All pediatric patients who present with complaints of joint pain or swelling should be thoroughly examined to confirm or exclude the presence of an acute arthritis. The presence of warmth, swelling, tenderness, effusion, or erythema on physical examination indicates joint disease. Although there are many common causes of arthritis in childhood, the emergency physician must be vigilant to assess for the possibility of septic arthritis, especially in patients with monoarticular involvement. Elements of the history, persistence of fever, physical exam findings, and laboratory analysis may aid in making the diagnosis. Inadequately treated septic arthritis carries a high rate of long-term morbidity (1,2). Arthrocentesis (the aspiration of synovial fluid from a joint cavity using a needle and syringe) is the only way to establish a definitive diagnosis of an acute arthritis. Arthrocentesis can be performed by emergency physicians safely, inexpensively, and painlessly with the judicious use of topical and local anesthetics and procedural sedation (1,3,4,5).

Any child with an acutely warm, tender, swollen, or erythematous joint requires immediate evaluation. Detecting an effusion is an important part of the physical examination. The knee is perhaps the simplest joint to evaluate for effusion, and knowledge of the technique for examination of this joint will aid the clinician in identifying other joint effusions. To properly examine the knee, the patient is placed in the supine position. Under normal circumstances, there is a concavity on the medial surface of the knee posterior to the medial patellar margin. If this concavity is absent, an effusion is present. The extremity should be externally rotated 30 to 40 degrees. Joint fluid should then be milked from the medial aspect to the dependent lateral aspect of the joint space using the fingers of both hands. Using both thumbs, the operator then presses the fluid back toward the medial side. A positive “bulge sign,” indicating the presence of a knee effusion, occurs when a delayed medial bulge is observed (3,6). Obese patients and patients with tense joints may have large effusions that are difficult to detect using this method. Balloting the patella (i.e., firmly pressing the center of the bone posteriorly) may cause a “click” as the patella contacts the underlying femur. In these patients, ballottement may be a superior method for detecting an effusion (3).

Several laboratory tests may assist the clinician in defining the risk for septic arthritis and thus the need for arthrocentesis. An elevated white blood cell count (WBC), an elevated erythrocyte sedimentation rate (ESR), or an elevated C-reactive protein (CRP) may be useful (7,8,9,10,11). A plain radiograph of the joint in question showing a wide joint space (or a widened joint space compared to the contralateral side) or ultrasound-proven effusion is also consistent with septic arthritis (12). However, normal or mild to moderate increases in lab values should not deter the clinician from performing joint aspiration if the clinical presentation raises the suspicion of infection. Therefore, arthrocentesis should be performed in virtually every patient with monarthritis at risk for septic arthritis in order to rule out infection.

There are no absolute contraindications to arthrocentesis, only relative contraindications. The presence of known or suspected cellulitis or skin lesions, which may harbor pathogenic organisms, should not keep the clinician from aspirating the joint. Although some authors recognize that this may introduce infection into a joint space, the benefits to the patient must be weighed against the risks of the potential morbidity of undiagnosed septic arthritis (5,13). More importantly, however, there are other points of entry into the joint that
may bypass the lesion overlying the area of the commonest approach. Some authors recommend that if the needle must penetrate the joint through a potential source of infection, the patient should be admitted to the hospital to receive intravenous antibiotics (5). As septic arthritis is thought to be caused primarily by hematogenous spread, arthrocentesis may theoretically seed a sterile joint in patients with bacteremia (5,14,15). Nevertheless, if septic arthritis is suspected in such patients, joint aspiration should be performed (16).

Landmarks and scar tissue in patients with artificial joints may make the procedure difficult, although aspiration may be clinically warranted because of an increased risk of septic arthritis (5). Given the rarity of pediatric patients with artificial joints, orthopedic consultation should be sought for these patients prior to arthrocentesis.

Arthrocentesis may be indicated in patients with hemophilia and other coagulopathies for painful, tense traumatic hemarthroses and is indicated if septic arthritis is suspected (4,13,17). However, the coagulation defect should be corrected prior to performing the procedure. Additionally, the physician should consider consultation with the hematologist and/or orthopedist who cares for the child prior to performing arthrocentesis for hemarthrosis.

Several general considerations relevant to arthrocentesis at any joint will first be reviewed, followed by information needed to perform the procedure for specific joints (4,5,6,13,14,18,19,20,21). Prior to beginning, the operator should prepare the equipment for the procedure. Next, he or she should attempt to select the best site. The site should be as far as possible from other body structures (tendons, large nerves, blood vessels) and in a location that maximizes the chances of successful entry into the joint space. Before prepping and draping, the bony landmarks should be identified. This may be done with the joint in the best position for insertion or in a position better suited to the identification of the landmarks, with subsequent repositioning for insertion. As repositioning and draping can cause these landmarks to be obscured (to vision and touch), the insertion site can be marked prior to prepping. Marking can be done with indelible ink or by indenting the skin with a needle cap or a similar object. The joint should be positioned to maximize the size of the joint cavity (e.g., by distraction) and to stretch the capsule and ligaments so that the needle tip does not penetrate any significant structures.