Fat pad separation sign (knee joint effusion)

Last revised by Mostafa Elfeky on 9 May 2023

Fat pad separation sign refers to the separation of the anterior suprapatellar and posterior suprapatellar (prefemoral) fat pads on lateral knee radiograph. This sign is useful in diagnosis of knee joint effusion on cross-table (horizontal beam) lateral knee radiographs with more sensitivity than standard knee lateral radiographs 1,2. Knee joint fluid can be detected as small as 2 mL with diagnostic accuracy near 90% 1,2.

This sign represents distension of suprapatellar bursa with fluid which is the first site to be filled with fluid in knee joint effusion.

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