Shoulder Instability

Anatomy

The glenohumeral joint is made up of a ball-like humeral head rotating on a shallow, dishlike surface, the glenoid. The bony anatomy of the glenohumeral joint allows the shoulder the widest range of motion of any joint in the body. Because of this wide range of motion, the shoulder is very dependent on soft tissue restraints to prevent dislocation.

The soft tissue restraints of the shoulder include the glenoid labrum, the surrounding capsule and ligamentous structures, and the rotator cuff musculature.

The glenoid labrum is a border of soft tissue that surrounds the bone of the glenoid and effectively deepens the glenohumeral articulation. The glenoid labrum may provide stability against humeral head translation.

The capsule extends from the periphery of the glenoid around the humeral head to the periphery of the articular cartilage. The capsule is thickened in three distinct areas. These thickenings make up the glenohumeral ligaments. The ligaments function to stabilize the shoulder by becoming taut in different positions of shoulder motion.

The superior glenohumeral ligament and middle glenohumeral ligament stabilize the shoulder against inferior subluxation or dislocation when the arm is at the patient's side. They also assist in resisting posterior translation. The primary function of the middle glenohumeral ligament is to limit external rotation with the arm at 45° of abduction. The inferior glenohumeral ligament forms a sling with an intervening axillary pouch that tightens anteriorly as the shoulder extends, preventing anterior dislocation, and tightens posteriorly as the shoulder flexes, preventing posterior dislocation.

The surrounding rotator cuff musculature provides dynamic shoulder stabilization. Weakening of the rotator cuff musculature can contribute to shoulder instability.

In 1923, Bankart described a lesion that occurs during a shoulder dislocation and ultimately leads to shoulder instability. The lesion is a detachment of the anterior-inferior portion of the labrum from the rim of the glenoid. The lesion occurs as the labrum is sheared from the glenoid rim during a shoulder dislocation.

In multidirectional instability, the capsule is often enlarged and may be weakened from multiple episodes of subluxation or dislocation.

...