Computational Analysis of Subscapularis Tears and Pectoralis Major Transfers on Muscular Activity

Muscle transfers are commonly performed to restore joint function after muscle tears. However, there are not many biomechanical studies of muscle transfers, with the available ones often limited to passive movements in anatomical planes. Using data from three activities of daily living (ADL) and an available computational musculoskeletal model of the shoulder, we analyse the impact of a subscapularis tear and its treatment by a pectoralis major (PMA) transfer of the clavicular, sternal, or both segments. The shoulder model is validated against experimental data: the model kinematics with motion capture, muscle activity with EMG measurements, and model joint reaction force with in-vivo data from an instrumented prosthesis. Our results indicate that subscapularis tear requires a compensatory activation of the supraspinatus and is accompanied by a reduced co-contraction of the infraspinatus, both of which can be partially recovered after PMA transfer. Furthermore, although the PMA acts asynchronously to the subscapularis before the transfer, its patterns of activation change significantly after the transfer. Overall, this study demonstrates that muscle transfers have a significant impact on the neuromuscular system of the joint during active motion, beyond mere kinematics. Capability of a transferred muscle segment to activate similarly to intact subscapularis is found to be dependent on a considered motion. Differences in the activation patterns between intact subscapularis and segments of PMA may explain the difficulty of some patients in adapting their psycho-motor patterns during the rehabilitation period. Thereby, rehabilitation programs could benefit from targeted training on specific motion patterns and biofeedback programs. Finally, the condition of the anterior deltoid should be considered to avoid adding limitations to the joint function before a transfer of the clavicular part.