Changes of pennation angle in rotator cuff muscles with torn tendons

Objective: To determine the changes of pennation angle that occurred after rotator cuff tears, and find evidence for the functional deterioration of the involved rotator cuff muscles. Methods: Twenty embalmed cadaveric shoulders were used for the current study. Ten shoulders with various types of rotator cuff tears (tear group) were compared with other 10 shoulders having intact rotator cuff tendons (control group). The length and width of the tear were measured and the tear area was calculated by multiplying them. After removing the muscles from the scapula, the muscle fibers were removed slice by slice in the plane parallel to the surface of the muscle till the slice which contained the intramuscular tendon was reached. The photographs of this slice were taken and the digital images were analyzed using a computer software to get the pennation angles for each muscle. Then the correlation between the size of the tear and the pennation angles of the supraspinatus muscle and the infraspinatus muscle were analyzed statistically. Results: The pennation angles of the supraspinatus and infraspinatus muscles in tear group were significantly greater than those in control group (P<0.05). In tear group, a positive correlation was observed between the pennation angle of the supraspinatus muscle and the tear length (r=0.854, P<0.05). A positive correlation was also observed in tear group between the pennation angle of the infraspinatus muscle and the tear area (r=0.759, P<0.05). Conclusion: The pennation angles of the rotator cuff muscles increase with the increasing size of rotator cuff tears. It means that the larger the tear size, the more the function loss.