What is Posterior Shoulder Instability?
The glenohumeral joint, also known as the shoulder joint, is the most unstable joint in our bodies because there isn’t much bone holding it in place. Although there have been records of instability in the front of the shoulder dating back to the time of Hippocrates, it wasn’t until 1741 that instability in the back of the shoulder was reported. Over time, doctors noted a variety of shoulder instability levels, from frequently repeating instability to locked dislocations. However, it took until 1984 for medical professionals to clearly distinguish between stable dislocations and frequently repeating ones, noting that the latter is quite rare.
While instability in the back of the shoulder is less common than in the front, it’s increasingly seen among athletes due to improved understanding of its causes and ways to treat it with minimally invasive surgery. Patients may come to the doctor with instability after a dislocating injury or after suffering shoulder pain due to a blunt injury. However, patients more often report vague shoulder pain, making diagnosis difficult. To diagnose, doctors rely heavily on patient history and physical exams, so they have to be particularly attentive to signs of this issue. Depending on the cause and level of severity, treatment can range from physical therapy to surgery. While in the past open surgery was used, the more modern and less invasive arthroscopic method is quickly becoming doctors’ first choice.
Let’s talk a bit about the shoulder’s anatomy to understand this better. The shoulder joint is often likened to a golf ball on a tee because of its shape and the way it fits together. Typically, just about one-third of the upper arm bone (humeral head) interacts with the shoulder blade’s socket (glenoid) at any given moment. This gives the shoulder a great range of motion that we need for everyday activities. However, this also means that it relies heavily on a careful balance of various stabilizing elements to stay in place.
The shoulder’s fixed stabilizers include the glenoid labrum, a ring of cartilage that attaches to the edge of the socket and deepens it, other aspects of the joint’s fitting together, ligaments, the joint capsule, a type of connective tissue, and the negative pressure within the joint. The most crucial fixed stabilizers against instability in the back are the labrum in the back, the capsule, and the Posterior Inferior Glenohumeral Ligament (PIGHL). The PIGHL plays a key role in maintaining stability when we’re using the shoulder while it’s flexed and rotated inwards, like when blocking in football. There’s some debate about how much the rotator interval, a particular section of the shoulder, helps with stability in the back of the shoulder.
Besides these fixed stabilizers, the shoulder also relies heavily on muscle for stability. The group of muscles and their tendons known as the rotator cuff are especially crucial in keeping the shoulder stable. They hold the upper arm bone tight against the socket, aiding stability and increasing the force necessary to dislodge it. The subscapularis, one muscle in the rotator cuff, is particularly important as it’s the main muscle preventing instability in the back of the shoulder. Though their contributions vary depending on the position of the shoulder, other muscles like the long head of the biceps tendon and the deltoid muscle also help with stability.
What Causes Posterior Shoulder Instability?
Recurrent posterior instability, which commonly affects the shoulder, has several causes. It can broadly be categorized into repetitive small injuries, sudden major injuries, and causes that aren’t related to any injury.
When we’re trying to understand the cause of someone’s shoulder instability, it’s essential to consider these three categories. This is because the person’s treatment will depend on what’s causing their problem.
One common cause of the condition is repetitive small injuries to the shoulder, often resulting from certain activities that put the shoulder in a strenuous position. These include bench-pressing, blocking in American football, and overhead sports like swimming, tennis, and baseball. Over time, these repeated stresses can cause damage and changes to the shoulder’s structure, leading to instability.
Major injuries to the shoulder, such as those sustained in high-impact sports or accidents, can also cause the shoulder to become unstable. Patients often remember a specific injury happening just before their symptoms started. High-energy traumas or injuries from seizures or electricity can cause the shoulder to become dislocated, contributing to instability. This kind of instability can also be the result of injuries to the soft tissue around the shoulder, or damage to the bone itself.
Some people have shoulder instability without any history of injury. This is the least common cause and is usually due to loose ligaments in the shoulder. These people may experience pain and instability, especially in certain positions. These symptoms can sometimes even interfere with day-to-day activities.
