Shoulder Instability Print

Shoulder Instability, Dr. Allen F. Anderson, Nashville, Orthopaedic Sports Medicine, Figure 1
Figure 1
Shoulder Instability

The shoulder joint is notable in that it has the greatest range of motion of all the joints in the human body. Bony restraints to motion are minimal. Therefore, the surrounding soft tissue envelope is the primary stabilizer that holds the ball of the shoulder (humeral head) in the socket (glenoid). After injury or overuse, the soft tissues that protect the shoulder may become injured or stretched and the result may be that the shoulder begins to slip out of place.

When the shoulder comes completely out of place, this is known as a dislocation. If the shoulder slips only part way out of the socket and then comes right back into place, this is a condition known as "subluxation".


The Basic Science of Shoulder Stability

Most authors agree that shoulder instability is rarely associated with abnormalities in bony alignment. Instead, the most common causes of shoulder instability are related to soft tissue defects. The shoulder capsule is large, loose, and redundant to allow for the large range of shoulder motion.

The capsule contains discrete capsular ligaments which are important in understanding shoulder instability. There are three main ligaments in the anterior (front) part of the shoulder which help prevent subluxation of dislocation.

These ligaments are known as the "superior glenohumeral ligament (SGHL), the middle glenohumeral ligament (MGHL), and the inferior glenohumeral ligament complex (IGHLC). Damage to the IGHLC which supports the bottom part of the shoulder capsule like a hammock is related to most cases of shoulder instability. Defects or injuries to the SGHL and MGHL may also contribute to instability. (See Figure 1.)


The anatomical appearance of the labrum reminds one of the position that a washer occupies at the end of a water hose. The capsular ligaments attach to the labrum and the labrum also increases the depth of the socket. The labrum is often damaged in patients with instability. In some patients, injury to the labrum may cause painful clicking which needs to be addressed if surgical treatment is to be considered.  Fig. 2 shows an arthroscopic view of the labrum.

The shoulder musculature is also important in keeping the ball of the shoulder (humeral head) in the socket. The rotator cuff muscles (supraspinatus, infraspinatus, subscapularis and teres minor) help compress the ball of the shoulder into the socket and increase the force needed to translate the ball out of the socket. (See Figure 3.)

shoulder, dislocation, Dr. Allen F. Anderson, nashville, orthopaedic surgery, sports medicine, figure 2 new

Figure 2

In addition, the muscles that attach to the shoulder blade (scapula) are important to shoulder instability. The shoulder socket is actually part of the shoulder blade and the muscles which attach to the shoulder blade provide a stable pedestal for arm motion and stability. In most patients, discrete anatomical deficiencies in the capsular structures in the shoulder joint appear to be responsible for the great majority of unstable shoulders.

In some patients, poor muscle tone may also contribute to instability. Treatment is generally aimed at correcting deficiencies in capsular structures or in muscle strengthening.


Classification Of Shoulder Instability

Shoulder instability may be classified in several ways. Most important are the direction of instability and the cause. The shoulder may have abnormal motion in one of several directions. By far, the most common type of instability is the anterior type. In this situation, the ball tends to move abnormally toward the front of the body.

Less commonly, the ball can move abnormally toward the back of the body (posterior dislocation or subluxation), or toward the armpit (inferior instability). In addition, some patients may experience laxity of the shoulder in which the ball moves in several different directions at one time. This is a condition known as multi-directional instability.

In terms of the cause of the symptoms, most patients with instability will have a specific traumatic event such as a fall or an injury in a contact sport which causes the shoulder to become loose. However, there are other individuals who will have problems with their shoulder without a specific injury and still others who develop problems only after repetitive use such as throwing and swimming. It is important to differentiate the cause of instability in determining treatment.

Shoulder Instability, Dr. Allen F. Anderson, Nashville, Orthopaedic Sports Medicine, Figure 3

Figure 3

© Allen F. Anderson, M.D. 2017