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

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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.

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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)

  1. 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.
  2. Shoulder AP: is the head overlapping the glenoid like a half-moon? — loss of overlap or a wide gap suggests posterior dislocation.
  3. Scapular Y or axillary view: is the head centred on the glenoid? — in front means anterior, behind means posterior dislocation.
  4. Look for fractures around the joint: greater tuberosity, glenoid rim, humeral neck, clavicle — they change the reduction plan.
  5. After reduction, repeat the films — to confirm the joint is back in place and to look for fractures the dislocation hid.
  6. Elbow lateral: look for the posterior fat pad and a raised anterior pad — an effusion after trauma means a fracture until proved otherwise.
  7. Draw the anterior humeral line — if it passes in front of the capitellum, think of an extension supracondylar fracture.
  8. Draw the radiocapitellar line on both views — if it misses the capitellum, the radial head is dislocated (look for a Monteggia ulnar fracture).
  9. 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.

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.

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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 (StatPearls (NCBI Bookshelf), 2023); Posterior Shoulder Dislocations (StatPearls (NCBI Bookshelf), 2022); Elbow Fractures Overview (StatPearls (NCBI Bookshelf), 2025); Supracondylar Humerus Fractures (StatPearls (NCBI Bookshelf), 2023).

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