Repeated shoulder dislocations are referred to as chronic shoulder instability. Throwing athletes are the most likely group to suffer this problem. Microtrauma from overuse or acute trauma are two common causes. The dislocation is usually in the anterior (forward) direction. There are many factors that contribute to this type of dislocation. In this article, physicians specializing in sports medicine take a look at the role of the glenoid (shoulder socket) in this chronic instability.
The shoulder joint called the glenohumeral joint is made up of two main parts. On one side is the humerus (the upper arm bone). At the top of the humerus is a round ball-shaped bone that fits into a shallow socket of the scapula (shoulder blade). This shallow socket is called the glenoid fossa or just glenoid. Movement of the head of the humerus in the glenoid is what gives us our shoulder movements of flexion, extension, abduction (arm away from the body), and rotation (internal and external).
Any damage to the already very shallow glenoid can contribute to shoulder instability. Defects in the rim around the glenoid and bone loss within the socket are two ways the glenohumeral contact can be affected, adding to the problem of chronic dislocations. Sometimes these defects occur because the shoulder dislocates in a traumatic event. Bone is actually fractured and a fragment of the rim breaks off. In other cases, the bone just wears away from constant contact and compression. Remember, these are athletes who are practice and perform overhead throwing motions 100s and even 1000s of times each season.
Other athletes such as football, volleyball, or soccer players may suffer a traumatic injury with damage to the ligaments attached to the glenoid rim. Without that little rim of fibrocartilage around the joint, it’s much easier for the shoulder to pop out of the socket and dislocate again and again. And sometimes a small instability changes the biomechanics of the shoulder complex enough that over time, the bone wears away unevenly. In either case, rim defects get larger over time. The result is a worsening of the instability.
And so the vicious cycle gets set up and continues. Changes in the joint structure cause biomechanical alterations (i.e., the way the shoulder moves in the socket). Changes in the arc of shoulder motion wear the joint surface unevenly. This, in turn, alters forces within the glenohumeral joint, wearing the glenoid bone unevenly, and the cycle continues. This is how even a small defect can ultimately lead to chronic instability.
What can be done about it? Well, the first thing is that the problem has to be identified and recognized for what it is. Surgery will fail if the full extent of the injury isn’t treated. The patient’s history, a physical examination, and appropriate imaging tests are all used to define the problem. X-rays can show bone loss, MRIs show how much bone loss is present, and CT scans detect rim fractures.
From this information, it is possible to calculate how much of the humeral head is actually in contact with the glenoid surface. The surgeon then uses the arthroscopic exam to look at the bare spot on the glenoid and finish the exact measurements of the defects. The authors provide detailed formulas and examples of calculations to help surgeons compute the percent of bone loss.
Treatment is based on the percentage of bone loss. What is considered an insignificant amount (less than 15 per cent of the surface area) may respond to conservative (nonoperative) care. Bracing, strengthening exercises, and modifying activity level may work well for athletes who are not involved in overhead sports. Rehab supervised by a physical therapist helps the patients understand which movements should be avoided in order to prevent another dislocation from occurring.
If this approach fails to restore motion and function, then surgery to repair the damage and/or reconstruct the shoulder may be required. Anyone with moderate bone loss (15 to 25 per cent) or severe bone loss (25 to 30 per cent or more) will need surgery. The exact surgery planned depends on what type of damage is involved (e.g., bone factures with fragments, labral (rim) tears, amount and location of bone loss, or other defects). Sometimes the surgeon is able to piece the bone fragments back together. This is most likely when the defects are small — limited in number and size and providing that the bone fragments can be found.
Surgical treatment becomes more complex when bone loss affects one-fourth (or more) of the joint surface. In such cases, the surgeon must look at the patient’s activity level, how long it’s been since the injury, the condition of the surrounding soft tissues, number of bone fragments, and potential for healing. To aid in this process, the authors provide a treatment decision-tree (flow chart), which incorporates all of these considerations. Research shows that whenever possible, any bone fragments should be reattached. The risk of failure goes down for patients who are treated in this way.
There are numerous options to choose from including transfer of bone from some other area of the shoulder to make up for defects in the rim. This procedure is called a glenoid augmentation. Several ways to do this are described (e.g., the Latarjet procedure; the Bristow procedure, the Eden-Hubbinette procedure). Drawings to illustrate the procedures are also provided. The surgeon tries to match the bone graft to the contoured (curved) surface of the glenoid. Screws are used to hold the graft in place.
It’s not enough to just wire bone fragments together and reattach them. The surgeon must pay attention to the biomechanics of the shoulder as well. Restoring the normal bony arc of motion is essential for stability. Does the patient want to return to everyday activity and function? Or is he or she expecting to return-to-play? This is a key determining factor in patient satisfaction. Surgeons are still looking for the best way to restore the joint to near normal. They can’t always guarantee 100 per cent recovery and return to sports at a preinjury level.
Sometimes surgery fails to correct the problem. The patient continues to experience symptoms of pain and repeat dislocations. A second (
revision) surgery is needed. The biggest predictor of failed surgery the first time is just how much bone loss is present because it’s this bone loss that results in a failed stabilization. The more bone loss, the greater the risk of a failed repair.
In summary, chronic shoulder dislocations with moderate to severe bone loss of the glenoid fossa or glenoid rim must be repaired surgically to restore shoulder stability. It is possible for high-demand contact athletes engaged in overhead throwing motions to regain shoulder stability and function. The surgeon must pay close attention to the socket side of the shoulder joint and repair any damage to the rim that holds the humeral head in place. There are ways to do this, but finding the best approach for optimal results remains an area of research right now.