A dynamic shoulder model: reliability testing and muscle force study.

This study introduces a dynamic shoulder model, where forces were applied to individual muscles in ten cadaveric specimens. The model provided reproducible glenohumeral joint motion and thereby allowed the investigation of active, glenohumeral joint mechanics. Forces were created by servo-actuated hydrodynamic cylinders and applied to the deltoid muscle and to the rotator cuff through wire cables. Computerized regulation initiated precise, time controlled cycles of glenohumeral joint motion. The position of the glenohumeral joint in all spatial orientations was measured and recorded using an ultrasonic sensor device. Reproducibility of glenohumeral joint motion was demonstrated on the basis of five cycles of glenohumeral joint elevation. Repeatability variance of position measurements for five cycles of elevation averaged 0.80 degrees for abduction, 0.75 degrees for anteflexion and 1.36 degrees for internal rotation. Arm weight and force distribution at the shoulder musculature were estimated according to the literature. In comparison to estimated physiologic conditions, a one third increase of arm weight led to a significant (p < 0.05) decrease of elevation of 20%, a one third decrease of arm weight to an average increase of elevation of 18% (p < 0.05). Exclusion of the supraspinatus muscle caused a significant (p < 0.05) 6% decrease of elevation of the glenohumeral joint. Without force applied to the subscapularis and infraspinatus/teres minor muscles, elevation decreased 16% (p < 0.05). A decrease of glenohumeral joint elevation of 25% resulted when force was applied to the deltoid muscle alone (p < 0.05).