The Dog Leg Sign

The dog leg sign, seen on an arteriogram of the popliteal region, refers to an acute bend in the apparent course of the popliteal artery. It is characteristically produced by a mural thrombus associated with aneurysmal dilatation of the popliteal artery.

The dog leg sign is an important secondary sign of popliteal artery aneurysms, because about 25% of popliteal artery aneurysms are not associated with angiographically visible popliteal artery dilatation. In such cases, ultrasound may be used to detect an angiographically occult aneurysm. Depending on the presence of mural thrombus, ultrasound may show a dilated, cystic structure with flow or a complex, cystic structure with flow and areas of echogenic thrombus.

The presence of this sign should also prompt investigation for other aneurysms, since 40% of popliteal aneurysms are associated with abdominal aortic aneurysms, 25% are associated with iliac arterial aneurysms, 35% are associated with femoral arterial aneurysms, and 50% are bilateral.

Other conditions may mimic the dog leg sign by causing deviation (though not typically an acute bend) of the course of the popliteal artery lumen:

• Tortuous and atherosclerotic popliteal artery
• Popliteal arterial entrapment syndrome: Passive dorsiflexion at arteriography will accentuate arterial narrowing.
• Adventitial cystic disease: There is eccentric narrowing of the popliteal artery. Ultrasound will show a cystic structure without flow in the arterial wall.
• Baker cyst.

References

Uppal A. The dog leg sign. Radiology. 2000 Feb;214(2):339-40.