Hoffa's sign is pain and distress when straightening the bent knee while an examiner presses deeply with the thumbs on either side of the patellar tendon.

The thumbs are pressing into the region of the Hoffa fat pad behind the patellar tendon.
Relevance of a positive Hoffa's test
The test is known as the Hoffa's test. It is a test for non-specific irritation or inflammation of the Hoffa fat pad.
Synonyms:
Hoffa sign
Hoffa test
Hoffa's test
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CLINICAL PUBLICATIONS
Superolateral Hoffa’s Fat Pad Edema: Association With Patellofemoral Maltracking and Impingement. Subhawong TK, Eng J, Carrino JA and Chhabra A. AJR Am J Roentgenol. 2010 Dec; 195(6): 1367–1373..