Lipohemarthrosis is the result of extruded bone marrow fat and blood into joint space following an intraarticular fracture [1]. First described in 1929 by Kling, Holmgren demonstrated the “fat-fluid” level radiographically in 1939 [1,2]. In 1942, Pierce et al. described a fat-fluid level, which had a fluid-fluid component and suggested that this phenomenon was the result of separation of blood into serum and cellular elements [2]. In 1996, Lugo-Olivieri et al. performed a retrospective review of 41 patients with tibial plateau fractures and found that joint effusions with fluid levels resulted from blood separating into cellular elements and supernatant serum, confirmed with CT or MR imaging. Lugo-Olivieri concluded that a single fluid-fluid level on radiographs in a post-traumatic knee did not necessarily reflect lipohemarthrosis, as hemarthrosis could appear similarly. They suggested that a double fluid-fluid level is a more specific finding for intraarticular fat and an underlying fracture.
Lipohemarthrosis is more common in fractures about the knee, but has been described in the shoulder, elbow, and hip fractures [1]. CT and MRI are more specific than radiography in evaluating the composition of the effusion, but sonography may also have a role [1]. The anterior layer on ultrasound will appear hyperechogenic because of the fat. On CT, this area will be fat-density, and will follow fat signal on all MR pulse sequences [1,2]. The supernatant layer will be anechoic on US, fluid density on CT, and follow simple fluid signal on all MR sequences [1,2]. The hemorrhagic cellular layer will appear hypoechogenic on sonography, hyperdense (50 – 70 HU) on CT, and produce intermediate signal on T1 and hypointense signal on T2-weighted images [1,2].
Reference(s): 1. Costa DN, Cavalcanti CF, Sernik RA. Sonographic and CT Findings in Lipohemarthrosis. AJR Am J Roentgenol. 2007 Apr;188(4):W389.
2. Lugo-Olivieri CH, Scott WW Jr, Zerhouni EA. Fluid-fluid levels in injured knees: do they always represent lipohemarthrosis? Radiology 1996;198 : 499-502
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