Overview of Glenohumeral Instability
SpecialIssue ฆ S houlder instability is the common shoulder problems in the young and the contact athletes. The shoulder joint is composed of 4 articulations (glenohu-meral, acromioclavicular, sternoclavicular, and scapulo-thoracic) but the term "instability" is mainly focused on the glenohumeral articulation. The spectrum of the instability is varying upon dislocation, subluxation and the microinstability. The glenohumeral dislocation is clearly identified by severe pain, gross deformation and
... oss deformation and disability. In contrast, the clinical presentations of gleno-humeral subluxation and microinstability are obscure. Once the shoulder is dislocated, the common sequela is the recurrent dislocation. The factors that influence the recurrent dislocation are age, activities, bony defect, and immobilization technique. Biomechanics The glenohumeral joint is the most frequent dislocated joint in the body due to the greatest arc of motion and the unconstraint characteristic. The stabilizing mechanisms of the glenohumeral joint depends on the static and dynamic stabilizers. For the static stabilizers, the bony construct and the capsulolabral complex play the major role. The glenohumeral joint is considered a ball and socket joint; however, the glenoid bone is relatively flat when compared to the deep socket of acetabulum. The glenoid labrum is a fibrous tissue that deepens the glenoid rim and serves as the attachment for the glenohumeral ligaments and the long head of biceps tendon. The glenohumeral ligaments are lax during the mid-range of motion and become taut at the extreme position. From the anatomical study, the glenohumeral ligaments have a wide variation of the size and the attachment, so is the restraint function. The glenohumeral joint capsule is a thin tissue reinforced by the glenohumeral ligaments. The intact capsule and the labrum contribute to the stability by keeping the negative intraarticular pressure along with the suction cup effect. The capsuloligametous tissue between supra-spinatus and subscapularis called "rotator interval" is another area of interest. From the cadaveric study, the tightened rotator interval contributes to the glenohumeral contracture by decreaseing the external rotation. Lastly, the adhesions/cohesions are the stabilizing mechanism of Siriraj Med J 2009;61:113-116 E-journal: http://www.sirirajmedj.com the lubricated synovial fluid. Conditions that affect the wetability of the joint surface such as arthritis or displaced intraarticular fracture would compromise this mechanism. For the dynamic stability, the rotator cuff, the prime mover, and the periscapular muscles are the main stabilizers. The dynamic stabilizing mechanism that centers the humeral head to the glenoid during the midrange of motion can be explained by the net joint reaction forces and the optimal glenoid arc. Trauma is the most common etiology that compromises the stabilizing mechanism such as the labral defect (Bankart lesion), humeral head defect (Hill-Sachs lesion), glenoid bone loss (Fig. 1), capsular avulsion, and rotator cuff tear. Other than that, the deterioration of the neuro-muscular control for example stroke, cervical spondy-losis, or the brachial plexus injury also affect the muscles around the shoulder 1-4 (Fig. 2). Many classification systems have been proposed based on frequency, causes, directions, and the degree Fig. 1. The anteroinferior glenoid defect known as bony Bankart lesion (white arrow).