Rim Sign (Hip and Shoulder) | Radiology Signs

Answer:

Rim sign is used for two distinct musculoskeletal findings and the site must always be stated. In the hip, the rim sign is a high-signal fluid line sandwiched between two low-signal sclerotic lines at the margin of a subchondral osteochondral fragment in femoral head osteonecrosis, representing joint fluid that has dissected into the plane of a subchondral fracture; unlike the double line sign, which marks the stable reactive interface between necrotic and viable bone, the rim sign implies mechanical instability of the fragment and correlates with impending or established collapse, so the two signs should not be used interchangeably. In the shoulder, the rim sign describes a rim-rent tear (partial articular surface tendon avulsion, or PASTA lesion), a partial-thickness, articular-sided tear at the bony footprint of the rotator cuff, most often the anterior-most fibres of supraspinatus or the infraspinatus insertion, seen on MR arthrography or fluid-sensitive MRI as a thin linear fluid signal undermining the tendon at its attachment to the greater tuberosity. Because the tear is confined to the footprint rim and is easily obscured by anisotropy artefact and the normal convergence of supraspinatus and infraspinatus fibres, it is commonly overlooked and can progress to a full-thickness tear if untreated. The term is ambiguous and also overlaps with the soft tissue rim sign of a ureteric calculus and the hepatobiliary rim sign, so reports should describe the finding rather than rely on the eponym alone.

Why is it called so?

In the hip a rim of fluid signal outlines an unstable osteochondral fragment; in the shoulder the tear sits at the rim of the tendon’s bony footprint, hence a rim-rent.

Pathophysiology

In osteonecrosis, once a subchondral fracture develops, joint fluid tracks along the fracture plane between the sclerotic necrotic segment and the adjacent bone, producing a bright fluid line flanked by two dark sclerotic margins; because fluid, rather than granulation tissue, separates the fragment, its presence signals a mobile or separating segment rather than simple ischaemic demarcation, which is what distinguishes it mechanistically from the double line sign, a reparative vascular interface at a still-adherent necrotic-viable junction. In the shoulder, the anterior-most fibres of supraspinatus insert onto a narrow footprint at the greater tuberosity; repetitive tensile and shear stress at this insertional rim causes the articular-sided fibres to peel away from bone while the bursal-sided fibres remain intact, so contrast or joint fluid undermines the tendon locally without crossing its full thickness, which is why the defect is confined to a thin linear rent at the bony rim rather than producing a communicating full-thickness tear.

Alternative names: Rim sign of osteonecrosis (hip); rim-rent tear or PASTA lesion (shoulder)

Other associated named signs: Crescent sign and the bite sign of femoral head collapse, and the geyser sign of a chronic massive cuff tear

References

1. Vinson EN, Helms CA, Higgins LD. Rim-rent tear of the rotator cuff: a common and easily overlooked partial tear. AJR Am J Roentgenol. 2007;189(4):943-6. PMID: 17885069.