title: Imaging findings of Hoffa's fat pad herniation.
abstract: BACKGROUND
  We report the radiologic findings of herniation of Hoffa's fat pad through a defect in the lateral patellar retinaculum in young children who presented with painless masses visible by ultrasound (US) only with flexion of the knee.
  
  
  METHODS
  Six children, between the ages of 1-8 years, presented with an anterolateral knee mass that was not tender and was only seen and palpable with knee flexion. An US was performed in all patients, magnetic resonance imaging (MRI) in 2 patients and knee radiographs in 1 patient.
  
  
  RESULTS
  US imaging displayed focal herniation of Hoffa's fat pad within the infrapatellar region through a defect of the lateral retinaculum, visible only during dynamic imaging when the knee was in flexion. MRI performed in knee extension did not demonstrate a mass; however, it revealed a focal defect in the lateral retinaculum in the region of the abnormality. Radiographs were normal.
  
  
  CONCLUSIONS
  Focal herniation of Hoffa's fat pad is an uncommon cause of an anterolateral knee mass in young children. When a knee mass is only identified in flexion, focal fat herniation through a defect in the retinaculum should be suspected and a dynamic US should be performed.
authors.affiliation: Department of Radiology, Perelman School of Medicine at the University of Pennsylvania, The Children's Hospital of Philadelphia, 34th Street and Civic Center Boulevard, Philadelphia, PA, 19104-4399, USA. chauvinn@email.chop.edu.
authors.forename: Nancy A
authors.initials: NA
authors.lastname: Chauvin
authors.affiliation: Department of Radiology, Perelman School of Medicine at the University of Pennsylvania, The Children's Hospital of Philadelphia, 34th Street and Civic Center Boulevard, Philadelphia, PA, 19104-4399, USA.
authors.forename: Asef
authors.initials: A
authors.lastname: Khwaja
authors.affiliation: Department of Medical Imaging, Nemours Children's Hospital, Orlando, FL, USA.
authors.forename: Monica
authors.initials: M
authors.lastname: Epelman
authors.affiliation: Department of Radiology, Boston Children's Hospital, Harvard Medical School, Boston, MA, USA.
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Callahan
mesh_terms: D000273:Adipose Tissue / Q000000981:diagnostic imaging*; D000293:Adolescent; D002648:Child; D002675:Child, Preschool; D003937:Diagnosis, Differential; D005260:Female; D006547:Hernia / Q000000981:diagnostic imaging*; D006801:Humans; D007223:Infant; D007231:Infant, Newborn; D007592:Joint Diseases / Q000000981:diagnostic imaging*; D007719:Knee Joint / Q000000981:diagnostic imaging*; D008279:Magnetic Resonance Imaging / Q000379:methods; D008297:Male; D010329:Patella / Q000000981:diagnostic imaging*; D014463:Ultrasonography / Q000379:methods*
keywords: Children; Flexion; Herniation; Hoffa’s fat pad; Knee; Magnetic resonance imaging; Ultrasound