Risk Factors and Frequency for Posterior Shoulder Instability
Posterior shoulder instability, where the shoulder frequently becomes dislocated towards the back, is much rarer than anterior shoulder instability, where the shoulder dislocates towards the front. It accounts for just 2% to 12% of all shoulder instability cases. You’re about 15.5 to 21.7 times more likely to experience a shoulder dislocation to the front, rather than the back. Despite this, the condition is becoming more common in young athletes, especially those participating in overhead sports, weightlifting, and American football. The average person diagnosed with posterior shoulder instability is a male aged between 20 and 30 years old, who participates in overhead or contact sports.
Signs and Symptoms of Posterior Shoulder Instability
People experiencing a condition called posterior shoulder instability may show a range of symptoms, which can be hard to identify due to their nonspecific and subtle nature. Usually, these individuals are young athletes. The primary sign is a deep pain at the back of the shoulder. The person may also notice declining athletic performance or less shoulder strength and endurance. Some may encounter mechanical symptoms like a clicking or popping sound from the shoulder. It’s important for doctors to suspect this condition if a young athlete reports vague shoulder complaints, especially if they participate in sports involving repetitive small traumas to the shoulder, such as overhead sports, swimming, weightlifting, rowing, or American football. It’s also useful to ask the patients about connective tissue disorders like Ehlers-Danlos or Marfan syndrome, which may indicate a high level of joint flexibility.
A thorough physical exam is essential, as the problem might not be apparent from the patient’s medical history alone. The doctor should examine both shoulders to detect any differences between the two sides. They need to check the shoulder’s appearance, its range of movement, the patient’s strength, how well the nerves and blood vessels are working, and also perform some tests specifically designed to provoke symptoms. The person’s range of motion, strength testing, and neurovascular examination will likely appear normal in cases of posterior shoulder instability. The doctor might find that the patient displays tenderness when the back of the joint is pressed, possibly due to inflammation resulting from multiple episodes of instability. It’s also important to evaluate for any poor movement mechanics in the shoulder blade, as this could result in shoulder pain and weakness.
There are specific examination methods to evaluate posterior shoulder instability. These include the jerk test and the Kim test.
- The jerk test is done with the patient either standing or seated. The doctor stands beside the affected shoulder, holding the elbow and shoulder bone in their hands. They move the patient’s arm and apply a backward force to the shoulder joint, while the other hand applies a forward force through the shoulder joint. The patient experiencing a sudden jerk and pain as the shoulder bone settles back into the socket indicates a positive test.
- The Kim test is used to detect damage in the labrum, a cartilage cushioning the shoulder joint. In this test, the patient is seated with their arm stretched out in the air. The doctor applies a downward force by pushing the patient’s elbow into the shoulder joint. While maintaining this force, the doctor lifts the patient’s arm a bit more, simultaneously applying a backward and diagonal force through the upper arm. Sudden pain in the shoulder signifies a positive test.
Testing for Posterior Shoulder Instability
If a doctor suspects a problem with your shoulder, the first step most often involves using plain X-rays to get an overview of the shoulder. X-rays can show the doctor several views of your shoulder, and this can help them identify problems like a dislocation, bone loss or fractures, or abnormal bone shaping. However, not all problems are visible on X-rays. For example, if your shoulder problem did not result from a trauma (a sudden event like a fall or a blow), the X-ray would likely appear normal.
If the condition persists, more advanced imaging tests might be required. An MRI (Magnetic Resonance Imaging) test can provide a more detailed image of the soft tissues in your shoulder (muscles, tendons, ligaments). An MRI involving a special dye injection, also known as a magnetic resonance arthrogram (MRA), is the best way to spot damage to the soft tissues that form a cup-like structure for your shoulder joint (posterior labral and capsular lesions). In people with posterior instability (when the shoulder feels loose and slips out of place repeatedly), it’s common to find injuries such as a reverse Bankart lesion or an enlarged posterior capsule. Although it’s rare, an MRA can also reveal other injuries like a posterior humeral avulsion of the glenohumeral ligament and a specific type of injury called a Kim lesion, which is a partially hidden tear in the rear part of the labrum.
