Posterior shoulder dislocation accounts for approximately 10% of all shoulder dislocations and is one of the most commonly missed diagnoses in orthopedic emergencies — 50–79% are missed on initial presentation. [1-2] A high index of suspicion, appropriate orthogonal imaging, and awareness of classic mechanisms (seizure, electric shock, trauma) are essential.
1. History
- Mechanism of injury: Posterior-directed force on a flexed, adducted, internally rotated arm; seizure (34–38% of cases); electric shock; high-energy trauma (MVC, falls); contact sports (e.g., football linemen blocking) [2][4-5]
- "Vulnerable position": Arm in flexion, adduction, and internal rotation at time of injury [1][5]
- Acute onset of severe shoulder pain with inability to move the arm, particularly inability to externally rotate
- Ask about witnessed seizure activity, loss of consciousness, alcohol/drug use, history of epilepsy
- Bilateral posterior dislocations are classic for seizure or electrocution — always ask about the contralateral shoulder [2][6]
- Prior instability episodes, hand dominance, sport/occupation, prior shoulder surgery [5]
- Important negatives: No fall on outstretched hand (more typical of anterior dislocation), no direct blow to anterior shoulder
2. Alarm Features
- Bilateral shoulder dislocation → pathognomonic for seizure or electrocution until proven otherwise [2][7]
- Neurovascular compromise: absent distal pulses, axillary nerve deficit (lateral deltoid numbness, deltoid weakness) [5]
- Associated fracture-dislocation (proximal humerus, glenoid) — may preclude closed reduction [8]
- Locked dislocation with large reverse Hill-Sachs lesion (>25–40% articular surface) — requires operative management [8-9]
- Concomitant cervical spine or brachial plexus injury [5]
- Signs of compartment syndrome in the arm (rare)
3. Medications
- Pre-reduction analgesia: Intra-articular lidocaine (1% lidocaine, 10–20 mL injected into the glenohumeral joint) is effective and may avoid procedural sedation [4][10]
- Procedural sedation: Propofol, ketamine, midazolam/fentanyl per institutional protocol if intra-articular block insufficient
- Post-reduction pain management: Acetaminophen and short-course NSAIDs first-line; opioids should be used sparingly [5]
- Seizure-related: If seizure etiology, evaluate need for anticonvulsant initiation — AED treatment has a protective effect against recurrent dislocation even if seizure-freedom is not achieved [7]
- Medications to consider as contributors: Antipsychotics, medications lowering seizure threshold, alcohol withdrawal
4. Diet
- No specific dietary considerations in the acute setting
- Adequate calcium and vitamin D for bone health in patients with associated fractures
- Hydration important if procedural sedation is planned (NPO considerations)
5. Review of Systems
- Neurologic: Seizure history, witnessed convulsions, tongue biting, incontinence, postictal confusion, headache, focal deficits
- Musculoskeletal: Contralateral shoulder pain (bilateral dislocation), neck pain, arm numbness/tingling
- Vascular: Hand color changes, coolness, weakness
- Psychiatric: Substance use (alcohol withdrawal seizures), medication compliance
- Constitutional: Fever (infection-related seizure), weight loss
6. Collateral History and Family History
- Witnesses to the event — was there a seizure, fall, or trauma?
