TRANSLATION OF THE GLENOHUMERAL JOINT WITH SIMULATED ACTIVE ELEVATION

Translation of the glenohumeral joint was measured with a dynamic shou lder model during elevation of the arm in 8 cadaveric specimens. Contr olled hydrodynamic actuator forces were applied to the deltoid muscle and the rotator cuff through wire fables. Using a constant force ratio , the glenohumeral joint was elevated to 90 degrees. The position of t he arm in all spatial orientations was measured with an ultrasonic dev ice. Reproducibility of glenohumeral joint motion was demonstrated on the basis of 5 cycles of glenohumeral joint elevation. The rotational center of the humeral head was used as the reference point for transla tion. Translation during elevation of the glenohumeral joint between 2 0 degrees and 90 degrees averaged 9.0 mm +/- 5.2 mm superiorly and 4.4 mm +/- 1.3 mm anteriorly. Physiologic variation of muscle activity, w hich is not yet fully understood, was not included in the model. In vi vo, translation may be diminished by coordinated activity of the rotat or cuff. The presence of significant glenohumeral joint translation un derlines the importance of active, muscular guidance at the shoulder. Physiologic translation must be considered in the design of total shou lder arthroplasty, in shoulder instability, and in the impingement syn drome.