Effusion Synovitis and Cartilage Evaluation

The presence of joint fluid is easily recognized by sonography, but may require optimal patient positioning [32]. A joint effusion can present on ultrasound as anechoic fluid or as hypoechoic fluid, possibly containing debris (Fig. 13). Pan-nus or thickened synovium appears more hypoechoic on sonography than the adjacent soft tissues [32]. Power Doppler can also demonstrate hyperemia in acutely inflamed synovium, which is helpful in distinguishing an acute from a chronic process. Sonography has the added value in providing guidance for joint aspiration and synovial biopsy; however, MRI provides a better thorough evaluation of effusion and synovitis when performed without and then after intravenous injection with gadolinium contrast. MRI also allows better evaluation of intra-articular erosions that can be assessed in only a limited manner using ultrasound. For this reason, the authors prefer MRI of the knee when synovitis is suspected.

On sonography, articular cartilage has a homogeneous hypoechoic appearance, a smooth contour, and a parallel echogenic interface with bone [32]. Unfortunately, in knee imaging, not all the articular surfaces can be evaluated. Therefore, MRI is the best imaging modality for cartilage assessment.

Knee Joint Effusion
Fig. 13. Knee joint effusion. (A) Longitudinal sonographic image of the right knee show fluid within the suprapatellar recess (arrow). (B) Color Doppler image off midline shows areas of increased flow representing mild synovitis in this patient with a total knee replacement.

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