- History of epilepsy, prior seizures, medication compliance with AEDs
- Family history of epilepsy or connective tissue disorders (Ehlers-Danlos, Marfan) predisposing to joint laxity
- Social history: Alcohol use (withdrawal seizures), recreational drug use (cocaine, synthetic cannabinoids), occupational exposures (electrical work)
- Prior shoulder instability events in the patient or family [5]
7. Risk Factors
- Seizure disorders (most common non-traumatic cause, 34–38% of cases) [2][11]
- Electric shock / electrocution [4][12]
- Contact/collision sports — especially American football linemen, rugby [5][13]
- Male sex (predominant) [2][14]
- Age 20–49 years (peak incidence) [14]
- Generalized ligamentous laxity / multidirectional instability [5]
- Increased glenoid retroversion (anatomic predisposition) [15]
- Prior posterior dislocation or instability
- Alcohol use disorder (seizure risk)
8. Differential Diagnosis
- Anterior shoulder dislocation — far more common; arm held in abduction and external rotation (vs. internal rotation in posterior); distinguish with axillary view [4]
- Proximal humerus fracture — may coexist; assess on radiographs
- Rotator cuff tear (acute massive) — inability to move arm but no dislocation on imaging
- Acromioclavicular joint separation — different location of tenderness and deformity
- Adhesive capsulitis — chronic presentation, no acute trauma
- Posterior labral tear without dislocation — more insidious onset, provocative tests positive [13]
- Cervical radiculopathy — referred shoulder pain with neck involvement
- Fracture-dislocation — must be identified as it changes management (may require open reduction) [8]
9. Past Medical History
- Epilepsy or seizure disorder (most important)
- Prior shoulder dislocations or instability (recurrence risk: 17.7% within first year) [14]
- Prior shoulder surgery (Bankart repair, capsulorrhaphy)
- Connective tissue disorders (Ehlers-Danlos, Marfan)
- Psychiatric conditions (ECT history, medication-induced seizures)
- Chronic alcohol use
- Osteoporosis (fracture risk)
10. Physical Exam
- Inspection: Arm held in adduction and internal rotation; flattening of the anterior shoulder; posterior fullness; coracoid process may be prominent anteriorly
- ROM: Loss of external rotation is the hallmark finding — the patient cannot externally rotate past neutral. Forward flexion and abduction are also limited [1][8]
- Palpation: Humeral head palpable posteriorly; anterior shoulder feels "empty"
- Neurovascular exam (mandatory pre- and post-reduction): Axillary nerve (lateral deltoid sensation, deltoid strength), distal pulses, hand sensation and motor function [5]
- Provocative tests (in subacute/chronic setting): Kim test, Jerk test — reproduce posterior pain/clunk [13][16]
- Contralateral shoulder: Always examine for bilateral dislocation
- Beighton score: Assess for generalized ligamentous laxity if multidirectional instability suspected [5]
11. Lab Studies
- Labs are generally not required for isolated posterior shoulder dislocation
- If seizure-related:
- BMP (electrolytes — hyponatremia, hypocalcemia, hypomagnesemia as seizure triggers)
- Glucose
- AED levels (if on anticonvulsants)
- Urine drug screen, blood alcohol level
- CK/myoglobin if prolonged seizure (rhabdomyolysis risk)
- Prolactin (if seizure vs. pseudoseizure distinction needed)
- CBC, coagulation studies if operative intervention anticipated
12. Imaging
- First-line: Plain radiographs — minimum AP + axillary lateral (or scapular Y) views [5][17]
- AP view alone misses posterior dislocations frequently — the "lightbulb sign" (internally rotated humeral head) and loss of the normal half-moon overlap are subtle clues
- Scapular Y view is the most clinically significant single view for diagnosis [1]
- Axillary or Velpeau view definitively shows posterior displacement [4][17]
- CT scan: Indicated for suspected fracture-dislocation, glenoid bone loss quantification, reverse Hill-Sachs lesion sizing, and surgical planning [5][18]
- MRI: Evaluates soft tissue — posterior labral tears (58%), rotator cuff tears (19%), reverse Hill-Sachs lesions (86%), biceps pathology [19]
- Point-of-care ultrasound: Emerging tool; can diagnose posterior dislocation when radiographs are non-diagnostic and confirm reduction in real time [10][12]
- When imaging is unnecessary: Prereduction imaging is not essential in recurrent dislocators with a clear mechanism on the field; postreduction films are always needed for first-time dislocations [5]
13. Special Tests
- Kim test: Axial load applied to the elbow with the arm at 90° abduction; positive if posterior pain or clunk [13][16]
- Jerk test: Arm at 90° forward flexion and internal rotation; axial load applied with horizontal adduction; positive if posterior clunk or apprehension [13][16]
- Load and shift test: Assesses degree of posterior humeral head translation [5]
- Posterior drawer test: Posterior translation of humeral head on glenoid [5]
- Beighton score: Generalized ligamentous laxity screening [5]
