Serpiginous tract sign | Radiology Signs

Answer:

The serpiginous tract sign is a wandering, snake-like line of low T1 signal with a corresponding high signal inner line on T2-weighted images, demarcating a geographic area of medullary bone. It is the classic appearance of a bone infarct, that is, medullary osteonecrosis, most often in the distal femoral and proximal tibial metaphyses and in the humerus. The line is the reactive interface between necrotic and viable marrow, and the paired T2 appearance is the same double line sign seen at the margin of femoral head osteonecrosis. Inside the tract the marrow signal is often preserved and follows fat, which is a helpful discriminator from tumour. The principal differential is chondroid tumour, enchondroma or low-grade chondrosarcoma, which shows lobulated T2 hyperintense matrix with ring-and-arc enhancement and rings-and-arcs calcification on CT, and osteomyelitis, which shows diffuse marrow oedema, cortical destruction, periosteal reaction, an abscess with a penumbra sign and a sinus tract. Causes to consider are corticosteroid therapy, alcohol, sickle cell disease, dysbarism, radiation, pancreatitis, Gaucher disease, systemic lupus and vasculitis, and a search for a cause is warranted when infarcts are multiple. Old infarcts calcify densely along the tract on radiographs, and malignant transformation to a sarcoma is rare but recognised.

Why is it called so?

The demarcating line winds through the marrow like a snake, giving a serpentine or serpiginous outline.

Pathophysiology

When medullary blood supply is interrupted, the fatty marrow and the osteocytes within the affected region die. At the boundary between dead and living tissue the body mounts a repair response: hyperaemic granulation tissue with abundant free water forms on the viable side and a rim of reactive woven and sclerotic bone forms as osteoblasts lay new bone on dead trabeculae. The dense mineralised rim has few mobile protons and is dark on all sequences, while the vascular granulation layer immediately inside it is bright on T2, generating the double line. Because repair advances irregularly around the necrotic zone, the interface is not circular but sinuous. Over years the interface mineralises fully and appears as a dense serpiginous calcification on radiographs.

Alternative names: Serpentine sign; double line sign of bone infarct

Other associated named signs: Double line sign in femoral head osteonecrosis and the penumbra sign of subacute osteomyelitis

References

1. Umans H, Haramati N, Flusser G. The diagnostic role of gadolinium enhanced MRI in distinguishing between acute medullary bone infarct and osteomyelitis. Magn Reson Imaging. 2000;18(3):255-62. PMID: 10745133.