Implanted Miniature Engineering Mechanisms in Tendon-Transfer Surgery Improve Robustness of Post-Surgery Hand Function

INTRODUCTION Upper-extremity tendon transfer surgeries have been routinely performed since the 1970s for conditions such as stroke, paralysis, spinal muscle atrophy, nerve or muscle trauma, and congenital disorders. The surgery involves re-routing one or more tendons from an non-functioning muscle and directly suturing it to a functioning donor muscle in order to partially restore hand function [3, 4]. However, a fundamental aspect of tendon-transfer surgery has gone unaddressed. Oftentimes, a single donor muscle is directly sutured to multiple recipient tendons in order to actuate multiple joints. For example, take the case of tendon-transfer surgery for high median-ulnar palsy, a severe condition that disables the flexor digitorum profundus (FDP) muscle bellies and results in an inability to fully close the fingers, leading to weak grasps. In order to restore finger flexion capability, the current surgical procedure is to directly suture the FDP tendons of all four fingers to a functioning donor muscle, such as the extensor carpi radialis longus (ECRL) (see Figs. 1a and 1b). While the direct suture is a simple method of attachment, it results in directly coupling the movement of the distal joints of all four fingers. As a result, the direct suture method prevents the fingers from adapting independently during physical interaction tasks such as grasping an object, fundamentally impeding post-surgery hand function. Specifically, when the hand closes in on an object during the grasping process, if one finger makes contact and stops, all the other fingers will stop before making contact since the motion of all the fingers is coupled (see Fig. 1b). Thus, the direct-suture attachment method results in poor multi-finger power/enveloping grasping ability and may require the patient to use unnatural wrist and arm movements to complete the grasp. This is a significant issue since the ability to perform power grasps is fundamental to the activities of daily living, such as when holding objects to feed oneself [2]. In order to address this fundamental issue in tendontransfer surgery, our group is exploring the use of implanted passive miniature differential mechanisms1 called “adaptive coupling mechanisms” to attach the donor muscle to the recipient tendons (see Figs. 1c, 1d, and 1e). Inspired by the application of these adaptive coupling mechanisms in underactuated robotic hands [1], the key idea is that these adaptive coupling mechanisms, such as a hierarchical pulley system or seesaw mechanism, will enable