Shoulder dislocation on X-ray
The glenohumeral joint is the most mobile joint in the body and the one that dislocates most often. Anterior dislocation is usually obvious, both clinically and on the film. The real danger is the posterior dislocation, which can look almost normal on a single AP view and is frequently missed at first presentation. This guide covers the views you need, how to tell anterior from posterior from inferior, the associated fractures to look for before and after reduction, and the pitfalls that catch people out.
1. Views: why one is never enough
A shoulder series for trauma should always include two views at right angles. The usual combination is:
- AP view: either a standard AP of the shoulder or a "true AP" (Grashey) view angled so the beam runs along the glenoid surface. On a true AP the joint space is seen clear, with no overlap of the humeral head and glenoid.
- Scapular Y-view (scapular lateral): the scapula is seen side-on, forming a "Y" shape. The body of the scapula is the stem, and the coracoid (anteriorly) and the acromion/spine (posteriorly) are the two arms. The glenoid sits where the arms meet.
- Axial (axillary) view or a modified axial: the most reliable view for showing whether the head is in front of or behind the glenoid, but it needs some abduction, which an injured patient may not tolerate.
This is the "rule of twos" from the MSK approach applied to the shoulder: an AP alone can show a dislocated head sitting in a position that looks deceptively normal. If you only have one view, say so in your report and ask for the second.
2. The normal relationship
On a standard AP, the humeral head overlaps the glenoid slightly, producing an elliptical area of overlap. The head should sit level with the glenoid, and the inferior cortex of the neck should form a smooth arc with the lateral border of the scapula. On the Y-view, the humeral head should sit over the centre of the "Y", where the glenoid is. On an axial view, the head sits in the glenoid "cup", with the coracoid pointing anteriorly.
Pearl: find the coracoid
On both the Y-view and the axial view, the coracoid points forward. Once you have identified it, you know which way is anterior, and you can say confidently whether the head has gone in front of or behind the glenoid.
3. Anterior dislocation
Anterior dislocation accounts for the large majority of glenohumeral dislocations, commonly quoted as about 95% or more. The usual mechanism is a force applied to the arm while it is abducted and externally rotated.
On the AP
- The humeral head lies medial and inferior to the glenoid, usually beneath the coracoid process (subcoracoid). Less often it sits further inferiorly (subglenoid) or medially (subclavicular).
- The normal overlap with the glenoid is lost or abnormal.
On the Y-view or axial view
- The head lies anterior to the centre of the "Y", on the same side as the coracoid.
Associated injuries
| Injury | What it is | What to look for |
|---|---|---|
| Hill-Sachs lesion | An impaction fracture of the posterolateral humeral head, where it has struck the anterior glenoid rim | A notch or flattening of the posterolateral head, often best seen on an AP in internal rotation, or after reduction |
| Bony Bankart lesion | A fracture of the anteroinferior glenoid rim | A small bone fragment near the inferior glenoid, or an irregular rim. The soft-tissue (labral) Bankart is invisible on X-ray. |
| Greater tuberosity fracture | Avulsion or shear of the greater tuberosity | A fragment lateral to the head. It usually reduces with the dislocation, but must be documented before and after. |
| Proximal humerus fracture | A fracture through the anatomical or surgical neck | Changes management: a fracture-dislocation usually needs senior or orthopaedic input before any reduction attempt |
The axillary nerve is at risk, so check and document sensation over the regimental badge area and deltoid function before and after reduction. This is a clinical check, not a radiographic one.
4. Posterior dislocation
Posterior dislocation is uncommon (about 2–4% of shoulder dislocations), but it matters out of proportion to its frequency because it is so often missed. Classic causes are seizures and electric shock, where violent muscle contraction drives the head backwards, and direct trauma to the front of the shoulder. Bilateral posterior dislocation after a seizure is a recognised pattern, so look at both shoulders.
Clinically the arm is held in internal rotation and the patient cannot externally rotate it. On the AP, the head may be in roughly the right place in the vertical plane, so the film can look nearly normal unless you look for the specific signs.
Signs on the AP
- Lightbulb sign: the head is locked in internal rotation, so the greater and lesser tuberosities are no longer seen in profile and the head looks round and symmetrical, like a lightbulb or an ice-cream cone.
- Rim sign: the gap between the medial border of the humeral head and the anterior glenoid rim is widened. A gap of more than about 6 mm is the usual threshold quoted.
- Trough line sign: one or two vertical sclerotic lines in the medial head, caused by a reverse Hill-Sachs lesion (an impaction fracture of the anteromedial head against the posterior glenoid rim).
- Loss of the normal elliptical overlap of head and glenoid.
Confirming it
The Y-view or axial view shows the head behind the glenoid, on the opposite side from the coracoid. If the views are equivocal and suspicion remains, CT is the definitive test.
