Posterosuperior Displacement Due to Rotator Cuff Tears
Su WR, Budoff JE, Luo ZP. Arthroscopy. 2011 Sep 10. [Epub ahead of print]
Massive rotator cuff tears commonly occur and if left untreated lead to further complications within the shoulder joint. Complications range from long head of the biceps degeneration to glenohumeral osteoarthritis. It is thought that these complications are caused by mechanical alterations of the joint, stemming from the lack of dynamic restraint normally provided by the rotator cuff. Previous studies of shoulders with and without tears of the supraspinatus and infraspinatus have only examined the amount of humeral head displacement with loading in an anteriosuperior direction. However, none have examined the amount of displacement when the shoulder is loaded in a posteriosuperior direction. In addition, this has never been studied with a tear of the supraspinatus and the subscapularis, which has been increasingly reported (see clinical pearls). Therefore, Su et al used 10 cadaver shoulders to examine the amount of humeral head displacement following varying degrees of anterosuperior (i.e., supraspintatus + subscapularis) and posterior/superior (i.e., supraspintus + infraspinatus) rotator cuff tears when loaded in a posterosuperior direction. The effect of bicep tendon loading was also examined. The cadaver specimens were mounted and secured in a material testing system. Simulated muscle activity of the intact rotator cuff was performed based on muscle cross-sectional area and previous muscle activity (electromyography) data. The shoulder was loaded in a posterosuperior direction by the material testing system at 10, 20, 30, 40, and 50 Newton of load with the following combinations of tears 1) supraspinatus 2) supraspinatus and superior half of the infraspinatus 3) complete supraspinatus and infraspinatus 4) supraspinatus and superior half of the subscapularis 5) complete supraspinatus and subscapularis. For each condition testing was performed with and without 55N of biceps loading. They found that there was more posterosuperior translation at 50N when the superior half of the infraspinatus was cut compared to the superior half of the subscapularis. The same was found when each tendon was completely cut. The addition of a loaded biceps decreased translation in every condition and the amount of decreased translation increased with larger tears ranging from 16-43%. Translations at the lower loads were found to be minimal.
The results of this study are very interesting and raise our understanding of the mechanical consequences of different combinations of rotator cuff tears. This suggests that the infraspinatus is an important restraint to posterosuperior loading (pushing motions). This is more obvious with a complete rupture, although complete tears of the subscapularis also resulted in large translations. This is in agreement with Burkhart’s suspension bridge hypothesis. It states that there is a band of tissue called the rotator cable that extends from the subscapularis to the infraspinatus and acts like a suspension bridge. He suggests that if this cable is intact then the anterior/posterior force couple of the rotator cuff will be maintained and translations in the anterior/posterior direction will be minimized. However, if it is compromised then the force couple is disturbed and severe translations will occur and alter normal shoulder kinematics. The other interesting finding is the significant role of the biceps to minimize humeral head translation. The role of the biceps at the shoulder joint has not been completely understood. However, this study suggests that it plays a significant role in maintaining dynamic stability especially when the anterior/posterior force couple is disrupted. The increased reliance (overuse) on the biceps to provide joint stability may also be the mechanism of biceps degeneration in rotator cuff tear patients. Do your rotator cuff tear patients commonly present with biceps pain? Do you feel the biceps should still be released in these patients after observing these results? Is there anything we can do to prevent the biceps from becoming injured in these patients?
Written by: Stephen Thomas
Reviewed by: Jeffrey Driban
Su WR, Budoff JE, & Luo ZP (2011). Posterosuperior Displacement Due to Rotator Cuff Tears. Arthroscopy : the journal of arthroscopic & related surgery PMID: 21908156