Stability and instability of the glenohumeral joint: the role of shoulder muscles.

Shoulder muscles contribute to both mobility and stability of the glenohumeral joint. To improve treatments for shoulder instability, we focused on the contribution of the shoulder muscles to glenohumeral joint stability in clinically relevant positions. Both computational and experimental models were used. A computational model of the glenohumeral joint quantified stability provided by active muscle forces in both mid-range and end-range glenohumeral joint positions. Compared with mid-range positions, the resultant joint force at end-range positions was more anteriorly directed, indicating that its contribution to glenohumeral joint stability was diminished. In end-range positions, simulated increases in rotator cuff muscle forces tended to improve stability whereas increases in deltoid or pectoralis major muscle forces tended to further decrease stability. To validate these results, a cadaveric model, simulating relevant shoulder muscles, was used to quantify glenohumeral joint stability. When infraspinatus muscle activity was decreased, compressive forces decreased. When pectoralis major muscle activity was increased, anteriorly directed forces increased. If anteriorly directed forces increase or compressive forces decrease, stability of the glenohumeral joint decreases. This cadaveric model was then used to evaluate the effect of placing the joint in the apprehension position of abduction, external rotation, and horizontal abduction. Consistent with the results of our computational model, apprehension positioning increased anteriorly directed forces. Knowledge gained from these models was then used to develop a cadaveric model of glenohumeral joint dislocation. Dislocation resulted from the mechanism of forcible apprehension positioning when the appropriate shoulder muscles were simulated and a passive pectoralis major muscle was included. Capsulolabral lesions resulted that were similar to those observed in vivo. Shoulder muscle forces are usually powerful stabilizers of the glenohumeral joint, especially in mid-range positions when the passive stabilizers are lax. However, muscle forces can contribute to instability as well. Certain muscle forces decrease glenohumeral joint stability in end-range positions. We found this to be the case with both active and passive pectoralis major forces. Improved understanding of the contribution of muscle forces not only toward stability but also toward instability will improve rehabilitation protocols for the shoulder and prove useful in the treatment of joint instability throughout the body.