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# Shoulder and elbow X-rays: dislocation and the fat pad sign

- Reading

Two views at right angles show whether the shoulder is out of joint; at the elbow, a dark strip of fat pushed up by fluid can betray a hidden fracture.

Updated Nov 13, 2026

## Key points

- Shoulder: never one view; most dislocations are anterior, posterior ones are missed on the AP.
- «Light bulb» head and a wide glenohumeral gap: posterior dislocation (seizure, electric shock).
- Visible posterior fat pad = elbow effusion = occult fracture (radial head in adults, supracondylar in children).
- Anterior humeral line through the middle third of the capitellum; radiocapitellar line through the capitellum always.

## What it is

A shoulder series is an AP view plus at least one view at right angles to it: the scapular Y view or the axillary view. One view is never enough, because a posterior dislocation can look almost normal on the AP. Anterior dislocation is by far the commonest: the humeral head sits in front of and below the glenoid, under the coracoid. The elbow series is an AP view and a true lateral with the elbow bent to 90 degrees. On the lateral you look at the fat pads, the anterior humeral line and the radiocapitellar line; in a child you also check the ossification centres in their expected order. A joint effusion after trauma with no fracture line visible is treated as a hidden fracture.

## What to check

- Shoulder: an AP view and a scapular Y or axillary view; the head should sit centred on the glenoid in both.
- Anterior dislocation: head below the coracoid on the AP and in front of the glenoid on the Y view.
- Hill–Sachs lesion: a notch on the back and outer side of the humeral head; Bankart lesion: injury to the front and lower glenoid rim or labrum.
- Posterior dislocation on the AP: «light bulb» head (fixed internal rotation), widened gap between head and glenoid (rim sign) and loss of the normal overlap.
- Elbow lateral: a thin anterior fat pad is normal; a raised anterior pad («sail») or any visible posterior fat pad means an effusion.
- Anterior humeral line should cross the middle third of the capitellum; the radiocapitellar line should pass through the capitellum in every view.

## Reading it step by step (and why)

- Check you have two views at right angles and a true lateral elbow at 90 degrees — a rotated film hides fat pads and misplaces lines.
- Shoulder AP: is the head overlapping the glenoid like a half-moon? — loss of overlap or a wide gap suggests posterior dislocation.
- Scapular Y or axillary view: is the head centred on the glenoid? — in front means anterior, behind means posterior dislocation.
- Look for fractures around the joint: greater tuberosity, glenoid rim, humeral neck, clavicle — they change the reduction plan.
- After reduction, repeat the films — to confirm the joint is back in place and to look for fractures the dislocation hid.
- Elbow lateral: look for the posterior fat pad and a raised anterior pad — an effusion after trauma means a fracture until proved otherwise.
- Draw the anterior humeral line — if it passes in front of the capitellum, think of an extension supracondylar fracture.
- Draw the radiocapitellar line on both views — if it misses the capitellum, the radial head is dislocated (look for a Monteggia ulnar fracture).
- In a child, check the ossification centres in CRITOE order — a centre present out of order may be an avulsed medial epicondyle in the joint.

## Common mistakes

- Relying on the AP view alone misses posterior dislocation, classically after a seizure or an electric shock.
- Calling a visible posterior fat pad normal: it is never seen in a normal elbow.
- Not looking at the radiocapitellar line in a forearm fracture misses the radial head dislocation of a Monteggia injury.
- Forgetting the neurovascular exam: axillary nerve after shoulder dislocation; brachial artery and median nerve after a supracondylar fracture.

## Next step

- Suspected posterior dislocation with a doubtful AP: ask for an axillary or scapular Y view, or CT if the views cannot be obtained.
- Effusion without a visible fracture: splint as a fracture and repeat the X-ray in about 7 to 10 days.
- Displaced supracondylar fracture, a pale or pulseless hand, or a nerve deficit: call orthopaedics urgently.
- Fracture-dislocation, a glenoid rim fracture or a failed reduction: CT and orthopaedic referral.
- Recurrent dislocation or suspected labral or rotator cuff tear: MRI (or MR arthrography) planned with the specialist.

