Hip effusion


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Hip effusion,
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increased amount of synovial fluid within the joint capsule. The cause of the effusion has to be correlated with the clinical presentation. The most frequent cause is idiopathic transient synovitis, also known as irritable hip syndrome. Effusions are also seen in septic arthritis and juvenile idiopathic arthritis, and at some stages in the evolution of Perthes disease.

Fluid within the joint is usually transonic (Fig.1), and has high signal on T2-weighted MRI. Mixed echoes within an effusion may be seen with infection or haemorrhage (Fig.2). The ultrasound examination should be performed with a high resolution linear array probe, the probe being placed in the plane of the femoral neck. A difference of more than 2 mm between the joints is pathological and indicates an effusion. The position of the limbs during the examination should be similar. The fluid may be aspirated under ultrasound guidance. The fat planes, as seen on radiographs, may be displaced by an effusion within the hip joint. These fat planes, however, lie between muscles and are not immediately adjacent to the joint capsule and are only displaced by an effusion if it is large. See antalgic gait and transient synovitis of the hip


HC


The Encyclopaedia of Medical Imaging Volume VII
Hip effusion, Fig. 1
Ultrasound of the left hip with an effusion measuring 13 mm. The dense, bowed white line is the femoral neck.
Hip effusion, Fig. 2
Haemorrhagic effusion in a boy with haemophiliac disease. The echoes are due to the haemorrhage.