Automated Fitting of an Elastokinematic Surrogate Mechanism for Forearm Motion from MRI Measurements

Abstract

Forearm rotation (pro-supination) involves a non-trivial combination of rotation and translation of two bones, namely, radius and ulna, relatively to each other. Early works regarded this relative motion as a rotation about a fixed (skew) axis. However, this assumption turns out not to be exact. This paper regards a spatial-loop surrogate mechanism involving two degrees of freedom with an elastic coupling for better forearm motion prediction. The model parameters are not measured directly from the anatomical components, but are fitted by reducing the errors between predicted and measured values in an optimization loop. For non-invasive measurement of bone position, magnetic resonance imaging (MRI) imaging is employed. We present a method to self-calibrate the arm position in the MRI scanning tube and fitting the model parameters from a few, coarse MRI scans. Results show a good concordance between measurement and simulation.