Subscapularis tendon loading during activities of daily living.

BACKGROUND: The purpose of this study was to determine the relative amount of load that is transmitted through the superior portion of the subscapularis during activities of daily living as compared with the load that is transmitted through the middle and inferior portions in a normal shoulder and in a shoulder with a supraspinatus tear.

METHODS: By use of the Newcastle shoulder model, the subscapularis was modeled with 3 lines of action encircling the humeral head. The load was measured in the entire subscapularis, and the percentage of this load in each of the 3 tendinous bands was calculated. Subsequently, a supraspinatus tear was simulated, and the forces generated by the subscapularis and glenohumeral joint contact forces were measured.

RESULTS: The maximum force produced by the entire subscapularis muscle for the various activities ranged from 3 to 43 N. Load sharing between the 3 subscapularis bands showed that the superior band bore the largest percentage of the total load of the muscle (95% ± 2%). The load in the subscapularis, particularly in the superior band, increased significantly when a supraspinatus tear was simulated (P < .0001).

CONCLUSION: The superior band of the subscapularis tendon bears the highest percentage of load compared with the middle or inferior band. The load in the subscapularis increased significantly in the presence of a simulated supraspinatus tear. Because a disproportionate amount of force is transmitted through the superior subscapularis, more clinical research is warranted to determine whether tears in this region should be routinely repaired.