Arthrocentesis and synovial fluid analysis in clinical practice: value of sonography in difficult cases.

Joint aspiration, or arthrocentesis, is one of the most useful and commonly performed procedures for the diagnosis and treatment of joint diseases. The synovial fluid aspirated may be examined to evaluate the degree of inflammation and, mainly, to detect the presence of some relevant pathogenic agents, such as crystals or microorganisms. In these cases, synovial fluid analysis still represents the best diagnostic procedure. Arthrocentesis is thus particularly required for the diagnosis and management of the acute "hot red joint," which may be considered a true medical emergency because of the morbidity and mortality related to septic arthritis. The most recent recommendations on arthrocentesis confirm the need for the procedure in the presence of synovial effusion of unknown origin, especially if septic or crystal arthritis is suspected. Owing to the importance of this analysis, it is clearly recommended that ultrasonography should be used to facilitate arthrocentesis in difficult cases. Furthermore, ultrasonography may be useful in revealing the presence of synovial fluid before the joint aspiration and, subsequently, distinguishing some aspects characteristic of crystal-induced arthropathies.