Double Line and Rim Signs

The double line sign (open arrow) is seen at the interface between osteonecrotic bone and normal bone. It consists of a bright T2 line (intermediate T1) on the living sided - representing granulation tissue, next to a dark line (on both T1 an T2) - representing sclerotic bone.
The rim sign should not be confused with the double line sign of osteonecrosis. The rim sign (simple arrow) comprises a high T2 or intermediate T1 signal line sandwiched between two low signal lines, and represents fluid between sclerotic borders of an osteochondral fragment, and implies instability. (Stage III)
Additionally (not shown) is the crescent sign, seen immediately beneath the subchondral bone of the femoral head, as collapse secondary to AVN begins.
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Reference: Bone and Joint imaging - Resnick
Credit: Dr Frank Gaillard
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