Mechanical environment associated with rotator cuff tears.

A simplified 2-dimensional finite element model was used to investigate the stress environment in the supraspinatus tendon. The extrafibrillar matrix and collagen fiber were modeled with fiber-reinforced composite elements. The stress was evaluated at humeroscapular elevation angles of 0 degree, 30 degrees, and 60 degrees. Two acromion conditions were simulated. In the first set of conditions there was no subacromial impingement. In the second set there was subacromial impingement of the bursal side. Impingement was simulated by producing a 1-mm indentation on the bursal surface, an indentation similar to the type of impingement associated with deltoid contraction. The results demonstrated that subacromial impingement generates high stress concentrations in and around the critical zone. Such high stress could initiate a tear; tears that result from stress point to an extrinsic mechanism. However, we found that high stress and potential tears caused by impingement may occur on the bursal side, the articular side, or within the tendon. This result is unaccounted for by traditional mechanical models in which only bursal-sided partial tears are initiated by subacromial impingement.