Lipohemarthrosis

• Mixture of fat and blood in joint capsule following trauma
• Lipohemarthroses occur in approximately 40% of all intra-articular fractures of the knee
• May take up to 3 hours after trauma to appear

• Gravity and a period of rest are needed to depict fluid-fluid layer which is characteristic of lipohemarthrosis
• Fat and blood enter joint from marrow space through an osteochondral defect at articular surface of joint
• Fat is less dense than blood so fat floats on the surface with the heavier, denser blood beneath it
• Can only be seen with a horizontal x-ray beam (beam is parallel to the floor)
• CT and MRI have been used to diagnose lipohemarthrosis
• Also to identify occult fractures not detected by radiography
• Lipohemarthrosis is not seen in all cases of intracapsular fracture
• Presence of a fat-fluid level is nearly diagnostic of a fracture, even when that fracture is radiographically occult
  
  
• Knee joint
  
  
• Most commonly, lipohemarthroses are produced with minimally displaced fractures of the tibial plateau
• Since cross-table lateral views of the knee in which the x-ray beam is horizontal are commonly performed in trauma patients, lipohemarthroses are more commonly seen with this joint

• Three bands can normally be distinguished
  
  
• The top band consists of fatty material
• The next band below is composed of serum and serous joint effusion
• Cellular parts of blood, i.e., erythrocytes and leukocytes settle to the bottom layer due to gravity

Lipohemarthosis. Upper image shows fat floating atop blood in the knee joint;
the lower image demonstrates a markedly comminuted fracture of the proximal tibia (white arrows) from which the marrow entered the joint

The CT, MRI, and Radiographic Appearance of Lipohemarthrosis Sorenson SM, Wolfson K, Gentili A, Masih S, Seeger LL   UCLA School of Medicine  AJR On-Line

  Lipohemarthrosis of the knee: specific imaging findings  Christoph Schick · Martin G. Mack · Ingo Marzi · Thomas J. Vogl  European Radiology