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Lipohaemarthrosis

Lipohaemarthrosis results from an intra-articular fracture with escape of fat and blood from the bone marrow into the joint, and is most frequently seen in the knee, associated with a tibial plateau fracture or distal femoral fracture. They have also been described in the hip, shoulder and elbow 1-2.

Radiographic features

As is well recognised fat floats on water. The fatty marrow therefore separates from the water based blood and layers above it, forming a fat-fluid level (also known as FBI sign) which can be seen on all modalities. 

Plain film

The fat-fluid level is seen on any horizontal beam radiograph, such that the beam is tangential to the fat-blood interface. In the knee this best achieved with a cross-table horizontal lateral view, where a long horizontal line is seen in the suprapatellar pouch. Ideally the patient has been lying in that position for 5 or so minutes to allow the fat and blood to adequately separate 1.

In patients with a prominent suprapatellar plica, a double fat-fluid level may be seen 1.  

It is important to remember that up to 64% of tibial plateau fractures do not have a x-ray visible lipohaemarthrosis, but rather a simple haemarthrosis 1, thus absence of the finding does not exclude an intra-articular fracture.

It is also important to remember that a simple haemarthrosis can separate into serum and red-cells (haematocrit effect) and create a subtle fluid-fluid level. This should not be mistaken for a lipohaemarthrosis 2. In some cases all three layers can be seen.

CT and MRI

CT and MRI having much higher sensitivity to density differences are not only very sensitive at identifying intra-articular fat, but also identify a haematocrit effect, with three layers visible (fat above, serum/synovial fluid middle, red blood cells below)2.

The upper layer will follow fat on all sequences and saturate on fat-saturated sequences.

Ultrasound

Although ultrasound is not very widely used in the assessment of the post trauma knee, the fat-fluid level can be identified when scanning the suprapatellar pouch as an echogenic layer above the hypo/anechoic blood/fluid. Fat should not be confused with the suprapatellar fat pad which lies anterior to the pouch and posterior to the quadriceps tendon. The two can be distinguished by massaging the suprapatellar pouch causing the fat-fluid level to disappear as a result of mixing 3.

Differential diagnosis

General imaging differential considerations include:

  • haematocrit effect: separation of serum and red-cells
  • suprapatellar fat pad: usually not a horizontal line; does not change on repositioning or agitation of joint fluid

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