A DYNAMIC SHOULDER MODEL - RELIABILITY TESTING AND MUSCLE FORCE STUDY

This study introduces a dynamic shoulder model, where forces were appl ied to individual muscles in ten cadaveric specimens. The model provid ed reproducible glenohumeral joint motion and thereby allowed the inve stigation of active, glenohumeral joint mechanics. Forces were created by servo-actuated hydrodynamic cylinders and applied to the deltoid m uscle and to the rotator cuff through wire cables. Computerized regula tion initiated precise, time controlled cycles of glenohumeral joint m otion. The position of the glenohumeral joint in all spatial orientati ons was measured and recorded using an ultrasonic sensor device. Repro ducibility 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 d egrees for abduction, 0.75 degrees for anteflexion and 1.36 degrees fo r 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 weig ht 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 1 8% (p < 0.05). Exclusion of the supraspinatus muscle caused a signific ant (p < 0.05) 6% decrease of elevation of the glenohumeral joint. Wit hout force applied to the subscapularis and infraspinatus;teres minor muscles, elevation decreased 16% (p < 0.05). A decrease of glenohumera l joint elevation of 25% resulted when force was applied to the deltoi d muscle alone (p < 0.05).