Abduction moment arm of transposed subscapularis tendon

Objective. The purpose of this study was to analyze the effects of the procedure of superior transposition of the subscapularis on the biomechanics of glenohumeral abduction. Design. The abduction moment arms of the subscapularis muscle for the normal attachment and transposed tendon were measured on 10 cadaver shoulders and compared to that for the normal supraspinatus tendon for which it is intended to substitute. Background. Superior transposition of the subscapularis tendon has been recommended for surgical repair of massive tears of the rotator cuff, but the effect of this procedure on shoulder biomechanics has not been reported. Methods. The moment arm about an instantaneous center of rotation was derived, based on the slope of tendon excursion-glenohumeral angle curve. To simulate the insertion of the transposed subscapularis tendon, pseudo-insertion sites were created. Results. Superior transposition of the subscapularis tendon significantly increased its abduction moment arm. The effect was optimal when the simulated insertion site was lateral rather than medial and, to a lesser extent, anterior versus posterior. Conclusions. The results provided a biomechanical rationale for subscapularis tendon transposition in restoring the loss of abduction strength of the shoulder in a massive cuff tear.