They are difficult to identify on plain radiographs with only one reliable sign described 5:Īsymmetrical widening >1 mm in teardrop distance (between the lateral aspect of the pelvic teardrop and most medial aspect of the femoral head)Įvaluation begins with probe placement just cephalad to the patella, allowing identification of the quadriceps tendon in the near field and cortex of the femur in the far-field between these two structures lies the suprapatellar recess of the knee joint, intercalating between the suprapatellar and prefemoral fat pads. Caudad angulation allows visualization of the lateral and medial joint recesses. An effusion will cause distension of the visualized recesses with fluid, which may be anechoic or of mixed echogenicity. Hip joint effusions are easiest identified on US, CT and MRI. Plain films are sensitive for effusions >5 mL, with US and MRI being more sensitive for smaller effusions 3. Effusion within the posterior recess is usually less well-defined. On a lateral radiograph, effusion causes displacement of the anterior and posterior fat pads surrounding the distal humerus. The triangular appearance of the displaced low-density fat pad simulates the appearance of a sail.Īn elbow joint effusion in the setting of trauma is very often a sign of an occult fracture. In adults, the occult fracture is most commonly of the radial head, whilst in children, a non-displaced supracondylar fracture should be suspected.Īn ankle joint effusion is best seen as a teardrop-shaped soft tissue density displacing the anterior fat pad on a lateral film and lying superior to the talar neck. The sail sign is the key to recognizing an elbow joint effusion. Obliterate the normally crisp posterior border of the quadriceps tendonĭisplace the quadriceps tendon and patella anteriorly Radiographic features Plain radiograph Knee joint effusionĪ knee joint effusion appears as well-defined rounded homogeneous soft tissue density within the suprapatellar recess on a lateral radiograph. 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. shoulder) but are most readily identified in the knee. Lipohemarthroses can occur in other joints (e.g. 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. ![]() ![]() 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. ![]() As part of an arthrogram, deliberate injection into the joint space of a contrast medium results in an iatrogenic effusion. pus) or trauma and might be exudate, transudate, blood and/or fat. Abnormal fluid accumulation can result from inflammation, infection (i.e. There is normally only a small amount of physiological intra-articular fluid. A joint effusion is defined as an increased amount of fluid within the synovial compartment of a joint.
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