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.
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