The Kim classification helps doctors categorize the kind of tear, following this system:
- Type I: incomplete detachment
- Type II: incomplete and concealed avulsion (known as a Kim lesion)
- Type III: erosions on the cartilage and labrum
- Type IV: flap tear (a piece of labrum is torn but still partially connected)
If the doctor is worried about specific bone anomalies, they might use a CT (Computed Tomography) scan instead. This type of test is better at highlighting how your bones are shaped, especially if there were signs of abnormal shapes on the X-ray. Recognizing these abnormalities is crucial as they can affect the kind of treatment you would need.

posterior shoulder dislocation. Left image: Posterior Labrum tear, Middle image:
All-Suture anchors placed into the glenoid, Right Picture: Repaired Posterior
Labrum tear.
Treatment Options for Posterior Shoulder Instability
The ultimate goal in treating someone with posterior shoulder instability – instability at the back of the shoulder, which can lead to pain, reduced function, and recurrence of the instability – is to minimize pain, improve the shoulder’s function, and prevent further instances of instability. The best treatment plan will depend on many factors, including the cause of the instability and any bone or tissue damage.
A “posterior shoulder dislocation” (PSD) refers to when the shoulder joint is pushed or pulled out of place towards the back. These are typically caused by major injuries or seizures and can lead to fractures in the shoulder joint. A “fixed” or “locked” PSD is when the shoulder joint can’t be easily moved back into place due to involvement of the bone. The treatment approach for PSD generally considers the patient’s overall health, level of activity, and the extent of any bone defects.
For recent dislocations with minimal bone defects, doctors may opt for a procedure called a closed reduction, conducted under general anesthesia. This involves gently pushing the dislocated joint back into place without surgery. If this doesn’t work, the doctors may consider a surgical approach. After the reduction, the patient’s shoulder may be supported using a brace.
Surgery is typically the course of action for those who have persistent instability because of soft tissue lesions or defects in the humeral head (the ball-like top part of the upper arm bone). How exactly these are managed surgically will largely depend on the size and nature of these defects.
Non-surgical treatment, such as medication and physical therapy, is generally the first choice for managing posterior shoulder instability lasting around six months. The success of this method often depends on why the instability happened in the first place. If caused by injury, physical therapy results may not be as successful. The therapy includes exercises to improve proprioception (body awareness and movement) and strengthen the shoulder muscles. Rehabilitation may also involve managing the mechanics of how the shoulder blade moves, as this function can contribute to pain at the back of the shoulder.
When non-surgical treatments do not resolve the issue, particularly for people who regularly experience posterior shoulder instability despite avoiding activities that cause it, or for those with instability caused by trauma, surgery may be considered. Procedures for managing any bone or soft tissue defects in the shoulder can be carried out, often using an arthroscope – a small camera introduced through a small incision in the shoulder. The best surgical approach would depend on the individual patient’s condition and the doctor’s assessment.
Surgeons may also use various open surgical techniques to manage soft tissue issues, including transferring muscles or tightening the joint capsule. While arthroscopic treatment has become the more common option in many cases due to its less invasive nature, open surgical techniques may still be necessary in some situations.
When treating posterior shoulder instability and there are associated bone defects, such as fractures to the shoulder joint, the doctor may combine soft tissue procedures with bone reconstruction procedures. The type of procedure used would depend on the extent and severity of the defect.
Ultimately, doctors work on a case-by-case basis to determine the best treatment for each patient with posterior shoulder instability, weighing up factors like the patient’s health, the extent of instability, the presence of any bone or tissue defects, and the cause of the instability.
What else can Posterior Shoulder Instability be?
Posterior shoulder instability, or the inability to maintain the shoulder joint in its proper position, is a condition that’s not very common and often hard to diagnose, particularly if it’s caused by continual minor injuries. In people who have ongoing pain in the back of their shoulder but aren’t reporting symptoms of instability, the doctor will look at several possibilities, which may include:
- A trapped suprascapular nerve
- Quadrilateral space syndrome, a condition that affects the artery and nerves in your shoulder
- A bony growth on the back of the shoulder socket, also known as a Bennett lesion
- Early signs of osteoarthritis, an age-related joint condition
- A tumor
It’s also crucial for doctors to differentiate between loose joints and symptomatic instability. Research indicates that healthy individuals can also have a shoulder joint that moves fairly easily just like those with a symptomatic unstable shoulder. A study demonstrated that even among athletes, moving the upper arm bone onto the rim of the shoulder socket cannot be considered abnormal.
