Background. Subacromial impingement is a widely recognized mechanism of chronic shoulder pain. The magnitudes of the compressive forces that impinge the subacromial structures were often measured from cadaveric specimens, but it is questionable to use this data as a sole basis to determine the shoulder motions and/or shoulder configurations that induce impingement in live subjects performing active motion. The purpose of the present study was to determine in vivo the magnitude of the compressive force at selected shoulder configurations. Methods. The subacromial structures may be impinged by the downward-directed forces exerted by the coraco-acromial ligament. The reactions of these forces push the ligament upwards and deform it into a curved shape. A single resultant of these reaction forces was determined with an inverse approach to quantify the magnitude of the impingement force. An ultrasound unit was used to visualize the deformed shape of the coraco-acromial ligament for thirteen subjects with no symptomatic shoulder problem actively holding their shoulders in five configurations. Findings. The impingement force in 90° abduction + maximum internal rotation (mean = 21.3 N) and that in the Hawkins test position (mean = 18.3 N) were significantly greater than those in 90° abduction + neutral and external rotation (means ≤3 N). Interpretation. For young asymptomatic shoulders, the motions that induce impingement are not any arm abduction, but the arm abduction with a large internal rotation. Further study is indicated to examine the impingement force among various age groups.
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