THE COMBINED DYNAMIC AND STATIC CONTRIBUTIONS TO SUBACROMIAL IMPINGEMENT - A BIOMECHANICAL ANALYSIS

Ten human cadaveric shoulders were tested with a dynamic shoulder mode l simulating physiologic rotator cuff, deltoid, and biceps muscle forc es. The combined effect of the muscle forces and acromial structure on subacromial impingement was measured with minimally invasive, miniatu re pressure transducers. Shoulders with large acromial spurs had signi ficantly greater impingement pressures at the anterolateral acromion i n neutral, internal, and external rotation compared with those with fl atter acromia. Application of a biceps muscle force reduced anterolate ral acromial pressures by 10%. Failure to simulate a supraspinatus for ce decreased acromial pressure 52% in shoulders with type III acromia in neutral rotation. Without rotator cuff forces applied, the maximum deltoid muscle force required to elevate the arm increased by 17%. Acr omial pressures were increased when no rotator cuff forces were applie d, but the increases were not significant. After an anterior acromiopl asty, pressures decreased by 99% anteriorly. However, failure to achie ve a flat surface posteriorly increased pressures in this location, es pecially with the shoulder in external rotation. Modeling the rotator cuff and deltoid muscle forces demonstrated the importance of the musc ular force couple to center the humeral head during elevation of the a rm. The inferior forces of the infraspinatus, teres minor, and subscap ularis muscles were necessary to neutralize the superior shear force p roduced by the deltoid and supraspinatus muscles.