Shoulder Instability

The shoulder joint, also called the glenohumeral joint, is a complex joint where the ball (humeral head) and socket (glenoid) help the shoulder move through an incredible range of motion. This includes moving in front of the body and behind the body in flexion and extension, out from the body and in towards the body in abduction and adduction, and rotational movements with internal and external rotation. We are blessed to have such incredible range of motion of the shoulder to be able to do everyday tasks. However, although this provides increased mobility for the shoulder, the shoulder is also more liabile to potential instability. To prevent the shoulder from becoming excessively unstable while still providing that wonderful range of motion, we have several natural restraints to excessive motion. First, the fibrocartilaginous labrum is a ring of soft tissue around the socket that helps stabilize the shoulder joint and deepen how the humeral head sits in the glenoid. Secondly, the capsular covering of the shoulder joint helps to further reinforce and stabilize the ball during motion. The capsule forms distinct thickenings, or what we call glenohumeral ligaments, that are tensioned throughout different ranges of motion in the shoulder. Thirdly, the rotator cuff muscles are four distinct muscles that also further reinforce the shoulder joint by not only helping to initiate motion but also to help tension the shoulder joint in different extremes of motion to help prevent the ball from sliding too far out from the socket.

Shoulder instability develops either through a traumatic injury or through atraumatic laxity and stretching out of the shoulder stabilizers which include the labrum, shoulder capsule which forms ligaments around the shoulder, and the rotator cuff muscles.

If we think about traumatic injuries, these can occur through injuries such as a fall on an outstretched hand, direct impact to the shoulder joint, motor vehicle accidents, or even in contact sports such as football or rugby. The overall mechanism of these traumatic injuries is forceful pushing of the humeral out of the shoulder joint. The ball can be pushed either posteriorly out the back of the shoulder joint, anteriorly out the front, and less commonly directly superiorly or inferiorly in the shoulder joint. When a true shoulder dislocation occurs with the humeral head out of the socket, it is essential that the humeral head is quickly put back into the socket because excessive amount of time with the humeral head outside of the shoulder socket leads to stretching out of the blood vessels the loss of blood flow to the humeral head. If untreated, the humeral head can collapse in a process known as avascular necrosis which is irreversible and requires extensive reconstructive surgery to treat. Thus, it is essential that if a shoulder dislocation is suspected, prompt evaluation and treatment by a professional medical provider should be obtained.

When we discuss atraumatic instability, two prominent mechanisms include repetitive overhead use which stretches out and damages the shoulder stabilizers over time as well as congenital shoulder laxity. Congenital laxity refers to where some patients are born with soft tissues that are excessively loose. This can be present either just in the shoulder (isolated shoulder laxity() or throughout the body (generalized shoulder laxity). When the shoulder capsular ligaments are overstretched, the humeral head has a tendency to slide in and out more easily from the socket compared to the average population. One example of this congenital laxity is observed in patients with Ehlers Danlos syndrome who because of looser shoulder stabilizers are more prone to shoulder instability.

Instability can be subcategorized in multiple ways:

1. Direction of dislocation: Based on the direction of the humeral head in relation to the shoulder socket, shoulder dislocations can be categorized as anterior, posterior, superior, and inferior dislocations or a combination of the above.
2. Dislocation vs subluxation: A frank dislocation occurs when the humeral head is completely removed from articulating with the shoulder socket. On the other hand, a shoulder subluxation occurs when the ball is not fully centered on the socket but is not fully slipped out yet.
3. Unidirectional vs multidirectional: Unidirectional instability occurs either in one primary direction whereas multidirectional instability has instability multiple directions. Obviously, multidirectional instability represents a greater challenge for treatment as it has multiple anatomic stabilizers which need to be appropriately addressed to help restore proper shoulder stability

We can also classify injuries based on which anatomic stabilizers have been injured. If we think about the labrum the surrounds the shoulder socket, we can injure different parts of the labrum depending on the direction of dislocation. A common type of labrum injury is the Bankart lesion where during an anterior dislocation, the labrum in the front part of the shoulder becomes injured as it is ripped off the socket. Similarly, if there is a posterior dislocation of the humeral head sliding out the back of the socket that injures the posterior labrum, this would be a posterior or “reverse” Bankart lesion. On the other hand, injuries can occur to the humeral head being crushed against the bony socket during the dislocation event. When there is an anterior dislocation, the posterior-superior portion of the humeral head can become impacted which fors a a Hill-Sachs lesion. These named injuries of the Bankart and Hill-Sachs lesions are important because though they occur during a first-time dislocation, in patients who are younger, play contact sports or perform heavy lifting, or have congenital stretching out of their shoulder stabilizers, these injuries predispose them further dislocations in the future.

In shoulder instability, because the humeral head is not appropriately centered on the socket, the patient can have a very strange sensation that the shoulder is slipping out of place or giving way. These episodes of the shoulder not being appropriately stabilized can repeatedly occur during sports or strenuous daily activities especially heavy lifting for overhead motion. In more severe cases, the shoulder can even slip out at night while the patient is sleeping. For anterior based instability, when the hand is out and away from the body, this can often trigger apprehension that the shoulder is about to subluxate/dislocate again. This sensation can often cause patients to lose confidence in their shoulder function.

Because the humeral head is not well centered, the surrounding musculature of the shoulder cannot work efficiently and so patients may have a loss of strength or range of motion especially with reaching behind the back. If there is an associated labral tear, patients may complain of clicking or catching in the shoulder. Additionally, because the humeral head is not appropriately centered and is pushing on the surrounding soft tissues of the shoulder, the surrounding nerves that cross the shoulder can be excessively stretched causing nerve symptoms such as numbness, tingling, and weakness down the arm.

If a patient is concerned about having shoulder instability, this is first confirmed on their history because many patients will have the symptoms that we have already outlined above. It is essential to understand the mechanism of injury and if the patient has a personal or family history of diagnosed hypermobile conditions such as Ehlers Danlos. A physical exam would help by determining in what direction the instability is primarily driven by, help to localize the pain, calculate a Beighton score that reflects generalized hypermobility, and consider other causes of pain around the shoulder such as cervical radiculopathy or referred pain from the neck, trapezius strains, and thoracic outlet syndrome.

Radiographs (x-rays) are essential to look at the overall joint morphology, associated injuries such as fractures or AC separations, Hill-Sachs lesions, and to gain a rough idea of any glenoid bone loss or version changes. An MRI is useful to evaluate the soft tissues around the shoulder. The use of contrast dye injected into the shoulder can be very helpful during an MRI to help accentuate detection of soft tissue injuries such as labral tears, associated rotator cuff tears, capsular injuries, and humeral avulsions of the glenohumeral ligaments (HAGL) lesions. CT scans are more accurate than MRI in quantifying the degree of bone loss. A CT scan with 3D reconstruction can be very helpful in cases of complex deformity for the surgeon and patient to better understand the anatomy. It is not uncommon to obtain an XR, MRI, and CT scan to have the most complete information before considering surgical intervention.