sometimes the only indication of an abnormal knee radiograph are the secondary soft tissue signs
identifying the knee joint effusion and understanding how it occurs is one of the more important things to take away from this pathway
the joint effusion fills the suprapatellar bursa, pushing away the fat pads (effusion is normally defined as >10mm of displacement)
in some cases, an effusion will contain a fat-blood level known as a lipohemarthrosis where an intraarticular fracture has resulted in marrow fat leaking into the joint and, given fat is less dense than blood, it will settle on top
it is important that the lateral knee projection is performed as a horizontal beam lateral, that is, central beam 90° to the floor to properly demonstrate the fluid level
the small fluid-filled bursae also have a tendency to become inflamed; here we explore one of the more common presentations of bursitis: prepatellar bursitis
Knee joint effusion
Case credit: Andrew Murphy
A joint effusion is defined as an increased amount of fluid within the synovial compartment of a joint.
There is normally only a small amount of physiological intra-articular fluid. Abnormal fluid accumulation can result from inflammation, infection (i.e. pus) or trauma and might be exudate, transudate, blood and/or fat. As part of an arthrogram, deliberate injection into the joint space of a contrast medium results in an iatrogenic effusion.
Lipohemarthrosis is a particular type of effusion that occurs in the setting of intra-articular fracture where a fat-fluid level is seen due to marrow fat leaking into the joint via the fracture. Fat, being less dense than blood, will float to the surface and present as a 'fat-fluid' level on top of the blood on any radiographs with the horizontal beam parallel to the level. Lipohemarthroses can occur in other joints (e.g. shoulder) but are most readily identified in the knee.
Recognition of joint effusion on plain radiographs can be difficult, particularly for non-radiologist. Appreciation of the typical appearances and signs of joint effusions can assist diagnosis.
Radiographic features
Plain radiograph
Knee joint effusion
A knee joint effusion appears as well-defined rounded homogeneous soft tissue density within the suprapatellar recess on a lateral radiograph. The effusion will 2:
separate periarticular fat pads >10 mm
obliterate the normally crisp posterior border of the quadriceps tendon
Findings: Minimally displaced intra-articular fracture of the posterior tibia is associated with a large hemarthrosis displacing the fat pads. Fracture is at the expected site of the tibial insertion of the posterior cruciate ligament.
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 1,2,4.
Radiographic features
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.
Plain radiograph
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 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 an x-ray visible lipohemarthrosis, but rather a simple hemarthrosis1; thus absence of the finding does not exclude an intra-articular fracture.
It is also important to remember that simple hemarthrosis can separate into serum and red cells (hematocrit effect) and create a subtle fluid-fluid level. This should not be mistaken for lipohemarthrosis 2. 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 have a 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) 2.
The upper layer will follow fat on all sequences and saturate on fat-saturated sequences.
Findings: Secondary to a tibial plateau fracture, fat-fluid level has developed in the supra-patella joint recess, note how it is displacing the fat pads and compare to the case below.
Findings: On the lateral horizontal beam image a fat-fluid level is seen within the supra-patella joint recess consistent with a lipohemarthrosis. This indicates that there must be an intra-articular fracture despite the fact that no fracture can be confidently identified on either the lateral or the AP image.
If a lipohemarthrosis isn't present, but a joint effusion is, does that mean there is not intraarticular fracture?
No, the absences of a lipohemarthrosis does not imply there is no fracture, there still could be one present or at least something abnormal occurring. Remember the marrow fat has to leak out (it may not), settle on the blood and the radiograph needs to be performed horizontal beam. That's a lot of things that have to go right, for you to see something go wrong!
Prepatellar bursitis is inflammation and fluid collection within the prepatellar bursa, located between the patella and the overlying subcutaneous tissue. It has been historically referred to as "housemaid's knee".
Chronic irritation due to trauma or repetitive kneeling leads to accumulation of fluid within the prepatellar bursa and sometimes hemorrhagic transformation. Less common causes include:
clumps of calcifications in the prepatellar soft tissues in chronic cases 3
Ultrasound
Hypoechoic fluid, sometimes containing debris, is noted anteriorly to the patellar surface 2.
MRI
Oval shaped fluid-filled sac is seen anterior to the patella and displays low T1 and bright T2/STIR signal intensity. When hemorrhage occurs T1 signal increases and T2 GRE signal reduces. The wall of the bursa may show increased thickness and irregularity.
Treatment and prognosis
Local injection of non-steroid anti-inflammatory drugs and steroids or local application of ice, and finally bursectomy in resistant cases.
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