Light bulb sign | Radiology Signs

Answer:

The light bulb sign is a humeral head that appears symmetrically round on the AP shoulder radiograph, because the arm is locked in fixed internal rotation and the greater tuberosity has rotated out of profile, removing the normal asymmetric lateral bulge. It indicates posterior shoulder dislocation, an injury missed on the frontal film in up to half of cases because the head still appears to sit at the glenoid. It should be sought with the other signs: the rim sign, widening of the glenohumeral joint space beyond about 6 mm; the trough line, a dense vertical line on the anteromedial humeral head representing the reverse Hill-Sachs impaction; and loss of the normal half-moon overlap of the head and glenoid. An axillary or scapular Y view, or CT, confirms the diagnosis. Classic settings are seizure, electrocution and major trauma. A separate light bulb sign exists in the liver, where a haemangioma is markedly and homogeneously hyperintense on heavily T2-weighted MRI, as bright as fluid.

Why is it called so?

The rounded, symmetric humeral head with no tuberosity profile looks like the glass envelope of an electric light bulb.

Pathophysiology

In the normal anatomical position the greater tuberosity projects laterally and gives the humeral head an asymmetric, tapered outline on the AP view. Posterior dislocation drives the head behind the glenoid and locks the arm in marked internal rotation, usually because the anteromedial head is impacted on the posterior glenoid rim. That rotation swings the greater tuberosity anteriorly out of the tangential plane of the beam, so only the smooth articular contour of the head is projected and it appears as a symmetric circle. The same impaction against the posterior glenoid rim produces the trough line.

Alternative names: Light bulb appearance, drumstick sign, ice cream cone sign (of the same appearance)

Other associated named signs: Rim sign, trough line sign, reverse Hill-Sachs lesion, and the Hill-Sachs deformity of anterior dislocation

References

1. Koutserimpas C, Piagkou M, Karaiskos I, Chronopoulos E, Arkoudis NA. Posterior Dislocation of the Shoulder: The Light-Bulb Sign. Cureus. 2023;15(10):e47800. PMID: 38022099.