Joint aspiration or arthrocentesis is one of the more rewarding procedures in medicine because it can be both diagnostic and therapeutic. When a patient presents with a red, painful, inflamed joint that is shown by examination to be full of fluid, it is important that the joint be drained, if possible. The removal of fluid is almost always of great relief to the patient, but fluid collected from a joint aspiration can provide volumes of important information about the cause of the inflammation and the health of the patient.
The typical joint aspiration technique begins by sterilizing the site and a site around the joint. Many times a sterile dressing is placed over the joint which is essentially just a sterile, absorbent pad with a window cut out of it. While wearing sterile gloves, the physician identifies anatomical landmarks in the joint. For example, in a knee joint aspiration, the knee is slightly flexed and an area at the top or bottom of the knee cap in examined for fluid. The area is usually numbed with lidocaine or similar drug, which is a local anesthetic. The injected anesthetic usually numbs the skin first, then deeper structures. After the area is numb, the arthrocentesis proceeds by advancing a needle into the joint space. As the needle is advanced, the syringe is pulled back to create a suction at the tip of the needle. Once the needle enters the joint space, synovial fluid will pour into the syringe. In most cases, as much fluid is drawn off as possible both for maximum patient relief but also to get the best fluid sample.
The synovial fluid is sent to the laboratory for a variety of tests. Joint aspiration fluid will be tested for cells of various types; red blood cells indicate that there is bleeding in the joint and white blood cells indicate that there is the presence of infection. The number of these various cell types tells doctor information about the joint aspiration as well. The fluid is examined for crystals, too. If gout or pseudogout is present in the joint, there will be crystals of uric acid or calcium pyrophosphate respectively, as seen under a microscope. The arthrocentesis fluid will also be subjected to a Gram stain and cultured—these are tests to see if there is a bacterial infection present in the joint.
In order for a joint aspiration to take place, the joint must be large enough to place a small needle into it. Commonly performed arthrocenteses are knee joint aspirations, ankle joint aspirations, and hip joint aspirations. While smaller joints of the fingers and toes become inflamed in gout and arthritis, they are usually too small to allow a needle to be safely introduced into the joint at the bedside. The procedure may not be able to be performed if the skin over the joint is infected or if the patient has a problem with their blood not clotting quickly enough.
Joint aspiration may need to be repeated periodically if fluid accumulates very rapidly. Patients with chronic joint disorders may also benefit from arthrocentesis plus the infusion of corticosteroids. In skilled hands, joint aspiration is safe, diagnostic, and therapeutic.
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