Biomechanical data concerning the shoulder impingement syndrome.

This study evaluates forces underneath the coracoacromial vault during elevation of the arm, using a dynamic shoulder model. Muscle forces at the glenohumeral joint were simulated with hydrodynamic cylinders and applied to the deltoid muscle and to the rotator cuff through wire cables in ten anatomic specimens. Computerized regulation initiated precise, time-controlled cycles of glenohumeral joint motion. The position of the arm in all spatial orientations was measured with an ultrasonic device. Forces underneath the coracoacromial vault were recorded with capacitive sensors. The mean force during one cycle of elevation averaged 13.9 N +/- 12.5 N underneath the acromion, 0.43 N +/- 0.51 N underneath the coracoacromial ligament, and 3.44 N +/- 4.37 N underneath the coracoid process. The peak force averaged 37.8 N +/- 33.2 N underneath the acromion, 3.03 N +/- 2.62 N underneath the coracoacromial ligament, and 6.93 N +/- 7.38 N underneath the coracoid process. Forces under the coracoid process exceeded forces under the acromion in some specimens. A marked increase in forces was observed at the final stage of arm elevation and during early reverse-elevation in most specimens. The authors believe that these force values represent the pathomechanics of the shoulder impingement syndrome.