Posterior Shoulder Dislocation and Luxatio Erecta
On this page
Direct answer
A posterior shoulder dislocation is the humeral head displacing behind the glenoid, classically after a seizure or electric shock that forces internal rotation and adduction. It is easily missed on a single view, and the light bulb appearance of the internally rotated head on an AP radiograph is the key clue. Luxatio erecta is a rare inferior dislocation with the arm held overhead.
What you must remember
- Mechanism: seizure or electric shock produces powerful internal rotation and adduction; a fall onto an adducted, internally rotated arm can do the same.
- Frequency: posterior dislocation is a small minority of shoulder dislocations and is often missed at first presentation.
- Light bulb sign: the internally rotated humeral head appears ovoid on the AP film, with the lesser tuberosity seen in profile, which should prompt a second view.
- Trough line: a linear impaction of the anteromedial humeral head, a reverse Hill-Sachs lesion, produced when the head strikes the anterior glenoid rim.
- Confirming view: the axillary or scapular Y view confirms posterior displacement of the head.
- Missed dislocation: closed reduction becomes unlikely after about three weeks, so open reduction or arthroplasty is considered.
- Luxatio erecta: an inferior dislocation with the arm abducted overhead and the head lying in the axilla; neurovascular status is checked before reduction by traction.
A stepwise approach to the shoulder that will not move
Posterior dislocation is a diagnosis that depends on looking for it, so the pathway starts with suspicion and ends with reduction and immobilisation. Each step protects the patient from the commonest error, which is a missed injury.
- Suspect from the history: a seizure, electric shock or unusual fall, with the arm held in internal rotation and painful restricted external rotation.
- Confirm on radiographs: the AP film shows the light bulb sign and trough line, and an axillary or Y view confirms posterior displacement.
- Check the neurovascular status: the axillary nerve and vessels are examined before any manipulation is attempted.
- Reduce promptly: closed reduction under sedation uses axial traction with the arm forward flexed and gently externally rotated.
- Immobilise in external rotation: a sling is worn with the arm held in slight external rotation for a period of protection.
- Treat the neglected case: beyond about three weeks, open reduction or arthroplasty is weighed against the age and function of the patient.
The exam tests recognition more than the procedure. The light bulb and trough line are the two images most worth remembering.
Where the exam tries to catch you
A posterior dislocation is a classic example of a sign that candidates overlook.
- Trap: missing the injury on one AP film because the shoulder looks almost normal Correct: look for the light bulb sign and obtain an axillary or Y view.
- Trap: attempting late reduction of a dislocation that is weeks old Correct: after about three weeks closed reduction is unlikely, and open or arthroplasty options apply.
- Trap: forcing external rotation on a locked posterior dislocation Correct: the head must first be disimpacted from the glenoid rim by gentle traction, as forced rotation can fracture the head.
Frequently asked questions
Which view best shows a posterior shoulder dislocation?
The AP view shows the light bulb sign and the trough line, while the axillary or scapular Y view confirms posterior displacement of the humeral head. These signs are easy to miss on a single standard shoulder film.
What is a trough line in posterior shoulder dislocation?
A trough line is a linear impaction of the anteromedial humeral head, a reverse Hill-Sachs lesion created when the head strikes the anterior glenoid rim. It is seen on the AP view and supports a diagnosis of posterior dislocation.
How is luxatio erecta different from other shoulder dislocations?
Luxatio erecta is an inferior dislocation in which the arm is held overhead and the humeral head lies beneath the glenoid in the axilla. It is rare, and the neurovascular bundle must be checked before reduction by traction and gentle manipulation.
What is the management of a posterior dislocation missed for several weeks?
Closed reduction becomes unlikely after about three weeks, so surgeons choose open reduction or arthroplasty, such as hemiarthroplasty, depending on the articular damage. The decision weighs age, function and the condition of the humeral head.