordinated muscle actions by human subjects [1] or the magnitudes of the forces produced by individual muscles as quantified in cadaveric specimens [2]. However, compared to the literature, these models either underestimate the forces produced by human subjects [1] or overestimate the magnitudes quantified in cadaveric specimens [3, 4]. Specifically, the model developed by Valero-Cuevas et al (2003) was reported to be four times weaker than the endpoint forces produced during maximum effort by human subjects experimentally [1]. In contrast, Towles et al., (2008) reported simulated endpoint forces substantially greater than the forces produced by individual muscles in cadaveric experiments for six of nine muscles studied. [3]. Similarly, Goehler and Murray (2010) simulated the endpoint forces produced by individual extrinsic muscles and reported results that were approximately 60% larger than the forces measured in cadavers [4].
Objective: The purpose of this work was to develop an open-source musculoskeletal model of the hand and wrist and to evaluate its performance during simulations of functional tasks. Methods: The musculoskeletal model was developed by adapting and expanding upon existing musculoskeletal models. An optimal control theory framework that combines forward-dynamics simulations with a simulated-annealing optimization was used to simulate maximum grip and pinch force. Active and passive hand opening were simulated to evaluate coordinated kinematic hand movements. Results: The model's maximum grip force production matched experimental measures of grip force, force distribution amongst the digits, and displayed sensitivity to wrist flexion. Simulated lateral pinch strength fell within variability of in vivo palmar pinch strength data. Additionally, predicted activation for 7 of 8 muscles fell within variability of EMG data during palmar pinch. The active and passive hand opening simulations predicted reasonable activations and demonstrated passive motion mimicking tenodesis, respectively. Conclusion: This work advances simulation capabilities of hand and wrist models and provides a foundation for future work to build upon. Significance: This is the first open-source musculoskeletal model of the hand and wrist to be implemented during both functional kinetic and kinematic tasks. We provide a novel simulation framework to predict maximal grip and pinch force which can be used to evaluate how potential surgical and rehabilitation interventions influence these functional outcomes while requiring minimal experimental data.
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