Anatomy and pathomechanics of shoulder instability

The mechanisms contributing to glenohumeral stability are complex and varied. The rotator cuff is the dominant contributor to stability through the mid arcs of motion through concavity compression. At the end ranges of motion, the capsular ligamentous system becomes responsible for shoulder stability. As the shoulder position varies from adduction to abduction and internal to external rotation, varying components of the capsular ligamentous system become responsible for static shoulder stability. The nature of the individual ligament contribution to overall static stability has become better known through biomechanical cutting studies of cadaveric shoulders. Further insight into the pathoanatomy of shoulder instability can be gleaned from MR imaging studies that defined which tissues have been injured and which have not. This more detailed understanding of the capsular ligamentous system will eventually result in a more precise nomenclature of defining shoulder instability. The current use of the words anterior shoulder instability is not unlike the use of the term internal derangement of the knee from the late 1970s. In the future, there will be discussions of clinical diagnoses of IGHLC injury or incompetence. The more precisely we know all the details of the mechanisms of shoulder stability, the more precisely we can clinically define the various injuries and syndromes that afflict our patients.