Multidirectional Shoulder Instability

Multidirectional instability (MDI) of the shoulder was first described in 1980 as a complex condition of the shoulder defined by instability in 2 or more planes of motion. The shoulder joint is unique in the way it provides a tremendous range of motion. In fact, it has the greatest mobility of any joint in the human body. However, mobility and stability are inversely proportional, and the complex interplay between the stabilizers of the shoulder works with little margin for error before instability occurs. Therefore the balance between the extraordinary physiologic range of motion and shoulder stability has proved to be delicate.The primary responsibility of the shoulder is to position the hand in space. Hence, some activities show preference toward mobility (swimming) while others favor stability (weight lifting, football lineman). Shoulder stability is maintained through both dynamic and static stabilizers. The dynamic structures responsible for joint stability include the rotator cuff muscles, the tendon of the long head of the biceps, and the periscapular musculature. The static stabilizers include the glenohumeral articular congruity, glenoid labrum complex, glenohumeral ligaments as well as the negative pressure created within the joint. Shoulder instability becomes symptomatic once the motion of the humeral head exceeds the boundaries set by the glenoid labrum complex. This is a result of pathology within the static and/or dynamic stabilizers.Shoulder instability was originally thought to fall within one of two categories. The first is Traumatic Unilateral with Bankart lesion treated Surgically (TUBS). The second category is Atraumatic, Multidirectional, Bilateral, which typically responds to Rehabilitation or Inferior capsular shift (AMBRI). Although these two categories offer a simple classification system for MDI, they are oversimplified and do not fully represent the full spectrum of pathologic laxity. As we have discovered more about the MDI process, modifications have been made to the original simplistic classification.