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Joint effusion

This article is dedicated to the humble joint effusion, particularly the plain radiographic appearances. 

A joint effusion is defined as an increased amount of fluid within the synovial compartment of a joint. There is normally only a small physiological amount of fluid. Abnormal fluid accumulation can result from inflammation, infection or trauma and may be an exudate, transudate, blood and/or fat.

Recognition of a joint effusion on plain radiographs can be difficult, particularly for the non-radiologist. Appreciation of the typical appearances and signs of joint effusions can assist diagnosis.

Radiographic appearance

Knee joint effusion

A knee joint effusion appears as well-defined rounded homeogeneous soft tissue density within the suprapatellar recess on a lateral radiograph. The effusion will displace the quadriceps tendon and patella anteriorly.  

Lipohaemarthrosis 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. Lipohaemarthrosis can occur in other joints (e.g. shoulder) but is most readily identified in the knee. 

Elbow joint effusion

The sail sign is the key to recognising an elbow joint effusion. On a lateral radiograph, an 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.

An 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 while in children a non-displaced supracondylar fracture should be suspected. 

Ankle joint effusion

An ankle joint effusion is best seen as a teardrop-shaped soft tissue density displacing the anterior fat pad on a lateral film. Effusion within the posterior recess is usually less well defined.

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