Modelling the Extrinsic Finger Flexors: Tendon Excursions and Moment Arms

Abstract

<p>A musculoskeletal model of the hand is needed to investigate the pathomechanics of tendon-related disorders and carpal tunnel syndrome. The purpose of this thesis was to develop a model with realistic extrinsic finger flexor tendon excursions and moment arms. An existing upper extremity model served as a starting point, which had programmed movement for the index finger. Movement capabilities were added to the middle, ring and little fingers. Metacarpophalangeal linkages were modelled as universal joints to simulate flexion/extension and abduction/adduction. Interphalangeal linkages were modelled as hinge joints to simulate flexion/extension. Extrinsic finger flexor tendon paths were modelled using two different approaches. The first method used control points fixed in the metacarpal and phalangeal coordinate systems to represent the annular and cruciate pulleys. The second method used wrapping surfaces at the metacarpophalangeal and interphalangeal joints to model constant moment arms with finger movement. Extrinsic finger flexor tendon excursions and moment arms in both the control point and joint wrapping models were iteratively adjusted to match the anthropometric regression model developed by Armstrong and Chaffin (1978) for a 50^th percentile male. Musculoskeletal scaling algorithms were also used to further evaluate the control point and joint wrapping models. More specifically, metacarpal and phalangeal segments were adjusted to determine the effects of length and thickness scaling on tendon kinematics. Tendon excursions and moment anns in the joint wrapping model best matched the anthropometric regression model. However, anatomical features of the tendons paths at the finger joints were not preserved in the joint wrapping model as noted by ultrasound imaging. Depending on user needs, both anatomic fidelity and model outcomes should be considered as compromises may be necessary in the modelling process.</p>