Overview
What is shoulder instability?
Shoulder instability is a condition that occurs when the shoulder, partially or fully, slips out of its socket.
The shoulder is a very mobile joint. It is designed to allow for motion in many directions, enabling us to perform many different tasks.
This increased mobility however comes at a cost. Because it moves easily in so many directions, it is as risk for slipping out of place. When the shoulder slips out partially, it is called shoulder subluxation. When it comes out completely, it is called a shoulder dislocation.
To understand what causes shoulder instability, it is important to understand the normal anatomy of the shoulder. Specifically we will discuss those structures that help to keep the shoulder in place. Then we will discuss those injuries and conditions that disrupt those structures and lead to shoulder instability.
Anatomy
The shoulder is a ball and socket joint. The humeral head (ball) is held and stabilized in the glenoid (socket) by tendons, ligaments and the labrum. Shoulder instability can develop when there is an injury or an abnormality of these supporting structures.
Tendons: The tendons, or rotator cuff, connect to the top of the humerus and help to center the ball of the shoulder within the socket. Rupture of the tendons or weakness of the muscles can lead to shoulder instability. Individuals with loose shoulders can often reduces symptoms of instability by strengthening the rotator cuff.
Ligaments: Underneath the rotator cuff is another layer made of thick connective tissue called a capsule. The capsule is composed of multiple ligaments that stabilize the shoulder. Some people have naturally loose ligaments and can develop shoulder instability without any injury.
Labrum: The labrum is an elevated layer of cartilage that lines the outer rim of the glenoid. It also helps to keep the shoulder within the socket. Tearing of the labrum (called a Bankart Lesion) can occur following a traumatic shoulder dislocation. A torn labrum following a shoulder dislocation may need to be repaired surgically to stabilize the shoulder.
Causes of Shoulder Instability
What causes shoulder instability? There are two many categories of shoulder instability. Traumatic and atraumatic.
Traumatic shoulder dislocation occurs following an injury. The shoulder can dislocate when being struck with the arm in a vulnerable position. For example, when a quarterback is tackled while throwing a football.
Atraumatic shoulder instability occurs in some individuals who have loose ligaments from birth. These individuals may have many joints that are loose or may only involve the shoulders.
It also can occur in individuals with normal ligaments but have developed looseness from repetitive stress over time. Examples include swimmers, gymnasts and volleyball players.
Evaluation and Treatment
If the shoulder is dislocated, the most important first step is to relocate the shoulder. Some shoulders will automatically fall back into the socket after a dislocation.
If the shoulder is fully out it is imperative to get to an emergency department to “relocate” the shoulder. This often requires medicine to relax the muscles around the shoulder. It is often necessary to obtain x-rays prior to the relocation to make sure there is not also a fracture which may complicate the ability to put the shoulder back in place.
After the shoulder has been relocated back into place, repeat X-rays are necessary to confirm that the procedure has been successfully performed.
Treatment is dependent on a number of factors.
Traumatic Dislocation
For a traumatic dislocation, a sling may be necessary for a few weeks to allow for the supporting structures to heal. Physical therapy may also be necessary to regain motion and to strengthen the muscles around the shoulder.
For younger individuals -- teenagers and those in their early 20’s -- surgery may be recommended after a first time dislocation. This is especially true for overhead athletes. It is known that the younger you are when you have a first-time dislocation, the risk of repeated dislocations is quite high.
For older individuals, and those with limited sports participation, physical therapy is an option in an effort to avoid surgery.
Atraumatic Instability
For individuals with atraumatic instability the best treatment is physical therapy to strengthen the rotator cuff and other muscles around the shoulder.
This treatment is often successful.
When physical therapy fails to eliminate the symptoms of instability (the feeling that the shoulder is going to come out), arthroscopic surgery may be necessary to tighten the ligaments to stabilize the shoulder.
Other Considerations
Direction of Instability
The direction of instability is also important in the evaluation and treatment of instability. Most traumatic dislocations occur with the ball coming out the front of the shoulder. This is called anterior instability. The injury occurs to the ligaments and labrum (Bankart Lesion) in the front of the shoulder.
The direction of instability may also be in the back of the shoulder. This is called posterior instability. This is less common than anterior instability. This is often seen in those who are electrocuted or who have seizure disorder. This will lead to injury of the ligaments and labrum (reverse Bankart) in the back of the shoulder. Posterior instability is also seen in football lineman and swimmers.
Individuals with atraumatic instability will typically have instability in more than one direction. This is called multidirectional instability.
Successful surgical and nonsurgical treatment of shoulder instability requires that the direction of instability be identified.
Associated Fractures
With traumatic dislocation of the shoulder there are sometimes fractures associated with the dislocation. Fractures can occur in the glenoid (socket) or the proximal humerus (ball).
Glenoid Fractures
These are called Bony Bankart lesions. The larger the fracture the more likely surgery will be necessary.
Proximal Humerus Fractures
Proximal humerus fractures are more common in shoulder dislocations in the older individual and can complicate relocation of shoulder dislocation. If the fracture is displaced, that is, separated significantly, surgery of the proximal humerus to fix the fracture may be necessary.
Hill-Sach’s Injury
What is a Hill-Sach’s Lesion? Harold Hill and Maurice Sachs were two radiologists who described this injury to the top of the humerus that occurs after a shoulder dislocation. When the shoulder dislocates anteriorly, that is when the ball of the shoulder comes out the front of the socket, it can impact against the socket causing an indentation.
Rotator Cuff Tears:
For a first-time dislocation after the age of 40, there is a significant risk of having a tear of the rotator cuff. It is important to obtain an MRI to evaluate for the integrity of the rotator cuff. Surgery to repair the rotator cuff is necessary to regain function and avoid long term shoulder problems.
Chronic Dislocators
Each time there is a shoulder dislocation there is further injury to the supporting structures. With repeated dislocations it may be more difficult to repair the torn structures which may impact the type of surgery that is necessary. It may also adversely affect the ability to have a successful outcome after surgery.