The shoulder joint is particularly at risk for dislocation. Chances are you know someone in your family, or your group of friends, who has suffered a dislocation of the shoulder. This is partly due to the difference in size between the larger humeral head (ball) and the smaller glenoid (socket), as discussed in our previous post on multidirectional instability. Most of the times, the humeral head dislocates in front of the socket (anterior instability), and many shoulder surgeons are very familiar with the various treatment alternatives for this condition. But what if your shoulder dislocates posteriorly? Much less common, but it can happen as well! And surgery will be of benefit for selected patients…
Posterior shoulder instability: what is it, anyways?
This term is used when the humeral head displaces abnormally over the back rim of the glenoid socket, generating pain and difficulty with use of the arm. A complete dislocation of the humeral head behind the socket represents the extreme of this condition, but in many individuals the instability episodes are subtler: the humeral head jumps in and out of the joint over the back rim of the glenoid in certain positions of the arm. Most of the times, posterior instability episodes occur when the arm is raised in front of the body with the palm facing forward (watch video below).
Who is at risk for posterior shoulder instability?
Posterior shoulder instability can be the result of an injury, typically when the arm is extended in the front of the body and, all of a sudden, the shoulder is forcefully driven backwards by accident. The typical example is represented by a football player (most commonly a defensive lineman) that gets injured while trying to stop with his arm a player running towards him. The collision will drive the humeral head to the back of the shoulder. As a result, in really bad injuries, the shoulder could possibly dislocate. More commonly, a structure called labrum is detached from the socket, with various degrees of capsular damage. These injuries allow the humeral head to come out of the socket in the back. As a consequence, the individual will experience repeated episodes of posterior shoulder instability that will slowly grind down and wear the bone on the back of the socket, starting a vicious cycle that will only lead to worse instability.
Posterior instability is also oftentimes seen in patients with multidirectional instability, collagen disorders, or glenoid dysplasia. What are all these terms? We discussed multidirectional instability (MDI) in a prior post; these are individuals with a normal skeletal structure but extremely lax ligaments: the joint dislocates posteriorly, but also anteriorly and inferiorly. Multidirectional instability is particularly common in individuals with poor collagen (hypermobility syndromes, such as Ehlers-Danlos syndrome). Finally, in some individuals the glenoid or shoulder socket does not develop normally, with the back of the glenoid being most commonly underdeveloped; this condition is called dysplasia, and it also predisposes to posterior shoulder instability, and also to osteoarthritis. It is also important to remember that poor positioning of the shoulder blade in space can contribute substantially to posterior shoulder instability (or any type of shoulder instability, for that matter): so-called scapular dyskinesis (scapular = shoulder blade; dyskinesis = poor motion) can contribute to the cause of instability, but also result from an attempt to compensate for the unstable shoulder joint (secondary scapular dyskinesis).
The evaluation and management of complete posterior dislocations, which can be locked in place (locked posterior dislocations), exceeds the content of this post. Today we will limit our discussion to recurrent posterior instability. Interestingly, although some individuals are aware of their instability episodes when they happen, in many individuals the only complaint is pain. Most of the times, patients with posterior instability experience pain when they place their arm extended in front of them, as well as with activities that will load the back of the joint, such as push-ups or bench pressing. Your shoulder specialist will use a number of physical examination maneuvers to unmask posterior instability.
Plain radiographs are a must, and they will identify individuals with dysplasia, as well as those who have developed acquired bone loss at the back of the glenoid as repeated episodes of instability wear down the bone. When bone loss is subtle, it can only be properly judged with computed tomography (CT). Finally, when the main goal is to evaluate the labrum and quality of the articular cartilage, as well as capsular stretching and the overall volume of the joint space, magnetic resonance imaging (MRI) can be of great value. The ability to visualize areas of damage can be improved if the MRI is performed after injection inside the joint of a solution called contrast (gadolinium).
Some patients with posterior shoulder instability can get better without surgery. This is particularly true for those individuals with no bone loss (congenital or acquired) as well as those with instability secondary to scapular dyskinesis or poor coordination of their rotator cuff in the presence of laxity. The mainstay of nonoperative treatment for posterior shoulder instability is physical therapy. Exercises are directed to improve proprioception (awareness of the position of a given joint in space and how to control it), coordination, and strengthening of both the muscles around the shoulder blade and the rotator cuff. However, some individuals will just not respond to physical therapy. For these individuals, surgical options typically include one of two procedures: either the torn labrum is repaired to bone and the capsule in the back of the joint shortened and strengthened or bone is added to the back of the glenoid.
Repair of the labrum, capsular tightening
Individuals with posterior instability, a torn posterior labrum, and no or minimal loss of glenoid bone can do extremely well with repair of the labrum (when torn) and/or tightening of the posterior capsule. Currently, most surgeons perform this procedure arthroscopically: a small skin cut is used to introduce a camera into the shoulder joint, and additional small cuts (also called portals) are used to introduce instruments to repair the labrum, imbricate the capsule, or both. Sutures are placed into the capsule and labrum and tied to complete the repair and tightening.
Posterior bone block
When a fair amount of bone is missing on the back of the glenoid, repair of the labrum and capsular plication are less likely to succeed. In these circumstances, extra bone can be added to the back of the glenoid to make it both wider and taller, so that the ball cannot jump over and out of the socket. These posterior bone block procedures may be performed using either open or arthroscopic techniques. The most common source of bone is the back of the pelvic bone (iliac crest, at the level of the waist line). Sometimes, bone from a cadaver donor is used, particularly when the amount of bone missing is large and replacement of cartilage is considered necessary.
What to expect after surgery
After surgery for posterior shoulder instability, most surgeons recommend protecting the arm in a shoulder immobilizer with the hand away from the body (external rotation). Otherwise, if the shoulder is placed in internal rotation, the posterior capsule gets stretched and the humeral head can more easily move to backwards, potentially endangering the surgical reconstruction.
Movement of the shoulder and physical therapy to restore motion and strength typically do not start until week six after surgery, although some surgeons recommend exercises to contract and tone the muscles around the shoulder blade (scapular isometrics) right away. It may take a year or longer before motion is completely restored, and in some individuals some motion loss is to be expected. But when surgery is successful, patients are really satisfied…, watch below!