Sagging rope sign | Radiology Signs

Answer:

The sagging rope sign is a thin, curvilinear sclerotic line running across the femoral neck on the frontal radiograph, concave upward, like a rope suspended between two points. It marks the anterior margin of the deformed metaphysis projected over the neck and indicates that a severe growth disturbance of the proximal femoral physis has occurred. It is classically described as a late finding in Legg-Calvé-Perthes disease, where it accompanies coxa magna, coxa breva, a short wide femoral neck, greater trochanteric overgrowth and, in the worst cases, hinge abduction. Importantly, the same sign occurs in achondroplasia and other skeletal dysplasias, where its cause is different and it does not imply previous avascular necrosis, and it is also reported after septic arthritis, slipped capital femoral epiphysis, sickle cell osteonecrosis and treated developmental dysplasia. The distinction is made on the rest of the skeletal survey, since the dysplastic patient has generalised changes. In Perthes the associated Stulberg classification of final femoral head shape at maturity predicts the long-term risk of osteoarthritis, and imaging of the residual deformity, with MRI or CT for hinge abduction and femoroacetabular impingement, guides osteotomy and joint-preserving surgery.

Why is it called so?

The sclerotic line hangs across the femoral neck like a rope sagging between two supports.

Pathophysiology

In Perthes disease the blood supply to the capital femoral epiphysis is interrupted, and the necrotic epiphysis fragments and collapses. Because the physis and the metaphysis are also damaged, growth of the femoral neck is disturbed and the metaphysis remodels into a broad, short and anteriorly extended shape. The anterior cortical margin of this deformed metaphysis is projected end-on through the femoral neck on the frontal radiograph and appears as a dense curved line superimposed on the neck. The same explanation applies in achondroplasia, where a metaphyseal deformity of different origin, defective endochondral ossification rather than necrosis, produces an identical projection, which is why the sign indicates a growth disturbance rather than any specific disease.

Alternative names: Sagging rope; hanging rope sign

Other associated named signs: Crescent sign of subchondral fracture, the Gage sign and the head-at-risk signs of Perthes disease

References

1. Shingade VU, Song HR, Lee SH, Suh SW, Oh CW, Hong JS. The sagging rope sign in achondroplasia–different from Perthes’ disease. Skeletal Radiol. 2006;35(12):923-8. PMID: 16775711.