Posterosuperior displacement due to rotator cuff tears.

Purpose: To compare the effects of different-sized anterosuperior and posterosuperior rotator cuff tears (RCTs) and the effects of long head of the biceps tendon (LHB) loading on posterosuperior glenohumeral translation.

Methods: Ten cadaveric shoulders were subjected to posterosuperior loading in the intact state and with sequentially larger anterosuperior and posterosuperior RCTs. Glenohumeral translation was measured with and without LHB loading.

Results: At 50 N of force, cutting the superior half of the infraspinatus led to significantly greater posterosuperior translation than did cutting the superior half of the subscapularis (8.9 mm v 5.9 mm, P = .0179), and cutting the entire infraspinatus led to significantly greater posterosuperior translation compared with cutting the entire subscapularis (12.7 mm v 8.0 mm, P = .0003). Fifty newtons of LHB loading reduced posterosuperior translation in the intact specimen, as well as for all sizes of anterosuperior and posterosuperior RCTs (P < .05). The percentage decrease in posterosuperior glenohumeral translation with LHB loading increased as the size of the RCT increased, varying from 16% to 43%. Comparing these results with those previously published, we found that for anterosuperior RCTs, loading the LHB decreased translation significantly more in the anterosuperior direction than the superior and posterosuperior directions for supraspinatus and 2-tendon tears. For posterosuperior RCTs, there was no difference in the effect of LHB loading between anterosuperior and posterosuperior translation.

Conclusions: The infraspinatus is a major dynamic restraint against posterosuperior glenohumeral translation, especially its inferior half. The LHB is less effective in constraining posterosuperior translation than anterosuperior translation.

Clinical Relevance: Knowledge of the biomechanics of RCTs enhances our ability to treat them.