The glenohumeral (shoulder) joint is inherently unstable, with the large humeral head articulating with the small and shallow glenoid. Static stability is provided by the orientation of the articular surfaces, the articular conformity of humerus and the glenoid, the glenoid labrum, the negative intra-articular pressure, the adhesion-cohesion of synovial joint fluid, and the glenohumeral joint capsule and its ligaments.
The glenoid labrum is a wedge-shaped fibrous ring attaching to the glenoid articular surface through a fibrocartilaginous transition zone. Below the glenoid equator, the inferior labrum is firmly continuous with the articular cartilage; above the equator it is more mobile and meniscal in nature.
The contribution of the labrum to glenohumeral (shoulder) stability has been clearly established: It acts as the anchor point for the capsuloligamentous structures, increases the depth of the glenoid socket, facilitates the concavity-compression mechanism as the humeral head is compressed in the glenoid during rotator cuff contraction. Loss of the labrum (Bankart lesion) has been reported to result in a 50% decrease in the glenoid depth. Lippit et al.
It demonstrated that the translational force required to dislocate the humeral head was 20% smaller after removal of the glenoid labrum. Although the functional anatomy of the glenoid labrum is becoming better understood, its role in the glenohumeral restraining mechanism, particularly at the extremes of motion, remains unclear.
Discrete thickenings of the joint capsule have traditionally been described as the glenohumeral ligaments. The nomenclature is based on their attachment from superior to inferior and anterior to posterior. Improved understanding of the anatomy and biomechanics of the glenohumeral ligaments has resulted from numerous cadaveric studies and intraoperative observations.
Because of the orientation of these ligaments, portions of the capsule reciprocally tighten and loosen as the glenohumeral joint rotates, thus limiting translation and rotation by load sharing.
In the middle range of motion, these structures are relatively lax with a surface area two times that of the humeral head;therefore, stability in the midrange rotation is maintained primarily by the action of the rotator cuff muscles compressing the humeral head into the conforming glenoid socket.