Posterior Shoulder Dislocation

Posterior shoulder dislocation is displacement of the humeral head behind the glenoid. It accounts for 2 to 4% of shoulder dislocations. It is missed at first presentation in up to 50% of cases, because the anteroposterior radiograph can look nearly normal.

Causes are seizures and electric shock (strong internal rotators overpower the weaker external rotators), alcohol intoxication, a direct blow from the front, a fall on an outstretched adducted and internally rotated arm, and sports injuries such as rugby and football. Bilateral dislocations are almost always from seizures or electrocution.

The injury includes a reverse Hill-Sachs lesion (anteromedial humeral head impaction, the "trough sign"), a reverse Bankart lesion (posterior labral tear), a posterior glenoid rim fracture, lesser tuberosity fracture, and rotator cuff tear. A reverse Hill-Sachs lesion of over 25 to 40% of the head surface makes the dislocation unstable.

Complications are chronic locked dislocation (missed for over 3 weeks), recurrent posterior instability, avascular necrosis of the humeral head, arthritis, stiffness, and nerve injury (axillary).

Pathophysiology.

Contraction of the subscapularis, pectoralis major, latissimus dorsi and teres major drives the head backward over the posterior glenoid rim when the arm is internally rotated and adducted. The head locks on the rim, and impaction of its anterior surface produces the reverse Hill-Sachs defect.

The arm is held adducted and internally rotated, with the shoulder looking flattened in front and prominent behind. External rotation is almost completely blocked (less than 0 to 10 degrees, which is the key sign), and forward elevation is limited to under 90 degrees. The coracoid process is prominent. Pain may be mild.

Ask about seizures, electric shock, alcohol use and previous episodes. Examine axillary nerve function, the radial pulse and the rest of the limb.

• Radiographs: anteroposterior (looks like a normal joint with an absent glenohumeral overlap, the "light bulb sign" of an internally rotated head, the "trough line" sign and "vacant glenoid"), axillary lateral (the definitive view, shows the head behind the glenoid) and scapular Y view.
• CT defines the reverse Hill-Sachs defect size, glenoid fractures and the position of the head. It is the investigation of choice before treatment.
• MRI for the posterior labrum, rotator cuff and capsule, and for avascular necrosis.
• Electroencephalography, calcium, magnesium, glucose and a toxicology screen when the cause is a seizure.

Differential diagnosis.

• Anterior shoulder dislocation and proximal humerus fracture.
• Frozen shoulder and acromioclavicular joint separation.
• Rotator cuff tear and brachial plexus injury.
• Luxatio erecta.

Acute dislocation (under 3 weeks) with reverse Hill-Sachs lesion under 25% of the head.

• Closed reduction under general anaesthesia or sedation with muscle relaxation: apply traction in line with the arm with the arm in slight forward flexion, then rotate it to external rotation while gently lifting the head from behind with the thumb. Do not force external rotation, because it can fracture a locked head.
• Immobilise in a sling or spica with the arm in neutral to slight external rotation for 3 to 6 weeks, then physiotherapy to strengthen the external rotators and posterior cuff.

Locked dislocation, or defect over 25%.

• Defect 25% to 40%: McLaughlin procedure (subscapularis transfer into the defect), or the modified McLaughlin (lesser tuberosity transfer), or a bone graft, or the reverse remplissage.
• Defect over 40%, or locked for over 6 months in a young patient: hemiarthroplasty or total shoulder replacement.
• Older patients with a long standing locked dislocation with little pain and low demand may be treated without surgery.

Recurrent instability. Posterior capsulolabral repair (arthroscopic), posterior bone block, or glenoid osteotomy for glenoid retroversion.

Seizure disorders. Treat the cause: optimise anti-seizure treatment with a neurologist and exclude electrical injury, metabolic causes and alcohol withdrawal.

Reduction under sedation. Ketamine 1 to 2 mg/kg intravenously over 1 to 2 minutes as a single dose (a further 0.5 to 1 mg/kg after 10 minutes if needed; 4 to 5 mg/kg intramuscularly without access), given by a clinician skilled in airway care with continuous oximetry and capnography, after fasting where time allows. Avoid with raised intracranial pressure, psychosis or acute upper airway infection. General anaesthesia with a muscle relaxant gives the best conditions for a locked or difficult reduction.

Operative antibiotic prophylaxis. Cefazolin 2 g intravenously (3 g when weight is 120 kg or over) within 60 minutes before incision as a single dose (repeat after 4 hours or blood loss over 1500 mL); with severe penicillin allergy, vancomycin 15 mg/kg intravenously starting 90 to 120 minutes before incision as a single dose, with levels and creatinine checked in prolonged cases.

Analgesia.

• Paracetamol 1 g orally every 6 hours as needed for up to 5 to 7 days (maximum 4 g daily; 3 g daily when weight is under 50 kg, with chronic alcohol use or hepatic impairment).
• Ibuprofen 400 mg orally every 8 hours with food for up to 5 days (maximum 1200 mg daily); avoid with eGFR under 30 mL/min/1.73 m2, peptic ulcer, heart failure or anticoagulant use.
• Oral morphine 5 to 10 mg every 4 hours as needed for severe pain for up to 3 days (start 2.5 to 5 mg with age over 75 or eGFR under 30 mL/min/1.73 m2; avoid exceeding 60 mg in 24 hours without senior review); monitor sedation and respiration and prescribe a laxative.

Monitoring. Neurovascular state before and after reduction. Post-reduction radiographs (axillary view) and CT. Radiographs at 1 week, 6 weeks and 3 months to confirm a stable reduction. Range of motion and rotator cuff strength at 6 weeks and 3 months. MRI at 6 to 12 months for avascular necrosis in locked dislocations. Seizure control at each visit.

Referral and escalation. Refer every posterior dislocation to orthopaedics on diagnosis. Emergency referral for an irreducible or locked dislocation, a fracture dislocation, vascular compromise, open injury or a progressive nerve deficit. Neurology referral for first seizure.

Special groups.

• Epilepsy: recurrent posterior dislocation is frequent; stabilise surgically only once seizures are controlled.
• Older adults: osteoporotic bone fractures easily during closed reduction.
• Children: rare; traumatic and sometimes voluntary.
• Kidney impairment: avoid ibuprofen and reduce morphine doses.