Infrapatellar synovial fold

The infrapatellar synovial fold (also known as the ligamentum mucosum or infrapatellar plica) is a thin, synovial membrane structure within the knee joint. It extends from the intercondylar notch of the femur, courses anterior to the anterior cruciate ligament (ACL), and inserts into the infrapatellar fat pad (Hoffa’s fat pad) with the two fringe-like folds termed the medial and lateral alar folds. This fold is lined by synovium and forms the roof of a horizontal cleft within the infrapatellar fat pad, which communicates with the knee joint cavity.

The infrapatellar synovial fold is a normal anatomical variant, present in the majority of knees, and can be visualized on MRI as a low-signal linear structure anterior to the ACL. Its morphology varies, and it may be absent in some individuals. Clinically, it is important to recognize this fold to avoid confusion with the ACL on imaging and during arthroscopy, and because it can be implicated in plica syndrome if pathologically thickened or fibrotic.