Anatomical variability naturally leads to multimodal of Denavit-Hartenberg parameters for the human thumb

A realistic biomechanical model of the thumb would enhance our understanding of the functional consequences of orthopedic and neurological diseases, and of treatments. Our work has shown that a kinematic description with five orthogonal and intersecting axes of rotation cannot predict realistic thumbtip forces. An alternative kinematic description proposes five axes of rotation that need not be orthogonal or intersecting. In order to make this description amenable for roboticist-use, we described the model in Denavit-Hartenberg (DH) notation. We explored the effects of reported anatomical variability on the DH parameters using Monte Carlo simulations. We report the DH parameters as statistical distributions that can be used for robotics-based models of the hand and stochastic analyses. We found three characteristic sets of kinematic descriptions. In 65.2% of the 3,140 simulations, the metacarpophalangeal flexion-extension axis was distal to the metacarpophalangeal adduction-abduction axis. We pose the question: Are multiple types of kinematic descriptions necessary to account for the natural anatomical variability of the thumb? This question is important for the biomechanical modeling of the hand, as the debate continues of whether patient-specific models are needed to simulate hand function for clinical applications, or if a single common model suffices.