Remember that instability isn’t always restricted to one direction. Hence, the doctor should examine the patient for instability in multiple directions. A past review discovered that around 24% had instability just in the back of the shoulder, and 19% had instability in multiple directions.
Doctors should also keep in mind that people with mental health conditions might be able to deliberately partially dislocate their shoulders. These individuals might develop instability during their teenage years for various reasons and can intentionally dislocate their shoulders when their arms are by their sides.
What to expect with Posterior Shoulder Instability
Understanding the research around the instability on the backside of the shoulder can be tricky due to several factors. These include the rarity of this condition, differences in patient details, various surgical methods and tools, diverse recovery plan, and the lack of a large number of high-quality studies. From the available studies, it shows that open surgery methods seem to have a good outcome with success rates between 80% to 95%. Though, some studies mention the failure rate might go up to 50%.
Initial findings showed that the arthroscopic method (a minimally invasive procedure) of managing the instability was not as promising as those treating the front instability. However, recent studies indicate success rates of around 90% for those who got the instability from sports injuries. An interesting study looked at this repair method on 107 shoulders of 98 athletes and found excellent results in 89% of throwers and 93% of non-throwing athletes after about 27 months post-surgery. However, throwers were less likely to get back to their pre-injury play level than non-throwers
A more noteworthy study, the biggest to date, reported on 200 shoulders in 183 athletes that were treated with the arthroscopic repair method for the backside shoulder instability. After an average follow-up of 36 months after surgery, the patients reported a significant improvement in their condition. 90% of athletes got back to their sport and 64% managed to return to their previous level of play. The study showed that those who were treated with suture anchors (a tool used in surgery) had significantly better results and a higher rate of return to their sport.
Another study focused on 56 American football players who had the arthroscopic repair method for backside shoulder instability. After roughly 45 months, there was a 93% return to sport, with 79% returning at the same level. There was a significant improvement in their shoulder condition and only a 3.5% failure rate.
In 2015, a comprehensive literature review explored both arthroscopic and open shoulder stabilization from 1946 to 2014. An analysis of studies on open shoulder procedures involving 321 shoulders showed 66.4% return to play, with only 36.9% returning to their previous level of play. The recurrence rate among these procedures was 19.4%. In contrast, studies on arthroscopic treatment of 817 shoulders postoperatively had a 91.8% return to play with 67.4% returning to their previous level of play. The recurrence rate among these procedures was 8.1%. The authors believe patients undergoing arthroscopic treatment have better outcomes in terms of the stability of the shoulder, the recurrence rate, patient satisification, and return to sports and previous level of play. However, athletes involved in throwing sports were less likely to return to their previous level of play compared to non-throwing athletes. Repairing with suture anchors resulted in fewer recurrences than those without it, especially for those involved in high physical activities.
Possible Complications When Diagnosed with Posterior Shoulder Instability
Dislocating your shoulder from the back can lead to various problems, like bone death (osteonecrosis), post-injury arthritis, and stiffness in the joint. It’s possible for osteonecrosis to occur in the shoulder’s humeral (round bone at the top) head after a simple dislocation, but it’s more frequently seen along with associated fractures at the anatomic neck, right below the head of the humerus. On the other hand, post-injury arthritis isn’t common with simple dislocations, but when it occurs after a posterior dislocation, it’s usually worse than arthritis following anterior dislocation; treatment could require a shoulder joint replacement surgery. Stiffness after the joint has been put back in place is commonly related to a delay in diagnosis. Additionally, stiffness may be associated with any additional stabilizing procedures, and treatment should target the root cause.
