Posterior Shoulder Dislocation

HomeShoulderPosterior Shoulder Dislocation

A shoulder that has been forced backward during a seizure, an electric shock, or a direct blow to the front of the shoulder, and that remains painful and stiff afterward despite seeming “in place” on a quick look, may be a posterior shoulder dislocation. It is one of the most frequently missed injuries in orthopaedics, often overlooked on initial X-rays and assessment, and one that requires a specialist’s trained eye to diagnose correctly and treat in time.

What is Posterior Shoulder Dislocation?

A posterior shoulder dislocation occurs when the humeral head is forced backward out of the glenoid socket, in the opposite direction to the far more common anterior dislocation. It accounts for only 2 to 4 percent of all shoulder dislocations, which is precisely why it is so often missed. Unlike anterior dislocations, which produce an obvious deformity that prompts immediate assessment, posterior dislocations can appear deceptively normal on standard examination and on a single plain X-ray view, leading to missed or delayed diagnosis in a significant proportion of cases.

As the humeral head dislocates backward, it impacts the posterior rim of the glenoid, creating an impression fracture on the front of the humeral head called a reverse Hill-Sachs lesion. The posterior labrum and capsule are torn in the process, a reverse Bankart lesion. Left undiagnosed, the joint can become locked in this dislocated position, a condition known as a chronic locked posterior dislocation, which is significantly more difficult to treat than an injury caught early.

 

Types of Posterior Shoulder Dislocation

Type I

Acute Posterior Dislocation: A recent dislocation, typically diagnosed and reduced within days of the injury. Often associated with a seizure, electric shock, or direct anterior shoulder trauma. Carries the best prognosis when identified and treated promptly.

Type II

Chronic Locked Posterior Dislocation: A dislocation that has been missed at the time of injury and remains unreduced for weeks or months. The humeral head becomes fixed against the posterior glenoid rim, and the reverse Hill-Sachs lesion enlarges over time as the joint remains locked. Significantly more complex to treat, often requiring open surgical reduction and reconstruction rather than a simple closed reduction.

Type III

Recurrent Posterior Instability: Following an initial dislocation or subluxation, the shoulder continues to sublux or dislocate posteriorly with specific arm positions, typically forward flexion combined with internal rotation, such as pushing open a door or bench pressing.

Type IV

Posterior Fracture-Dislocation: A posterior dislocation associated with a significant fracture of the humeral head or glenoid, most commonly a reverse Hill-Sachs lesion large enough to engage the glenoid rim, or in severe cases a full humeral head fracture. Requires careful surgical planning addressing both the instability and the bony injury.

Causes

Posterior shoulder dislocation has a distinctive set of causes that differ meaningfully from the typical sporting fall that produces an anterior dislocation.

Seizures are one of the most well recognised causes. During a generalised seizure, the powerful internal rotator muscles of the shoulder, which are stronger than the external rotators, can overpower the joint and force the humeral head backward, sometimes dislocating both shoulders simultaneously.

Electric shock and electrocution injuries produce a similar pattern, causing violent, involuntary muscle contraction that drives the shoulder into posterior dislocation.

A direct blow to the front of the shoulder, such as a fall onto an outstretched, internally rotated arm, or a direct impact during a contact sport collision, can also force the humeral head backward.

Axial loading of an adducted, internally rotated arm, for example landing awkwardly during a fall with the arm pinned across the body, is another recognised mechanism.

  • The arm is held close to the body in a fixed position of internal rotation and adduction, with the patient unable to externally rotate or lift the arm away from the body
  • A flattened appearance at the front of the shoulder, with prominence felt at the back, which can be subtle and easily missed compared to the obvious deformity of an anterior dislocation
  • Significant pain, particularly with any attempt at external rotation
  • Following a seizure or electric shock: bilateral shoulder pain and stiffness, which should always raise suspicion for posterior dislocation of one or both shoulders
  • In chronic, missed cases: a longstanding restriction of shoulder movement, particularly external rotation, that has been present for weeks or months and has often been attributed to frozen shoulder or a rotator cuff problem

Diagnosis

Diagnosis requires a high index of suspicion, particularly after a seizure, electric shock, or any shoulder trauma with persistent restricted external rotation. Clinical examination demonstrates a block to external rotation, with the arm fixed in internal rotation, and a posterior prominence may be palpable.

