Lipohemarthrosis 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; rarely a patellar fracture. They have also been described in hip, shoulder, elbow and wrist fractures .
It is well known that fat floats on water as it is less dense. The fatty marrow separates from the water-based blood and layers above it, forming a fat-fluid level (also known as the FBI sign). This can be seen on all modalities.
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 is 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 minutes to allow the fat and blood to adequately separate .
In patients with a prominent suprapatellar plica, a double fat-fluid level may be seen .
It is important to remember that up to 64% of tibial plateau fractures do not have an x-ray visible lipohemarthrosis, but rather a simple hemarthrosis , thus absence of the finding does not exclude an intra-articular fracture.
It is also important to remember that a simple hemarthrosis can separate into serum and red cells (hematocrit effect) and create a subtle fluid-fluid level. This should not be mistaken for a lipohemarthrosis . In some cases, all three layers can be seen, a so-called lipohydrohaemathrosis. This tri-level appearance is sometimes known as parfait sign.
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 hematocrit effect, with three layers visible (fat above, serum/synovial fluid middle, red blood cells below) .
The upper layer will follow fat on all sequences and saturate on fat-saturated sequences.
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 .
General imaging differential considerations include:
- hematocrit effect: separation of serum and red cells
- suprapatellar fat pad: usually not a horizontal line; does not change on repositioning or agitation of the joint fluid