title: Fluid-fluid levels in injured knees: do they always represent lipohemarthrosis?
abstract: OBJECTIVE
  To determine if fluid-fluid levels on horizontal-beam radiographs of posttraumatic knee joints always represent lipohemarthrosis.
  
  
  METHODS
  The files of 41 patients with tibial plateau fracture were reviewed retrospectively. Twelve with fluid-fluid levels on radiographs who underwent computed tomography (CT) or magnetic resonance (MR) imaging were selected; CT attenuation and MR signal intensity patterns were used to assess fluid-fluid levels. Simulated hemarthrosis and lipohemarthrosis underwent plain radiography, CT, and MR imaging.
  
  
  RESULTS
  Eight patients showed no evidence of fat in the supernatant fluid with either CT (n = 7) or CT and MR imaging (n = 1). Fluid levels resulted from blood separating into cellular elements and supernatant serum. Four patients showed evidence of lipohemarthrosis with either CT (n = 3) or MR imaging (n = 1); in one of these patients, a double fluid-fluid level could be demonstrated retrospectively on plain radiographs.
  
  
  CONCLUSIONS
  Single fluid-fluid levels in posttraumatic knee joints do not necessarily represent a lipohemarthrosis. Double fluid-fluid levels are a more specific finding.
authors.affiliation: Russell H. Morgan Department of Radiology and Radiological Science, Johns-Hopkins Hospital, Baltimore, MD 21287, USA.
authors.forename: C H
authors.initials: CH
authors.lastname: Lugo-Olivieri
authors.forename: W W
authors.initials: WW
authors.lastname: Scott
authors.forename: E A
authors.initials: EA
authors.lastname: Zerhouni
mesh_terms: D006395:Hemarthrosis / Q000175:diagnosis*; D006801:Humans; D007718:Knee Injuries / Q000175:diagnosis*; D007719:Knee Joint / Q000000981:diagnostic imaging / Q000473:pathology*; D008055:Lipids; D008279:Magnetic Resonance Imaging; D012189:Retrospective Studies; D013582:Synovial Fluid; D013978:Tibial Fractures / Q000175:diagnosis*; D014057:Tomography, X-Ray Computed
chemical_list: D008055:Lipids