Tendons are anatomic structures interposed between muscles and bones; they consist of dense fibrous connective tissue made up primarily of collagenous fibres embedded in a proteogycan-water matrix.
Synovial sheaths are serous envelopes formed by the parietal and visceral sheets,
that surround tendons at bone surfaces with peritendinous fluid for lubrication.
Their function is to contain the tendon allowing an easy glide.
Peritendinous fluid is normally less than 2 mm in thickness.1,2
A synovial sheath can be distinguished from the tendon only when the synovial fluid increases.
The fibrous sheaths or retinacula are the canals through which tendons slide during their course,
decreasing friction.1
Bursae are fluid filled sacs lined with synovial cells.
They are located between bony prominences and the surrounding soft tissues,
their function is to reduce friction and allow free movement. 3
When an intra-articular fracture occurs,
fat and blood from the bone marrow mix with synovial fluid within the injured joint,
resulting in a fat-fluid level called lipohemarthrosis.
Lipohemarthrosis was first described by Kling in 1929.
Ten years later,
fat-fluid levels were demonstrated radiographically by using the horizontal beam technique.
In 1942,
fat-fluid levels were visualized in the context of knee fractures,
by Pierce and Eaglesham.4
Lugo Olivieri et al.
suggested that a double fluid-fluid level could be more specific for lipohemarthrosis.5
In the next years,
lipohemarthrosis was reported in different joints,
depicted in radiographs,
ultrasound,
CT and MRI,
commonly known as a pathognomonic sign of an intra-articular fracture.
The presence of free-floating fat in a joint space could be seen in the first 2 hours after trauma,
as fat globules,
allowing a wavy appearence along the fat-fluid interface.6
A well depicted fat-fluid level develops after 3 hours of leakage of blood and fat bone marrow into the synovial fluid,
since fat is less dense,
it floats in the superior layer creating a level.6
In this context,
the bone marrow fat can also appear in the bursae or tendon sheaths being a secondary sign of intra-articular fracture,
which is not frequently reported.
There are few cases of fat effusion in tendon sheaths,
described in wrist and ankle joints,
and only one case of fat-fluid level in the iliopsoas bursa in literature.
Fat-fluid level in tendon sheath was attributed the term "floating fat sign" by Le Corroller et al.
Depending on the shape of the fat level,
Vergahen called "fat drops" and this could be interpreted as fat globules depicted in the first 2 hours.4,7
In a recent publication Kanieska et al.,
propose the term "lipidus migrans",
describing this finding in ankle joints associated to intra-articular fractures.8
The mechanism described for the presence of floating fat in tendon sheaths is discussed;although we agree that most of the time is associated with a traumatic injury of the tendon sheath due to the close relationship with the bone fracture.
Czuczman et al.,
found fat-fluid levels involving iliopsoas bursa results from extra-capsular extension of a lipohemarthrosis,
in the context of a fracture,
since bursal communication with the hip joint,
instead of laceration of the bursal wall by a bone fracture,
that is equivalent to the mechanism in tendon sheaths as previously mentioned.9
After a few days,
the synovial membrane resorbs fat and blood products.
Futhermore,
inmovilization and delayed imaging studies,
not performed in acute phase,
do not allow visualizing this sign and this could be a reason to be an underreported finding.10
