Multidirectional Shoulder Instability

Multidirectional instability (MDI) of the glenohumeral joint is a complex clinical entity characterized by symptomatic, involuntary subluxation or dislocation in 2 or more directions, typically including an inferior component. The condition, first described by Neer and Foster in 1980, highlights the delicate balance between mobility and stability in the human body's most mobile joint; a sophisticated interplay of static and dynamic stabilizers maintains this balance. The static stabilizers include the bony architecture of the glenoid and humeral head, the glenoid labrum, which deepens the socket, the capsuloligamentous complex, and the negative intra-articular pressure. The dynamic stabilizers consist of the muscles surrounding the shoulder joint, including the rotator cuff, the long head of the biceps tendon, and the periscapular musculature, which maintain stability through active concavity-compression and coordinated neuromuscular control. MDI represents a failure of this integrated system, wherein the humeral head translates beyond the physiologic boundaries of the glenoid, resulting in pain, apprehension, and functional disability. Historically, shoulder instability was conceptualized through a dichotomous classification system called TUBS (traumatic, unilateral, Bankart, surgical) and AMBRI (atraumatic, multidirectional, bilateral, rehabilitation, inferior capsular shift). While valuable for introducing the concept of atraumatic instability, this framework is now considered obsolete as it fails to account for the significant clinical heterogeneity and the spectrum of pathology observed in patients with MDI. A substantial portion of patients with MDI have a history of trauma, and many with traumatic instability have underlying ligamentous laxity, blurring the lines of this rigid classification. A modern approach, reflecting a more advanced understanding of the condition, emphasizes the underlying etiological factors. A recent systematic review proposed a more clinically relevant "AB classification," which categorizes patients based on the presence or absence of (A) significant trauma and (B) generalized hyperlaxity. This framework acknowledges that MDI is not a single entity but a heterogeneous condition. This moves the clinical focus from a rigid algorithm to a patient-specific diagnosis, which better guides treatment decisions and helps standardize future research. This shift in thinking, from a simple dichotomy to a multifactorial spectrum, is fundamental to the contemporary management of MDI.