[Etiology of sub-acromial impingement syndrome--a biomechanical study].

Eight fresh shoulder specimen were dissected and the functional relationship of the rotator cuff tendons to the acromion was demonstrated through various arcs of shoulder motion. The scapula was fixed and the arm was elevated, rotated, and abducted by pulling the muscle tendon according to EMG activities. Using FUJI compression films the subacromial compression of the rotator cuff was measured. Different forms of acromion were tested. While moving the shoulder with normal relationship of deltoid to rotator cuff, there is no -or at least less than 0.1 mPa compression of the rotator cuff in the subacromial space. Weakness of the rotator cuff results in a moderate compression of 0.6 mPa. In type III acromion there was significant increase up to 1.0 mPa between the acromion and the rotator cuff. This study suggests, that a mechanical subacromial compression only occurs in shoulder joints with a Bigliani acromion type III and/or in joints with a imbalance of the deltoid/rotator cuff relation. Thus an acromioplasty should be primarily undertaken only if the pathology affects the acromion. If there is a muscle imbalance, we recommend a special rehabilitation program in order to strengthen the cuff-muscles the first place.