## Memory aid

CRITOE, the order in which the elbow ossification centres appear (roughly at 1, 3, 5, 7, 9 and 11 years): Capitellum, Radial head, Internal (medial) epicondyle, Trochlea, Olecranon, External (lateral) epicondyle.

## Test yourself

Think of the answer, then tap the question to see it.

## Which shoulder dislocation is the commonest, and where does the humeral head lie?

Anterior dislocation: the head lies in front of and below the glenoid, under the coracoid.

## What signs on the AP view suggest a posterior shoulder dislocation?

A round «light bulb» head from fixed internal rotation, a widened gap between head and glenoid, and loss of the normal half-moon overlap.

## What does a visible posterior fat pad on a lateral elbow X-ray after trauma mean?

A joint effusion, which is treated as a hidden fracture: the radial head in adults, the supracondylar humerus in children.

## Where should the anterior humeral line cross the capitellum on a normal lateral elbow?

Through its middle third. If it passes in front of the capitellum, an extension supracondylar fracture is likely.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/radiology/shoulder-and-elbow-x-rays-dislocation-and-the-fat-pad-sign/)

## Elsewhere in DurgiPedia

- [Shoulder tests: Neer, Hawkins, Jobe and drop arm](https://durgical.com/en/clinical-exam/shoulder-tests-neer-hawkins-jobe-and-drop-arm/)
- [Joint examination: inspection, palpation and range of motion](https://durgical.com/en/clinical-exam/joint-examination-inspection-palpation-and-range-of-motion/)

## Connections

## Related 5

- [Describing a fracture on X-ray](https://durgical.com/en/radiology/describing-a-fracture-on-x-ray/)
- [Fractures in children: buckle, greenstick and the growth plate (Salter–Harris)](https://durgical.com/en/radiology/fractures-in-children-buckle-greenstick-and-the-growth-plate/)
- [Wrist and hand X-rays: distal radius and scaphoid](https://durgical.com/en/radiology/wrist-and-hand-x-rays-distal-radius-and-scaphoid/)
- [MRI: T1, T2, FLAIR and diffusion](https://durgical.com/en/radiology/mri-t1-t2-flair-and-diffusion/)
- [Imaging when abuse is suspected: the skeletal survey](https://durgical.com/en/radiology/imaging-when-abuse-is-suspected-the-skeletal-survey/)

## Related conditions 5

- [Shoulder dislocation](https://durgical.com/en/conditions/shoulder-dislocation/)
- [Proximal humerus fracture](https://durgical.com/en/conditions/proximal-humerus-fracture/)
- [Clavicle fracture](https://durgical.com/en/conditions/clavicle-fracture/)
- [Elbow dislocation](https://durgical.com/en/conditions/elbow-dislocation/)
- [Supracondylar humerus fracture](https://durgical.com/en/conditions/supracondylar-humerus-fracture/)

[← All of radiology](https://durgical.com/en/radiology/)

Educational content; it does not replace a radiologist's report and is not a clinical instruction. Each study is ordered on the patient's assessment and your institution's protocols.

Sources: [Shoulder Dislocations Overview](https://www.ncbi.nlm.nih.gov/books/NBK459125/) (StatPearls (NCBI Bookshelf), 2023); [Posterior Shoulder Dislocations](https://www.ncbi.nlm.nih.gov/books/NBK441919/) (StatPearls (NCBI Bookshelf), 2022); [Elbow Fractures Overview](https://www.ncbi.nlm.nih.gov/books/NBK441976/) (StatPearls (NCBI Bookshelf), 2025); [Supracondylar Humerus Fractures](https://www.ncbi.nlm.nih.gov/books/NBK560933/) (StatPearls (NCBI Bookshelf), 2023).

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/radiology/shoulder-and-elbow-x-rays-dislocation-and-the-fat-pad-sign/