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 lateral, overhanging margin of the deformed femoral head projected over the neck and indicates that a severe growth disturbance of the proximal femoral epiphysis and 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. As it reossifies the femoral head remodels into an enlarged, flattened shape, part of which comes to overhang the femoral neck anterolaterally beyond the true weight-bearing segment, while the neck itself is also shortened and widened by the associated physeal damage. The lower edge of this overhanging portion of the head is projected tangentially across the femoral neck on the frontal radiograph and casts a dense curved line; early authors mistook this for a condensation within the metaphysis itself, but Clarke’s critical appraisal and later three-dimensional CT confirmed the line is a shadow of the head margin, not a metaphyseal reaction. The same explanation applies in achondroplasia, where a spherical but circumferentially overhanging head of different origin, defective endochondral ossification rather than necrosis, produces an identical projection, which is why the sign indicates a growth disturbance of the head 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.