Changes in pennation angle in rotator cuff muscles with torn tendons

Although several authors have reported on the pennation angles of intact rotator cuff muscles, the relationship between their alteration and rotator cuff tears has not been fully clarified. The purpose of this study was to measure the pennation angles of human cadaveric rotator cuff muscles with torn tendons. Twenty embalmed cadaveric shoulders were studied. Ten shoulders with various types of rotator cuff tears (tear group) were compared with ten shoulders that had intact rotator cuff tendons (control group). In seven shoulders with full-thickness tears, the area of the tear was determined by multiplying its length and width. After removing the muscles from the scapula, the superficial muscle fibers of each muscle were removed layer by layer until the entire intramuscular tendon was exposed. Photographs were taken and the pennation angles were then measured on digital images. The correlation between the size of the tear and the pennation angles of the supraspinatus and the infraspinatus muscles were determined statistically. The pennation angles of the supraspinatus and infraspinatus muscles in the tear group were significantly greater than those in the control group (P = 0.027 and 0.007, respectively). In seven shoulders with full-thickness rotator cuff tears, a positive correlation was found between the pennation angle of the supraspinatus muscle and the tear length (r = 0.854, P = 0.014). Moreover, a positive correlation was found between the pennation angle of the infraspinatus muscle and the tear area (r = 0.759, P = 0.048). On the other hand, the pennation angle was not affected by the presence of the partial-thickness tears in the remaining three shoulders. In rotator cuff tears, the pennation angles of the involved rotator cuff muscles increased with increasing size of the tear.