Lipohaemarthrosis of the knee
Lipohaemarthrosis. This horizontal beam lateral (cross table) radiograph shows fluid in the knee joint. You will notice that there is a very straight line (arrow) at the superior aspect of the fluid, above which there is some low density material. This is fat floating on blood, and makes this a lipohaemarthrosis. The reason this is significant is that it means that there must be an underlying fracture, even if one cannot be seen on the radiograph (because the fat is actually marrow fat, and must have leaked into the joint through a fracture). In this example, a CT was arranged to assess for the presumed fracture and shows a sclerotic fracture line in the proximal tibia, which is an insufficiency-type fracture, with a component that extends to the cortex of the tibia (arrows). The patient was elderly and osteoporotic.