The glenohumeral joint (shoulder socket) is a ball-and-socket joint. The socket, known as the glenoid, is part of the shoulder blade and is a shallow cavity or dish for the ball of the joint to attach. The ball of the ball-and-socket joint is the top part of the upper arm bone (humerus) known as the humeral head. A layer of specialized cartilage known as the glenoid labrum forms a ring around the socket that adds depth to the joint, creates a seal that maintains joint fluid within the shoulder and increases overall stability to the shoulder socket. The capsule around the labrum has regions of thickening that add stability to the shoulder and these regions are known as glenohumeral ligaments. Collectively, this structure of thickened ligaments and the labrum that encircles the joint is called the capsulolabral complex. A dislocation occurs when the humeral head (the ball) comes out of the socket, frequently causing the labrum and ligaments to tear. Shoulder instability arises when the labrum or capsulolabral complex is torn. This can lead to many complications, pain, and recurrent dislocations. The instability can be in the front (anterior) or back (posterior) region of the shoulder. It can also be classified as multidirectional.

Anterior instability

Anterior instability of the shoulder is the most common. It occurs when the front (anterior) region of the capsulolabral complex is torn (this tear is commonly referred to as a Bankart lesion). A blow to the shoulder from behind or trauma to the arm while extended or rotated outward are common causes. This tear leads to pain, weakness, and recurrent partial or full dislocations of the humeral head in the forward direction. Anterior dislocations are obvious with an abnormal appearance of the shoulder “pressing out forward” and requires reduction (setting the shoulder back in place). Physical therapy is recommended to strengthen the muscles that stabilize the shoulder. Re-dislocation or persistent instability should be treated with surgical stabilization.

Posterior instability

Posterior instability is more rare than anterior instability. Repetitive minor trauma to the shoulder while the arm is lifted and rotated in towards the body can cause posterior labral tears (in the back of the shoulder socket).  A common example of this is a football lineman consistently jamming his arm while blocking. Posterior labral tears that lead to instability are also commonly associated with shoulder pain that results after a seizure or electrical shock. Posterior dislocations also require reduction (setting the shoulder back in place) and physical therapy is recommended, much like anterior instability treatment.  With recurrent dislocations or subluxations (partial dislocation) are present with pain, surgical fixation is often recommended.

Multidirectional instability (MDI)

Multidirectional instability occurs in more than one direction. It is caused by an increase in the volume of the capsule of the joint in the shoulder and thinning of the labrum. Multidirectional instability is most common in athletes who use repeated overhead movements like volleyball players and swimmers. Pain and instability when holding heavy objects at the athlete’s side is an indicator of instability in the lower (inferior) direction. Physical therapy is recommended to strengthen the muscles that stabilize the shoulder.