Biomechanical Modeling of Brachialis-to-Wrist Extensor Muscle Transfer Function for Daily Activities in Tetraplegia

We recently reported a novel case demonstrating the feasibility of a brachialis (BRA)-to-extensor carpi radialis brevis (ECRB) tendon transfer, but it is not yet known whether this transfer provides robust functional results across activities. The purpose of this study was to use biomechanical modeling to define the functional capacity of the BRA-to-ECRB tendon transfer in terms of enabling the performance of several activities of daily living.

A model of the transferred BRA-ECRB muscle-tendon unit was developed to calculate isometric elbow and wrist joint torque as a function of elbow and wrist angles resulting from different BRA reattachment locations from 50 to 80 mm proximal to the wrist joint crease. Using this model, mathematical optimization predicted the optimal location for BRA reattachment in order to perform each of a number of important upper extremity tasks as well as to calculate a global optimum for performing all of the tasks.

Analysis of active joint torque showed that the entire elbow torque-angle curve surface shifted "diagonally" toward elbow flexion and wrist extension as the attachment location approached the wrist joint; peak wrist torque was produced at extended wrist angles. Our model predicted that the optimal attachment location for each different task ranged from 54.3 to 74.6 mm proximal to the wrist joint, which is feasible given the anatomy of the muscle-tendon unit. The attachment location to optimize performing all tasks was calculated as 63.5 mm proximal to the wrist joint.

This study clearly demonstrates that the BRA, which is underused as a donor in tetraplegia surgery, is an excellent donor muscle to provide wrist extension. Biomechanical simulation further highlighted the need to consider not only donor-muscle appropriateness but the patient's desired function when planning surgical tendon transfers.

Quantitative evaluation of the way that surgery affects daily tasks rather than simply matching muscle properties may be a more appropriate approach for surgeons to use when choosing and tensioning donor muscles.