Passive and active contributions to glenohumeral stability

Fresh-frozen shoulder specimens were used to evaluate restraining forces provided by capsuloligaments and muscles crossing the glenohumeral (GH) joint to better understand various factors contributing to GH stability. The humeral head was translated in the posterior-anterior (Post-Ant), inferior-superior (Inf-Sup), and medial-lateral (Med-Lat) directions, and rotated about the humeral long axis relative to the glenoid from the neutral position, and restraining forces and moments acting on the humeral head by passive and active structures were recorded. Results showed that the GH capsuloligaments generated considerable resistance forces to passive displacements, which varied systematically in amplitude and direction. The restraining force increased more quickly with Post-Ant displacement than with Inf-Sup displacement. Furthermore, GH stiffness was higher in the anterior direction than in the posterior direction and stiffness in the superior direction was higher than that in the inferior direction. Loading muscles across the GH-joint made the joint considerably more stable and humeral axial rotation laxity was reduced markedly when the muscles were loaded moderately at ∼2% maximal muscle force. Contributions of the rotator cuff were found to be especially important in preventing excessive inferior humeral translations.