A joint effusion is an abnormal collection of fluid within the synovial cavity of a joint. On ultrasound, joint effusion appears as an anechoic or hypoechoic area within the joint capsule, displacing the normally apposed joint surfaces apart. Ultrasound is highly sensitive for detecting even small effusions in many joints, including the shoulder, elbow, wrist, hip, knee, and ankle. The presence and character of a joint effusion provides important diagnostic information: a simple, anechoic effusion may suggest a traumatic or degenerative process, while a complex effusion with debris or septations raises concern for infection or inflammatory arthritis. Ultrasound-guided aspiration of joint fluid is commonly performed to confirm the diagnosis.
Sonography Term