- Point-of-care ultrasound: Posterior approach — assess humeral head position relative to glenoid rim [10][12]
14. ECG
- Indicated if seizure etiology is suspected — rule out cardiac causes of syncope mimicking seizure (long QT syndrome, Brugada, arrhythmias)
- Indicated if electrocution — assess for arrhythmias, ST changes, conduction abnormalities
- No specific ECG findings from the dislocation itself
- Continuous cardiac monitoring if electrocution mechanism
15. Assessment
Posterior shoulder dislocation is a rare but frequently missed orthopedic emergency. The prevalence is approximately 1.1 per 100,000 population per year. [14] The classic triad of seizure/electrocution mechanism, arm locked in internal rotation, and inability to externally rotate should raise immediate suspicion. A majority of dislocations (65%) have associated injuries — fractures are most common, followed by reverse Hill-Sachs lesions (86%) and rotator cuff tears (~20%). [11][19] In the absence of fracture or reverse Hill-Sachs injury, the risk of rotator cuff tear increases nearly fivefold (OR 4.6). [11]
Key complications include:
- Recurrent instability (17.7% within the first year; risk factors: age <40, seizure etiology, large reverse Hill-Sachs >1.5 cm³) [14]
- Chronic locked dislocation if diagnosis is delayed (reported delays up to 25 years) [2]
- AVN of the humeral head
- Post-traumatic arthritis
16. Treatment Plan
Initial stabilization:
- Neurovascular assessment before any reduction attempt [5]
- Analgesia: Intra-articular lidocaine or procedural sedation [4][10]
Closed reduction technique for posterior dislocation:
- Apply axial traction to the arm in adduction and internal rotation
- Apply gentle anterior-directed pressure on the posterior humeral head
- Slowly externally rotate the arm while maintaining traction
- A "clunk" indicates successful reduction
- Early reduction before muscular spasm develops improves success [5]
- Ultrasound-guided reduction is an emerging option [10][12]
Post-reduction:
- Repeat neurovascular exam [5]
- Postreduction radiographs (AP + axillary) to confirm concentric reduction and rule out fracture [5][17]
- Immobilization in a sling (neutral or slight external rotation) for 2–4 weeks [4-5]
- Acetaminophen + short-course NSAIDs for pain; avoid opioids if possible [5]
- Physical therapy: Progress from ROM → scapular stabilization → rotator cuff strengthening → sport-specific conditioning [5]
Operative indications:
- Failed closed reduction / locked dislocation [8-9]
- Large reverse Hill-Sachs lesion (engaging) [20-21]
- Associated displaced fracture (reverse bony Bankart, proximal humerus fracture-dislocation) [8]
- Recurrent posterior instability refractory to conservative management [13][16]
- Surgical options: Arthroscopic posterior capsulolabral repair, modified McLaughlin procedure (subscapularis remplissage), bone block procedures, arthroplasty for chronic cases with arthrosis [9][13][20][22]
17. Disposition
- Discharge criteria: Successful closed reduction confirmed on radiographs, intact neurovascular exam, adequate pain control, reliable follow-up, no associated fracture requiring operative fixation
- Admission criteria: Failed reduction, fracture-dislocation requiring operative management, neurovascular compromise, seizure requiring workup/monitoring, bilateral dislocations
- Observation: Consider for patients with new-onset seizure requiring monitoring and neurologic workup
- Specialist consultation triggers:
- Orthopedic surgery: All first-time posterior dislocations (for follow-up), failed closed reduction, fracture-dislocations, large reverse Hill-Sachs lesions, recurrent instability [4]
- Neurology: New-onset seizure, uncontrolled epilepsy [7]
- Immediate orthopedic referral for posterior sternoclavicular dislocation [4]
18. Follow Up / Return Precautions
- Follow-up: Orthopedic follow-up within 1–2 weeks for all first-time posterior dislocations; earlier if any concern for instability or associated injury [5]
- Advanced imaging: MRI should be considered at follow-up to evaluate for labral tears, rotator cuff injury, and reverse Hill-Sachs lesion, especially in young/active patients [5][19]
- Return precautions — instruct patients to return immediately for:
- Recurrent dislocation or sensation of shoulder "slipping out"
- New numbness, tingling, or weakness in the arm/hand
- Worsening pain despite immobilization
- Inability to move fingers or hand color changes
- Expected recovery: Most patients have persistent mild functional deficits at 2 years. Athletes may return to play when pain-free with symmetric ROM and sport-specific function, typically 3–6 weeks for uncomplicated cases [4][14]
- Seizure patients: AED treatment should be discussed even after a first seizure given the protective effect against recurrent dislocation injury; neurology follow-up is essential [7]
- Recurrence risk: Highest in patients <40 years, seizure-related dislocations, and large reverse Hill-Sachs lesions — these patients warrant close surveillance and early surgical consultation [14]
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