FLUID-FLUID LEVELS IN INJURED KNEES - DO THEY ALWAYS REPRESENT LIPOHEMARTHROSIS

PURPOSE: To determine if fluid-fluid levels on horizontal-beam radiogr aphs of posttraumatic knee joints always represent lipohemarthrosis. M ATERIALS AND METHODS: The files of 41 patients with tibial plateau fra cture were reviewed retrospectively. Twelve with fluid-fluid levels on radiographs who underwent computed tomography (CT) or magnetic resona nce (MR) imaging were selected; CT attenuation and MR signal intensity patterns were used to assess fluid-fluid levels. Simulated hemarthros is and lipohemarthrosis underwent plain radiography, CT, and MR imagin g. RESULTS: Eight patients showed no evidence of fat in the supernatan t fluid with either CT (n = 7) or CT and MR imaging (n = 1). Fluid lev els resulted from blood separating into cellular elements and supernat ant serum. Four patients showed evidence of lipohemarthrosis with eith er CT (n = 3) or MR imaging (n = 1); in one of these patients, a doubl e fluid-fluid level could be demonstrated retrospectively on plain rad iographs. CONCLUSION: Single fluid-fluid levels in posttraumatic knee joints do not necessarily represent lipohemarthrosis. Double fluid-flu id levels are a more specific finding.