[Biomechanic results in impingement syndrome of the shoulder]

Forces underneath the coraco-acromial vault during elevation of the arm were evaluated with a dynamic shoulder model. The deltoid muscle and the rotator cuff muscles were simulated with a hydrodynamic device, in ten autopsy specimens. Controlled cycles of glenohumeral joint motion were initiated with computerised regulation. An ultrasonic device measured the position of the arm in all spatial orientations. Capacitive sensors recorded forces underneath the coraco-acromial vault. The mean force during one cycle of elevation averaged 13.9 Newton +/- 12.5 Newton underneath the coraco-acromial ligament and 3.44 Newton +/- 4.37 Newton underneath the coracoid process. The peak force averaged 37.8 Newton +/- 33.2 Newton underneath the acromion, 3.03 Newton +/- 2.62 Newton underneath the coraco-acromial ligament and 6.93 Newton +/- 7.38 Newton underneath the coracoid process. The force markedly increased at the final stage of arm elevation and during early reverse-elevation in most specimens, corresponding to the painful arc sign. In some specimens, the force under the coracoid process exceeded the force under the acromion. Osteophytes protruding into the subacromial space may lead to a concentration of force and to high regional pressures.