Asterisk sign | Radiology Signs

Answer:

The asterisk sign refers to the normal star-shaped pattern of condensed weight-bearing trabeculae in the femoral head, seen on a frog-leg lateral radiograph, on tomography or on coronal CT, and to its distortion in disease. In osteonecrosis the limbs of the star become thickened, irregular and blurred as necrotic trabeculae are resorbed and reinforced by creeping substitution, and this alteration was historically used as an early indicator before MRI was available. The sign is now of limited practical value because it is insensitive and observer-dependent, and MRI is the standard for early diagnosis, showing the serpiginous demarcating band and the double line sign long before trabecular architecture changes. It remains a useful illustration of the mechanics of the proximal femur, in which the principal compressive and tensile trabecular groups intersect and their relative density defines Singh’s index of osteoporosis, a grading that falls as the secondary trabecular groups disappear. When trabecular distortion is noticed, the report should proceed to the findings that actually change management: the size and location of the necrotic segment, the presence of a subchondral fracture giving the crescent sign, contour flattening, and acetabular involvement, which together determine ARCO stage and the choice between core decompression, osteotomy and arthroplasty.

Why is it called so?

The intersecting trabecular groups of the femoral head form the shape of a star or asterisk.

Pathophysiology

The proximal femur transmits body weight through an eccentric neck, so trabecular bone is laid down along the lines of principal stress described by Wolff’s law. The principal compressive group runs from the medial cortex of the neck to the superior weight-bearing dome and the principal tensile group arcs from the lateral cortex across the head, and where these and the secondary groups intersect they form the radiographic star. After infarction, the necrotic segment loses its osteocytes but retains its trabecular scaffold initially; during repair, osteoclasts resorb dead trabeculae while osteoblasts apply new bone to others, so some limbs of the star are thinned and others thickened. The resulting asymmetry blurs the pattern, and once the subchondral plate fails the architecture collapses altogether.

Alternative names: Star sign; trabecular star of the femoral head

Other associated named signs: Crescent sign, the double line sign on MRI and Singh index grading of trabecular loss