My Orthopaedic Surgeon

Multidirectional instability (MDI) is a relatively common, generally bilateral, atraumatic condition affecting shoulder function. MDI is caused by generalized capsular laxity—that is, insufficiency of the static ligament constraints. There is excessive mobility of the glenohumeral joint in all directions, although there may be a predominance of 1 direction, typically anteroinferior or posteroinferior.
Synonyms and related keywords: multidirectional shoulder instability, multidirectional shoulder laxity, atraumatic shoulder instability, MDI, shoulder patholaxity, AMBRII
Author: Daniel C Wnorowski, MD 2007

multidirectional instability implies three-way subluxations or dislocations either anteriorly, posteriorly, or inferiorly;
- these patients usually have no true Bankart lesion and incontrast patients have a capacious axillary pouch;
- affects overhead throwing atheletes, gymnasts, swimmers;
Wheeless' Textbook of Orthopaedics

Overview of Glenohumeral Instability, Michael L. Pearl, MD. Web Archive copy Last posted 2006
The shoulder faces unique challenges in comparison to other joints when it comes to stability. The arm moves through an extremely wide ROM, and often does so while under very heavy mechanical loads. To understand the mechanisms of glenohumeral stability, it is instructive to compare this joint with the hip and knee. The hip, with its ball and socket configuration, has intrinsic mechanical stability and a truncated spherical ROM. In contrast, the knee has minimal intrinsic bony stability and must rely on a complex array of ligaments for its stability, an arrangement that allows for essentially 2 degrees of freedom. The shoulder borrows from both of these mechanisms with modifications that provide less stability and greater ROM.

This article focuses on glenohumeral joint dislocation. While acromioclavicular (AC) joint separations are sometimes called shoulder dislocations by nonmedical persons, these are not true shoulder dislocations. Shoulder dislocations occur when the head of the humerus comes out of its socket, the glenoid.
Synonyms and related keywords: dislocation of the glenohumeral joint, glenohumeral joint dislocation, dislocated shoulder, posterior shoulder dislocation, anterior shoulder dislocation, acromioclavicular joint injury, humerus, glenoid, glenohumeral ligaments, rotator cuff muscles, glenoid labrum, negative intra-articular pressure
Seade, Bryan & Josey 2006

Shoulder dislocations account for almost 50% of all joint dislocations. Most commonly, these dislocations are anterior (90-98%) and occur due to trauma. Most anterior dislocations are subcoracoid in location. Subglenoid; subclavicular; and, very rarely, intrathoracic or retroperitoneal dislocations may occur.
Synonyms and related keywords: shoulder instability, anterior shoulder dislocation, posterior shoulder dislocation, inferior shoulder dislocation, subglenoid shoulder dislocation, subclavicular shoulder dislocation, intrathoracic shoulder dislocation, retroperitoneal shoulder dislocation, luxatio erecta, multidirectional shoulder instability, traumatic shoulder dislocation, atraumatic shoulder dislocation, atraumatic shoulder
Welsh & Veenstra 2004