Principles for the evaluation and management of shoulder instability.

During use of the normal shoulder, the humeral head is centered within the glenoid and the coracoacromial arch. When the shoulder cannot maintain this centered position during use, it is unstable. An unstable shoulder prevents normal function of the upper extremity. Shoulder instability is not the same as joint laxity. Joint laxity is a property of normal joints and allows the shoulder to attain its full range of functional positions. The concavity of the glenoid and the coracoacromial arch along with the passive and active forces that press the humeral head into the glenoid and the coracoacromial arch maintain the head in its centered position. This concavity-compression mechanism is dependent on the integrity of the glenoid and the coracoacromial arch, muscular compression, and restraining ligaments of the shoulder. Loss of any of these elements due to developmental, degenerative, traumatic, or iatrogenic factors may compromise the ability of the shoulder to center the humeral head in the glenoid.