What is Posterior Shoulder Instability and Dislocation?

Posterior shoulder instability (PSI) and dislocation is a very rare and complex problem. Due to the lack of research, it is very difficult to diagnose, and differentiating it from other shoulder problems becomes difficult. It has been observed that it is often misdiagnosed and there is a delay in identifying the problem.

PSI affects athletes participating in sports which require overhead motions. 10% of all shoulder-related problems in the active, young, and military population comprise of posterior shoulder instability and dislocation. People involved in weight lifting, volleyball, football, tennis, swimming, or any other contact sports are at high risk.

The shoulder is the most movable joint but also the least stable joint in the body. Static stabilization is provided by the articular cartilage surfaces, glenoid labrum, ligaments, and joints.

The labrum is a ring of cartilage at the end of the glenoid cavity that helps in stabilizing the joint. It acts as an anchor for the capsular ligaments that stabilize the glenohumeral joint at extreme motions. The most significant structure responsible for preventing posterior instability is the posterior capsule that is located between the biceps tendon and the posterior band of the glenohumeral ligament. 

It stops posterior translation when the arm performs internal rotation or adduction. This part is also the thinnest and the weakest part of the capsule making it more prone to instability and dislocation. It is a multi-directional instability. 

 

What causes posterior shoulder instability and dislocation?

Posterior instability and dislocation is a condition that rarely occurs alone. Some of the factors causing this are:

• Microtrauma: this is an important factor in the development of PSI and dislocation. It is caused by repetitive force applied to the posterior region of the shoulder in flexed, adducted, and internally rotated positions. This is caused when there is no healing period between intensive exercise routines and can lead to gradual degeneration of the structures. 
• Macro trauma: a sudden blow to the anterior region of the shoulder when the shoulder is flexed can cause PSI and dislocation. Macro trauma is common among patients involved in high contact sports such as football. 
• Structural abnormalities in the soft tissues, glenohumeral ligaments, posterior capsule, the rotator cuff muscles, and the biceps tendons.

 

Symptoms of posterior shoulder instability and dislocation

Patients affected with PSI and dislocation will experience severe pain and weakness along the posterior joint line and in the biceps tendons and rotator cuff.

The pain will intensify when the arm will perform 90° forward flexion, adduction, and internal motion. 

Swelling and continuous discomfort are typical symptoms.

 

Diagnosis of posterior shoulder instability and dislocation

Diagnosing PSI and dislocation may be a difficult task because symptoms can be similar to other shoulder conditions. 

The doctor will look into the medical history of the patient and past trauma inflicted to the shoulder if any. The doctor will then proceed with the physical examination of the shoulder to come to a clear diagnosis and rule out the suspicion of other shoulder-related problems such as SLAP, rotator cuff tear, or subacromial impingement. 

The shoulder examination will test the range of motion, strength, and flexibility of the affected shoulder and the unaffected shoulder. 

As the patients experiencing pain complain of general pain in the shoulder region, there are specific tests that help in understanding the accurate part of the shoulder that is afflicted. These tests include: 

1. Posterior Drawer Test: The examiner will stabilize the shoulder and the spine of the scapula with one hand and hold the humeral head with the other hand. The examiner will then proceed to press the humeral head into the center of the glenoid to evaluate the neutral position of the joint. Posterior stress is then applied, if the patient experiences pain and symptoms of instability, the result is positive. 
2. Posterior Stress Test: The examiner stabilizes the shoulder with one hand and then he will push the shoulder in 90° flexion, adducted and internally rotated by the elbow. If the patient experiences severe pain and instability, the test shows a positive result. 
3. Kim Test for Posteroinferior instability: In this test, the arm is adducted to 90° while the patient is sitting, and then the examiner will patiently raise the arm an additional 45° while applying downward and posterior pressure to the upper arm with an axial load to the elbow. Posterior displacement indicates a positive result.
4. Jerk Test: The examiner will grasp the scapular spine and the collarbone with one hand while holding the elbow with the other. The arm is then flexed 90° and internally rotated and the elbow is also flexed 90° while pressure is applied on the anterior region of the shoulder girdle.  This causes posterior dislocation of the humeral head. The arm is then adducted and pushed posteriorly, if the patient feels a sudden painful jerk when the humeral head relocates, the test result is positive. 

Physical examination is followed by X-rays and MRI scans. Any possible damage to the shoulder can be seen. It is recommended that patients get an axillary view as it reveals the most diagnostic information of the posterior dislocation and instability. Bony abnormalities can also be identified.

MRIs help in identifying the damage caused to the soft tissues and ligaments that could also be a plausible cause for instability. MRI Scans can also help in determining whether the patient will benefit from open surgery or arthroscopy.

Based on the tests and scan findings the doctor will come to a diagnosis and will plan the treatment approach.

 

Treatment for posterior shoulder instability and dislocation

Based on the findings by the doctor and the severity of the condition, the doctor will either opt for nonsurgical methods or surgical methods. 

Non-surgical methods include: 

• Rest: rest is necessary to speed up the healing process. Any activities that may aggravate the pain should be avoided. 
• Ice packs: applying ice packs twice a day will help in reducing the swelling and will also provide temporary relief. 
• Medication: muscle relaxants and nonsteroidal inflammatory drugs will help in reducing inflammation and also help in relieving pain. 
• Physical therapy: therapy focused on a range of motion and strengthening the shoulder should be performed under the expert guidance of a professional. 

If conservative methods fail and the patients continue to feel severe pain in the shoulder, surgical methods should be considered. Arthroscopic stabilization is considered as the first step of surgical treatment followed by open surgery. 

The success of surgical intervention also depends on many underlying factors such as the patients’ psychological problems, shoulder injury, and the degree of instability in the shoulder. It also depends on the structural abnormalities in the soft tissues.