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, transduate, blood 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.
A knee joint effusion appears as well-defined rounded homeogeneous soft tissue density within the suprapatella recess on a lateral radiograph. The effusion will displace the quadraceps tendon and patella anteriorly.
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.
The sail sign is the key to recognizing 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 simmulates 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.
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.