Shoulder Instability: Overview

Shoulder Instability: Overview

The shoulder joint is a ball and socket joint, considered to be the most mobile joint in the human body, and is often compared to a golf ball on a golf tee. Sometimes, the golf ball can slip off the tee. When this occurs, if the ball loses contact with the socket, it is referred to as a dislocation. If the ball only loses contact partially with the socket, it is referred to as a subluxation. These episodes of either subluxation or dislocation collectively are referred to as shoulder instability.

Following a dislocation event, damage to the surrounding soft tissues, such as the glenoid labrum of the shoulder, may occur. The shoulder is the most frequently dislocated major joint in the human body. The highest percentage of primary dislocation occurs in males age 10 to 20, followed by males age 50 to 60. Shoulder dislocations can occur in a posterior direction out the back, inferior direction out the bottom, or anterior direction out the front. Dislocations out the front of the shoulder joint are the most common, making up 90% to 98% of all dislocation episodes.

Focusing on dislocations out the front of the shoulder, these events are most often acute in nature and can occur in several different ways. The first is falling onto an outstretched arm, which can be seen during contact sporting events or from a fall from heights. Second, forceful positioning of the arm out to the side while the lower arm rotates backward, referred to as a 90/90 position seen here, can cause the ball to lever out the front of the socket. Lastly, a direct blow to the back of the shoulder can also result in a dislocation.

Although dislocations out the front of the shoulder tend to be traumatic, there are steps patients can take to lower their risk. Following fall prevention protocols when participating in high-risk activities at work or at home can help keep you safe. If you are participating in contact sports, wearing proper protective gear during play is advised. In addition, participation in an upper body strength program focusing on the rotator cuff and shoulder blade musculature may help to support the shoulder joint.

Looking at the skeletal anatomy, there are 3 bones that create the shoulder: the upper arm bone, known as the humerus; the shoulder blade, known as the scapula; and the collarbone, known as the clavicle. The top portion of the humerus has a rounded portion, known as the humeral head, which contacts with the shallow socket portion of the scapula known as the glenoid. These two structures together make up one of two joints in the shoulder, referred to as the glenohumeral joint. It is within this joint where shoulder dislocations occur.

The glenoid socket is small in relation to the humeral head. Due to the socket's small size, there are soft tissue structures that surround the shoulder joint to keep it stable. These structures include the glenoid labrum and glenohumeral ligaments. The labrum is a ring of tissue attached around the edge of the bony socket. The labrum helps deepen the socket so the ball, the humeral head, stays in place. The glenohumeral ligaments act as tethers, so when the shoulder is taken through extreme ends of motion, the ligaments tighten to prevent the ball from slipping out of the socket.

In cases of shoulder dislocation or subluxation in younger patients, these tissues may be stretched or torn. The labrum may tear off the socket at the front and bottom portions, referred to as a Bankart tear. Specifically in a dislocation out the front, dislocation events can happen in older patients, too. However, when they occur, these patients are more likely to experience a tear of their rotator cuff tendons.

People who experience a shoulder dislocation will have immediate onset of pain following a traumatic event, along with the inability to use the affected arm. Sometimes, the ball will go back into the socket on its own and other times, it will require reduction to be completed by a health care professional in the emergency department. After a shoulder dislocation, it is common that patients will have a feeling of looseness with movement of the affected shoulder even after it is back in its normal position.

For evaluation of a shoulder with suspected instability, the health care provider will gather information such as if they are right or left-handed, the activity the patient was participating in, if the dislocation was traumatic, and any prior dislocation events. The health care provider will perform a physical examination to evaluate the shoulder, which will include moving the shoulder into different positions to assess for instability.

Following shoulder dislocation, x-rays are obtained to evaluate for several things. First, x-rays help to determine the direction of the dislocation if the patient is seen in the emergency room. Second, they confirm if the ball has been placed back into the socket if reduction was required. Lastly, health care providers can evaluate for any breaks or damage to the bones. An MRI is subsequently ordered to evaluate for any damage to the glenoid labrum, or glenohumeral ligaments that may have stretched or torn because of the dislocation. In some cases, a CT scan may be ordered to further evaluate damage to the bone of the shoulder joint.

After review of the imaging tests and correlation with the patient's symptoms, the doctor will identify and share a treatment plan best suited to the patient's needs.