What is the incidence of posterior dislocation of the shoulder and what is the typical follow-up after closed reduction?

Posterior Shoulder Dislocation: Incidence and Post-Reduction Follow-Up

Incidence

Posterior shoulder dislocations are rare, accounting for only 2-4% of all shoulder dislocations, making them significantly less common than anterior dislocations 1, 2, 3. This rarity contributes to their frequent misdiagnosis—over 50-60% are missed on initial presentation 4, 5. The low incidence combined with subtle clinical findings makes posterior dislocations one of the most commonly missed orthopedic injuries.

Key Epidemiologic Points:

• Anatomically, 97.5% of posterior dislocations are subacromial in location 3, 5
• Common mechanisms include seizures, electrocution, direct trauma, and high-energy indirect forces 3, 5
• The diagnosis is frequently delayed, with an average interval of 66 days between injury and correct diagnosis in one series 1

Critical Diagnostic Imaging Requirements

Pre-Reduction Imaging

You must obtain proper radiographic views BEFORE attempting reduction—specifically anteroposterior (AP) views in internal and external rotation PLUS an axillary or scapula-Y view 4. This is non-negotiable because:

• AP views alone miss posterior dislocations in over 60% of cases 4
• Attempting reduction without radiographic confirmation can worsen occult fracture-dislocations 4
• Axillary or scapula-Y views are essential to differentiate anterior from posterior dislocations and identify associated fractures 4

Radiographic Findings Specific to Posterior Dislocation:

• Humeral head displaced posteriorly relative to glenoid on axillary/Y views 4
• Anterior Hill-Sachs lesion (anteromedial humeral head compression fracture)—the "reverse" Hill-Sachs 1, 5
• If radiographs are equivocal or symptoms persist, obtain CT scan to definitively characterize the dislocation and fracture patterns 4

Post-Reduction Management and Follow-Up

Immediate Post-Reduction Protocol

Obtain post-reduction radiographs immediately to confirm successful reduction and identify any fractures that were obscured by the dislocation 4. This step is mandatory before any immobilization or discharge planning.

Perform thorough neurovascular assessment following reduction, as axillary artery injury can occur, particularly with associated proximal humeral fractures 4. If vascular compromise is suspected, CT angiography is the preferred examination 4.

Follow-Up Imaging Strategy

For patients with successful closed reduction and no significant associated injuries:

• Conservative treatment with immobilization is appropriate if the shoulder remains stable after reduction 1
• In one series, 6 of 35 patients (17%) were successfully managed conservatively after closed reduction, achieving a mean Constant Score of 85 points 1

For recurrent instability or persistent symptoms:

• MR arthrography is the gold standard (rated 9/9 in appropriateness) for evaluating labroligamentous injuries, Hill-Sachs lesions, and glenoid bone loss 6
• Non-contrast MRI is a reasonable alternative (rated 7/9) when MR arthrography is unavailable 6
• Ultrasound has no defined role and should not be used as a primary diagnostic tool 6

Treatment Decision Algorithm Based on Timing and Defect Size

For acute posterior dislocations (< 2 months duration):

• Closed reduction is indicated when the anterior Hill-Sachs lesion is < 15% of humeral head size (measured on axillary view) 5
• Immobilization in neutral rotation or slight external rotation for 3-6 weeks (based on general orthopedic principles)

For locked/persistent posterior dislocations or larger defects:

• Surgical intervention is required, with options including elevation of defect, bone grafting, McLaughlin procedure (subscapularis transfer into defect), rotational osteotomy, or arthroplasty 1, 7
• The McLaughlin technique (suturing subscapularis tendon into the humeral head defect) provides satisfactory results for unreduced or recurrent dislocations 7
• Posterior glenoidplasty with capsulorrhaphy and infraspinatus advancement shows excellent results for recurrent posterior instability 2

For dislocations with significant glenoid bone loss (>20%):

• Latarjet procedure is recommended 6

Prognostic Factors

Time to diagnosis is the most critical prognostic factor—there is a high correlation between diagnostic delay and worse outcomes 1. In surgical series:

• Conservative treatment after successful closed reduction: mean Constant Score of 85 points 1
• Operative treatment for locked dislocations: mean Constant Score of 79 points 1
• Early surgical intervention is preferred, as delayed surgery is associated with progressive labro-ligamentous injury 6

Critical Pitfalls to Avoid

• Never rely on AP radiographs alone—this is the primary reason for the >60% missed diagnosis rate 4, 5
• Do not attempt reduction without proper imaging, as fracture-dislocations can be worsened 4
• Delaying reduction increases risk of neurovascular complications and makes subsequent treatment more difficult 4, 1
• In older patients, assess for rotator cuff tears, which are more common with shoulder dislocations 4
• Seizure-induced dislocations typically have larger anterior Hill-Sachs lesions and higher recurrence risk 5