If you need surgery for recurring instability at the back of your shoulder, complications are unique to the specific procedure. Risks with open treatment may include infection, pain, weakness, and stiffness in the shoulder. For treatment using an arthroscopic (minimally invasive) procedure, injury to the nerve in the armpit (axillary nerve) is possible, especially during the repair to the back and bottom of the shoulder capsule. This area should avoid thermal ablation. Incorrect placement and use of surgical instruments (portals) can also lead to damage to the cartilage (chondral) and accidental damage to the labral (cartilage ring around the shoulder socket).
The most frequent issue following surgical treatment for an unstable shoulder is recurrence of the instability. The frequency varies depending on the treatment method and the cause of instability. For instance, studies found that over 800 shoulders treated with arthroscopic methods had an 8.1% recurrence rate while over 300 shoulders treated with open surgical methods had a 19.4% recurrence rate. Other studies have noted that the average rate of recurring instability after arthroscopic treatment is around 5% and not more than 10%. The likelihood of recurring instability is higher if the traumatic posterior shoulder dislocation occurred in a person under 40, during a seizure, or if a large Hill-Sachs lesion (dent in the back part of the humeral head) is present. A glenoid retroversion (backward tilt of the shoulder socket) has also been associated with a higher risk of recurring instability in young athletes.
Potential Complications:
- Osteonecrosis: Bone death
- Posttraumatic arthritis: Joint inflammation after injury
- Joint stiffness
- Infection: From open surgery
- Pain: From the procedure
- Weakness: In the shoulder
- Axillary nerve injury: Risk from arthroscopic treatment
- Chondral damage: Damage to cartilage
- Labral transection: Damage to cartilage ring around the shoulder
- Recurrent instability: Risk of dislocation recurring
Recovery from Posterior Shoulder Instability
After having surgery for a condition called posterior shoulder instability, it’s important to properly care for the affected area to aid in recovery. To do this, the arm is usually fixed in a position that reduces pressure on the back part of the shoulder – specifically about 30 degrees away from the body and neither turned inwards nor outwards. Turning the arm inwards can stress the area, so it’s recommended to avoid this.
Although different doctors may have slightly different approaches, the general guideline is to keep the shoulder still for about 4 to 6 weeks. During this time, patients are typically encouraged to gently move their elbow and wrist, but without moving their shoulder. These non-strenuous movements can often begin within days after the surgery.
In the following weeks, care is needed to avoid both turning the arm inwards and moving it forward and inwards – both can put strain on the healing area.
About 6 weeks after surgery, it’s usually ok to start moving the shoulder. Strengthening exercises can often begin 2 to 3 months after the procedure. At this stage, those who participate in sports can typically start engaging in custom rehabilitation programs once they have regained around 80% of the strength of their shoulder on the unaffected side. This often happens around 6 months after surgery.
When it comes to resuming sports, decisions are made on an individual basis. Generally, athletes can go back to their sport once they have a full, pain-free range of motion and normal strength in the shoulder. This milestone is commonly reached between 6 to 9 months after surgery.
Preventing Posterior Shoulder Instability
Issues with the back part of the shoulder (posterior shoulder instability) are less common than issues with the front part (anterior shoulder instability). However, they often tend to be misdiagnosed or missed. Overhead athletes, like those who play basketball or tennis, can develop this condition due to constant strain on the back part of the shoulder. It can also happen due to a direct, forceful hit to the shoulder while it’s in a vulnerable position, which is often seen in athletes involved in contact sports like football.
Athletes who often throw in their sport should closely monitor their throwing technique, as any flaws can increase the risk of injuring the shoulder again, even after surgery. Young athletes involved in any kind of sport should talk to a doctor who specializes in bone and muscle injuries (orthopedic specialist) if they start to feel a gradual increase in pain in the back part of their shoulder, signs of instability, a decrease in their throwing performance, or similar symptoms after an injury.
Athletes whose symptoms are due to constant minor injuries might find the most improvement with non-surgical treatment. These athletes may be advised to avoid throwing for at least 4 to 6 weeks and join a rigorous rehab program. This program usually involves strengthening the shoulder muscles and fixing the throwing technique.
If surgery is needed, a particular type, called arthroscopic treatment, is now widely used. Athletes treated with this type of surgery can generally get back to their sport quickly, especially if they don’t have to do a lot of throwing in their sport.