Pitfalls

In all joints, it is important to assess both the affected joint, and the contralateral side, as a small amount of fluid within the spaces can be normal. In addition, cartilage can appear hypoechoic within these spaces, but will be non-compressible [1]. Fluid within an effusion can be simple (anechoic) or complex (heterogenous in echogenicity) depending on what caused the effusion. Examples of potential complex effusions include hemarthrosis with clots or loculated septic arthritis [1, 3]. Synovitis can mimic a complex effusion, but can be differentiated with the addition of color to look for low grade venous flow. Finally, in pediatric patients, age-specific differences in cartilage thickness and growth plate appearance can be mistaken for joint fluid or other pathology. It is therefore important to keep the expected appearance at different stages of skeletal development in mind when interpreting findings.