What causes trough sign in the humeral head on frontal shoulder radiographs??
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A vertical line adjacent to the medial cortex of the humeral head—the trough line sign—occurs due to a compression fracture of the humeral head caused by posterior glenoid impaction during posterior shoulder dislocation. This trough line sign is an important radiographic marker for posterior shoulder dislocation, which is often subtle and can be easily overlooked on routine anteroposterior (AP) radiographs.
Why is it called so?
The sign is termed the “trough line” because it represents the margin of a trough-like impaction fracture on the antero-medial aspect of the humeral head, resembling the edge of a trough.
Pathophysiology
During posterior shoulder dislocation, the internally rotated humeral head is driven against the posterior glenoid rim, resulting in a compression fracture of the antero-medial humeral head. This creates a vertical fracture margin that is radiographically visible as a dense line parallel to the medial cortex of the humeral head. The outer line is the normal medial cortex, while the inner (trough) line is the edge of the impacted fracture—yielding two parallel vertical lines, a classic feature of this sign.
Alternative names: Trough line sign
Other associated named signs: Reverse Hill-Sachs lesion (the osseous defect corresponding to the trough line on cross-sectional imaging)
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