Pitfall: a normal-looking AP does not exclude posterior dislocation
A patient who has had a seizure and cannot externally rotate the arm has a posterior dislocation until a second orthogonal view proves otherwise. An AP that "looks fine" is not enough. A normal shoulder simply X-rayed in internal rotation can also produce a lightbulb-like head, so the diagnosis rests on the second view, not on the AP appearance alone.
5. Inferior dislocation (luxatio erecta)
Luxatio erecta is rare. The arm is fixed in abduction, often with the hand above or behind the head. On the AP the humeral head lies below the glenoid, and the shaft of the humerus points upwards and outwards. It is often associated with greater tuberosity fractures, rotator cuff tears and neurovascular injury, particularly to the axillary nerve and artery, so careful distal examination is essential.
6. Pre- and post-reduction films
Practice varies, and some clinicians reduce clinically obvious, recurrent anterior dislocations without a pre-reduction film. Where films are taken, they answer two questions:
- Before reduction: is there a fracture that changes the plan? A fracture of the humeral neck seen with a dislocation is a reason to stop and seek senior or orthopaedic advice, because reduction can displace it.
- After reduction: is the joint reduced on two views, and has a fracture appeared or become displaced? A Hill-Sachs or greater tuberosity fracture is often easier to see once the head is back in place.
Follow your local policy on which films are required. Describe the result in plain terms: "the humeral head is now congruent with the glenoid on AP and Y-views; there is a small Hill-Sachs defect; no other fracture."
7. Pitfalls and mimics
- Pseudosubluxation: after a proximal humerus fracture, a haemarthrosis and muscle atony let the head drift inferiorly. It can look like an inferior dislocation, but the head is still in front of the glenoid on the Y-view and the joint is not truly dislocated. Recognise it and do not attempt a reduction. See shoulder and clavicle fractures.
- Internal rotation of a normal shoulder mimicking the lightbulb sign. The second view resolves it.
- Single view only. The commonest source of a missed posterior dislocation.
- Satisfaction of search: after identifying the dislocation, people forget to look at the glenoid rim, the greater tuberosity, the clavicle, the AC joint, the ribs and the lung apex (a pneumothorax can sit at the edge of a shoulder film). See the chest X-ray guide for checking the apex.
- Old, locked dislocations in older or cognitively impaired patients can present weeks later with a large impaction defect. Ask about the time course.
Key measurements and signs
- Anterior: about 95% or more of dislocations. Head medial and inferior on AP, anterior on Y-view.
- Posterior: about 2–4%. Rim sign gap more than about 6 mm; lightbulb sign; trough line (reverse Hill-Sachs).
- Hill-Sachs: posterolateral head (anterior dislocation). Reverse Hill-Sachs: anteromedial head (posterior dislocation).
Sources: Radiopaedia (posterior shoulder dislocation); Radiology Masterclass shoulder gallery. Figures are approximate and vary between series.
Red flags: act now
Escalate immediately
- Any dislocation with an absent or reduced radial pulse, a cold or pale hand, or an expanding axillary swelling: possible axillary artery injury.
- A dislocation with a fracture of the humeral neck: seek orthopaedic advice before any reduction attempt.
- A seizure or electric shock patient with a painful shoulder held in internal rotation: get a proper second view, and check the other side.
- A neurological deficit, particularly axillary nerve, before or after reduction: document it and refer.
- Luxatio erecta: high rate of neurovascular injury.
What to focus on, by role
| Role | Priorities |
|---|---|
| Medical student | Know that anterior is common and posterior is missed. Learn the lightbulb, rim and trough line signs, and the Hill-Sachs and Bankart lesions. |
| Nurse / ENP | Document neurovascular status, especially the axillary nerve, before and after reduction. Recognise when the second view is missing. |
| Junior doctor | Never reduce from an AP alone if there is any doubt about a fracture. Ask for a Y-view or axial view in every seizure patient with shoulder pain. Check the post-reduction film on two views. |
| Radiographer | A modified axial or Y-view is the key to posterior dislocation. Consider the second shoulder after a seizure. Use preliminary clinical evaluation to flag a fracture-dislocation. |
| Advanced / FRCR | Subtle posterior dislocation, locked chronic dislocation, glenoid rim fragments and the distinction from pseudosubluxation. Practise in rapid reporting mode. |
Practise on real upper limb X-rays →
References
- Radiopaedia. Posterior shoulder dislocation. radiopaedia.org
- Radiology Masterclass. Shoulder X-ray gallery. radiologymasterclass.co.uk
- Radiology Masterclass. Fracture description. radiologymasterclass.co.uk
- RCEM Learning. Emergency medicine learning resources (shoulder injuries). rcemlearning.co.uk
- Royal College of Radiologists. iRefer: making the best use of clinical radiology. irefer.org.uk
For education only. Not a substitute for a formal report, senior review or your local policy. Never use this page to make decisions about a real patient.