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Objectives: The primary goals of surgical restoration of grasping are often to restore the thumb's ability to move and produce forces (i.e., thumb function). However, the anatomic and kinematic complexity of the thumb precludes knowing the quantitative improvements of thumb function a particular surgical modification will produce. The objective of this study is to design a musculoskeletal model of the human thumb that is capable of predicting functional consequences of any reconstructive surgery. Preliminary results of such a design are presented here.
Methods: The model includes the kinematics of the joints and a network of thumb muscles and tendons. Despite the published discrepancies over the number of degrees of freedom and the geometry of the rotational axes at each joint, most agree that there are two distinct motions about intersecting perpendicular axes at each of the two most proximal joints (carpo-metacarpal and metacarpo-phalangeal joints) and one at the last joint (interphalangeal joint). As a first approximation, the model contains these kinematic degrees of freedom. The model predicts the muscle coordination patterns that produce maximal thumb-tip forces based on the complex mechanical interactions among the kinematic, anatomic and physiologic elements of the thumb and numerical optimization techniques.
Results: Experimental coordination patterns and maximal forces are used to validate model predictions. Preliminary results indicate that a purely mechanical model can predict the salient features of muscle coordination for the thumb in several functional postures - particularly, key (thumb pad in contact with the medial side of the index finger's mid-phalanx) - producing maximal thumb-tip forces in several directions - particularly, palmar (force emanating from mid-distal phalanx of thumb used in key pinch). When producing palmar force, most muscles are activated at medium to high levels (extensor pollicis brevis and abductor pollicis long us, at low to silent levels).
Conclusions: Initial findings show that the coordination of thumb musculature can be explained by the principles of mechanics alone. When the model has been validated fully (i.e., for the other functional postures and directions), it may be used in a variety of ways; two of which include: elucidating control strategies in the thumb and acting as an "in-vitro" test-bed for designing new and modifying existing reconstructive surgical techniques for improving grasping ability for those with peripheral nerve injuries and the spinal-chord injured, like the veteran population with quadriplegia.
Acknowledgments: Rehabilitation R&D Service of the Department of Veterans Affairs, VA Merit Review B898-3RA