Standard anteroposterior shoulder X-rays are frequently reported as normal or near-normal in posterior dislocation, which is the principal reason this injury is so often missed. The axillary lateral view or scapular Y-view is essential to confidently identify a posterior dislocation, and should always be requested when this injury is suspected clinically, even if the standard AP view looks unremarkable. The classic “light bulb sign,” where the humeral head appears rounded and symmetrical on the AP view due to fixed internal rotation, is a subtle but important clue that should prompt further imaging.

CT scan is used in nearly all confirmed cases to accurately quantify the size of the reverse Hill-Sachs lesion, assess the glenoid for posterior bone loss or fracture, and guide surgical planning. MRI assesses the posterior labrum, capsule, and rotator cuff, and is particularly useful in subacute and chronic presentations.

Treatment

Closed Reduction (acute cases)

For acute posterior dislocations diagnosed promptly, Dr. Kushalappa performs closed reduction under sedation or general anaesthesia, gently guiding the humeral head back into the glenoid socket. Following successful reduction, the shoulder is assessed for the size of the reverse Hill-Sachs lesion and the stability of the joint, which determines whether surgery will subsequently be needed.

Arthroscopic Posterior Labral Repair (reverse Bankart repair)

For posterior instability with a torn posterior labrum and capsule, but without a large engaging reverse Hill-Sachs lesion, Dr. Kushalappa performs arthroscopic repair of the posterior labrum, reattaching it to the glenoid rim using suture anchors, in the same way an anterior Bankart repair addresses anterior instability. Recovery involves a period of bracing in external rotation, followed by a structured rehabilitation programme, with return to sport at 5 to 6 months.

McLaughlin Procedure or Reverse Latarjet (for engaging reverse Hill-Sachs lesions or significant glenoid bone loss)

When the reverse Hill-Sachs defect is large enough to engage with the posterior glenoid during arm movement, or when there is significant posterior glenoid bone loss, soft tissue repair alone is not sufficient. Dr. Kushalappa addresses the humeral-side defect using a McLaughlin-type procedure, transferring the subscapularis tendon and lesser tuberosity into the bony defect to prevent engagement, or reconstructs the posterior glenoid bone loss using a bone graft technique analogous to the Latarjet procedure performed for anterior instability. The choice between these techniques depends on the size and location of the bony defects identified on CT.

Open Reduction (chronic locked dislocation)

For chronic locked posterior dislocations that have been missed and remain unreduced for an extended period, closed reduction is rarely successful. Dr. Kushalappa performs open surgical reduction, addressing the enlarged reverse Hill-Sachs defect and reconstructing the joint to restore stability and movement. These cases require more extensive surgery and a longer recovery than acute presentations, which is why early and accurate diagnosis is so important.

Why Choose Dr. Kushalappa Subbiah in Bangalore?

Dr. Kushalappa Subbiah completed a Fellowship in Shoulder Surgery at the Sydney Shoulder Research Institute, where he trained in advanced arthroscopic shoulder procedures including labral repair and reconstruction. He holds the International Olympic Committee (IOC) Diploma in Sports Medicine, and has direct clinical experience managing shoulder injuries in Indian athletes across cricket, swimming, tennis, and contact sports. He is appointed as a Consultant Shoulder Surgeon at NH Hospital, Bangalore.

Frequently Asked Questions

Standard shoulder X-rays are taken from the front, and a posterior dislocation can appear almost normal on this single view because the humeral head remains roughly centred on the glenoid when seen from this angle. The arm is also held in a way that does not produce the obvious deformity seen with an anterior dislocation. An axillary or scapular Y-view, which looks at the shoulder from the side, is needed to confidently rule it in or out, and this view is sometimes skipped if posterior dislocation is not specifically suspected.

Yes, and this is one of the most well recognised causes. The powerful contraction of the internal rotator muscles during a seizure can force the shoulder into a posterior dislocation, sometimes affecting both shoulders at once. Anyone with persistent shoulder pain and restricted movement after a seizure should be specifically assessed for this injury.

If left unreduced, the joint can become a chronic locked posterior dislocation. The reverse Hill-Sachs defect on the humeral head enlarges over time, the surrounding soft tissues stiffen, and the shoulder becomes progressively more difficult to treat. What might have been a straightforward closed reduction in the first few days can become a complex open surgical reconstruction if diagnosis is delayed by weeks or months.

Not always. Many acute posterior dislocations are successfully treated with prompt closed reduction alone, particularly when there is no significant bone loss or engaging lesion. Surgery becomes necessary for recurrent instability, a large reverse Hill-Sachs lesion, significant posterior glenoid bone loss, or in chronic missed cases where the joint has remained dislocated for an extended period.