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Arthrocentesis is the insertion of a needle into a joint cavity for the removal of synovial fluid and/or the injection of pharmaceutical agents into the joint cavity. Fluid can be aspirated from almost any joint. Arthrocentesis is used to diagnose and make treatment decisions regarding a joint. Obtaining synovial fluid is safe, simple, relatively pain free, inexpensive, and extremely beneficial to the patient.

Arthrocentesis may be diagnostic or therapeutic. Diagnostically, it is performed to identify the cause of an acute arthritis, to identify an intraarticular fracture, to identify the causes of an effusion, or to give a therapeutic trial of pharmaceuticals. It can also be therapeutic by relieving pain from elevated intraarticular pressure, to drain septic or crystal-laden fluid, or to inject pharmaceuticals. Ultrasound (US)-guided joint aspiration is a supplemental technique that improves several aspects of the arthrocentesis procedure.

Synovial fluid analysis will provide unique and valuable information about the affected joint.32 It is the only method to definitively diagnose or rule out a septic arthritis. The fluid should be analyzed for the presence of crystals. A white blood cell count and differential may help identify the causes of an effusion. A Gram's stain can be quickly performed to identify bacteria in the synovial fluid. Bedside gross analysis of the fluid's physical properties such as clarity, viscosity, and color has been shown to be a reliable clinical predictor of a potentially septic joint.30 A culture of the synovial fluid should be performed to definitively identify any microbiologic pathogen in the joint.

There are several general principles that should be followed when performing an arthrocentesis (Table 77-1). The Emergency Physician (EP) must know the anatomical relationships around the joint. The needle should go around and not through any tendons. Avoid piercing the articular cartilage, which is avascular and may not heal. Do not bounce the needle off the bone, as this is extremely painful for the patient. Insert the needle through the extensor surface of the joint. The synovial cavity is closest to the skin over the extensor surface of the joint. The extensor surface has fewer tendons and ligaments than the flexor surface. Most of the blood vessels and nerves are located on the flexor surface of the joint. Using the extensor surface of the joint for the procedure will avoid potential injury to these structures. Place the joint in slight flexion to maximize the size of the joint cavity. Apply distal in-line traction to the small joints of the wrists, hands, and feet to enlarge the joint cavity. This allows the needle to more easily enter the small joint spaces. Compression of large joints will mobilize peripheral fluid. This is helpful when the volume of synovial fluid is small. Compression may be applied manually or with an elastic bandage.

Table 77-1 General Principles for Arthrocentesis

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