Knuckle sign | Radiology Signs

Answer:

The knuckle sign is abrupt tapering or frank cut-off of a pulmonary artery, with the vessel ending in a rounded, knuckle-like stump at the site of an occluding pulmonary embolus. It was described on the chest radiograph but is far better shown on CT pulmonary angiography, where the vessel is seen to terminate abruptly against the filling defect. It is usually considered alongside the Fleischner sign, dilatation of the central pulmonary artery immediately proximal to the obstruction, with which it frequently coexists; the knuckle describes the calibre change at the point of occlusion while the Fleischner sign describes the enlarged vessel upstream. It is distinct from the Westermark sign (regional oligaemia distal to the obstruction) and from Hampton’s hump (a peripheral pleural-based wedge of infarction). All of these radiographic signs are insensitive and non-specific, and none can be used to diagnose or exclude pulmonary embolism; CTPA remains the diagnostic test.

Why is it called so?

The abruptly truncated, rounded end of the occluded artery resembles a clenched knuckle.

Pathophysiology

A thromboembolus lodging at a pulmonary arterial bifurcation obstructs the lumen and arrests contrast or blood flow at that point. The vessel proximal to the clot remains distended by the column of blood held up behind the obstruction and may enlarge further from acutely raised pulmonary arterial pressure, while the segment beyond it is unopacified and collapses. On projection imaging the transition between the dilated proximal vessel and the unopacified distal one is rendered as a rounded, blunt truncation rather than the smooth progressive tapering of a normal artery.

Alternative names: Knuckle of the pulmonary artery

Other associated named signs: Fleischner sign, Westermark sign, Hampton’s hump, Palla sign, and the polo mint sign and railway track sign on CTPA

References

1. Kumaresh A, Kumar M, Dev B, Gorantla R, Sai PV, Thanasekaraan V. Back to Basics – ‘Must Know’ Classical Signs in Thoracic Radiology. J Clin Imaging Sci. 2015;5:43. PMID: 26312141.