Rim sign (musculoskeletal) | 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 refers to a low signal band demarcating a necrotic subchondral segment of the femoral head on MRI, corresponding to the reactive interface between dead and living bone; on T2-weighted images it is the outer line of the double line sign, whose inner high-signal line represents granulation tissue at the repair front. It supports a diagnosis of osteonecrosis and is used with the size and location of the necrotic segment, the presence of a subchondral fracture and any contour collapse to stage the disease and estimate the risk of progression. In the shoulder, the rim sign describes contrast or fluid tracking around the humeral head and outlining the undersurface of the acromion on conventional or MR arthrography, indicating a full-thickness rotator cuff tear with communication between the glenohumeral joint and the subacromial-subdeltoid bursa; on non-contrast MRI the equivalent is fluid in the bursa continuous with joint fluid through a tendon defect. Because the term is ambiguous and overlaps with the soft tissue rim sign of a ureteric calculus and the hepatobiliary rim sign, reports should describe the finding rather than rely on the eponym alone.

Why is it called so?

In both applications a rim of altered signal or contrast outlines the abnormality, at the margin of the necrotic segment or around the humeral head.

Pathophysiology

In osteonecrosis, the necrotic segment is separated from viable bone by a reparative interface of granulation tissue, fibrosis and reactive sclerosis. The mineralised sclerotic outer zone has very few mobile protons and appears dark on all sequences, while the inner vascular granulation tissue is bright on T2, which is what generates the paired lines. In rotator cuff disease, chronic degeneration and impingement of the supraspinatus tendon lead to a full-thickness defect; because the bursa lies immediately superficial to the cuff, joint fluid or injected contrast passes freely through the defect and distributes around the humeral head and under the acromion, which is why the appearance is a direct marker of a communicating tear rather than of tendinopathy alone.

Alternative names: Double line sign in the hip; rim sign of rotator cuff tear

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. Ikemura S, Yamamoto T, Motomura G, Nakashima Y, Mawatari T, Iwamoto Y. MRI evaluation of collapsed femoral heads in patients 60 years old or older: Differentiation of subchondral insufficiency fracture from osteonecrosis of the femoral head. AJR Am J Roentgenol. 2010;195(1):W63-8. PMID: 20566783.