Unravelling Triple Layered Concept of Lipohaemarthrosis

Background: Lipohaemarthrosis is the presence of intra-articular fat, synovial fluid, serum and blood products, almost always related to an intra-articular fracture, most commonly observed in the knee. Due to the layering of these components owing to density differences, lipohaemarthrosis has characteristic imaging appearances. Conventionally, it has been assumed that the components of a lipohaemarthrosis are influenced by gravity alone, resulting in layering of the contents parallel to the horizontal plane. Our anecdotal experience, however, suggests that the layers are not always gravity dependent, for reasons that are currently unknown. Aims and Objectives: It is the purpose of our study to investigate lipohaemarthroses of the knee and explore potential reasons that explain the associated imaging features. Materials and Methods: Computed tomography (CT) scans in 30 knees with a lipohaemarthrosis secondary to an intra-articular fracture were retrospectively analysed. Angles between the horizontal plane and the two interfaces between the three-layered components, as well as the angle of rotation of the knee relative to the horizontal plane in the axial CT images, were measured. A statistical analysis of the data was performed. Results: The three layers of lipohaemarthrosis were not perfectly aligned with the horizontal plane in any of the cases, with mean angles of 7 and 4 degrees relative to the horizontal for the two interfaces and the knees were also rotated by a mean of 16 degrees, which was statistically significant ( P < .0001). Conclusion: The contents of a traumatic lipohaemarthrosis within the knee joint are not always gravity dependent as previously thought.