Arthrocentesis and synovial fluid (SF) analysis produces valuable information that is important for the diagnosis of arthritis, arthrocentesis can help relieve the signs and symptoms of arthritis, especially if a large joint effusion is present.
Indications
- Infection – Arthrocentesis must be performed immediately if there is any suspicion of infection. An inflammatory monarticular arthritis should be considered infectious until proven otherwise.
- Crystal-induced disease – Arthrocentesis and SF analysis are the only way to identify unequivocally.
- Post-traumatic – Analysis of joint fluid is the only way to distinguish post-traumatic hemarthrosis from post-traumatic arthritis with bland SF.
- Inflammatory versus noninflammatory arthritides – SF analysis enables the clinician to differentiate inflammatory and noninflammatory arthritides.
- Therapeutic – Arthrocentesis can be therapeutic, and can increase the efficacy of intra-articular glucocorticoids. Therapeutic arthrocentesis is indicated in any patient with a hemarthrosis.
Technique
- Anesthesia – Local anesthesia with 1% lidocaine without epinephrine significantly reduces discomfort associated with the procedure. A number 25 or 27 needle should be used to infiltrate the skin, subcutaneous tissue, and pericapsular tissue.
- Needle choice – After the periarticular tissues have been anesthetized, a 20- or 22-gauge needle can be used to aspirate small- to medium-sized joints. An 18- or 19-gauge needle should be used for aspirating large joints, if there is a suspicion of infection or intra-articular blood, or if there is a likelihood of viscous or loculated fluid.
- Landmarks – Typical landmarks often are obscured around a swollen joint. Therefore, after a thorough physical examination and before anesthetizing the skin, it is often helpful to mark the approach. Position – Unlike injection, aspiration is best done when a joint is in a position of maximum intra-articular pressure.
- Radiographic assistance – Although most joints can be aspirated without radiographic assistance, some joints, such as the hips, sacroiliac joints, or zygoapophyseal joints, require aspiration by an interventional radiologist under computed tomography guidance.
- Collection – SF should be collected in an EDTA or sodium heparin tube for cell counts, and a sterile tube for Gram stain and microbiology culture studies.
SF Analysis
- Color – Normal SF is colorless and clear. The yellow color characteristic of SF from people with arthritis is due to xanthochromia.
- Opacity – Generally, it is the number of white blood cells (WBCs) that determines the opacity of inflammatory SF. Synovial fluid from people with osteoarthritis is clear, whereas the SF in inflammatory arthropathies is translucent, and SF from a septic joint will be opaque.
- Viscosity – Normal joint fluid is viscous due to the presence of hyaluronic acid. Enzymes present in inflammatory arthropathies digest hyaluronic acid, resulting in a decrease in fluid viscosity.
- Blood – The presence of blood in a joint usually is the result of acute trauma.
- Crystals – Although crystals can be identified in SF a few days old, optimal examinations for crystals are performed on wet preparations of SF soon after aspiration.
- Classes – There are four classes of SF, defined by differences in gross examination, total WBC count, WBC differential, the presence of absence of blood, and results of Gram stain and culture.