Arthrocentesis involves both the puncture of a joint and the aspiration of its synovial fluid. It is typically used to make an accurate diagnosis of a painful, warm, swollen joint. Removal of excess fluid can be therapeutic. Analysis of the removed fluid helps to decipher its etiology. 
The most emergent diagnosis of consideration, a septic joint, occurs less frequently in the elbow than in larger joints (5-10%), with the exception of cases in which Neisseria gonorrhoeae is the pathogen. [2, 3] See Aspiration Techniques and Indications for Surgery, Septic Arthritis for more information.
Before the procedure, periarticular processes such as bursitis, tendonitis, contusions, and cellulitis must be excluded on clinical grounds. Performing an arthrocentesis with the goal of obtaining synovial fluid to send for analysis is useful for the following purposes:
Evaluation of a nontraumatic, acute monoarticular arthritis
Differentiation of an occult fracture not clearly visualized on radiographs from an inflammatory cause by the presence of an hemarthrosis
A large effusion caused by fluid or blood is painful and leads to significant impairment of joint mobility. Removing synovial fluid as a therapeutic modality is useful for the following purposes:
Relief of pain
Improvement of mobility
Instillation of medications
Repeated arthrocentesis for a septic elbow in carefully selected patients as a means to decrease bacterial load and to avoid surgical debridement (only under the discretion of the orthopedic specialist)
Elbow arthrocentesis has been employed as an initial treatment measure in patients with radial head fractures, however, a Cochrane review did not find sufficient evidence to permit determination of the effectiveness or safety of this procedure in adults. 
Arthrocentesis through overlying cellulitis is controversial because of the concern of seeding the joint with bacteria. This decision should be made in consultation with an orthopedist and with consideration of the potential risks and benefits.
Relative contraindications for elbow arthrocentesis include the following  :
Overlying skin or soft tissue infection (eg, cellulitis, abscess; see above)
Overlying skin lesions (dermatitis, psoriasis)
Patient on anticoagulation medication
Prosthetic joint (refer to orthopedic specialist)
The synovial membrane contains pain fibers; therefore, customary practice is to instill a local anesthetic before the procedure to minimize pain. (See Local Anesthetic Agents, Infiltrative Administration.) Typically, lidocaine 1% is used. After the skin is prepared with a povidone-iodine solution or chlorhexidine, make a small wheal with a small (25-gauge) needle in the dermis at the determined entry point for aspiration. Do not inject anesthetic into the joint, because this may hinder synovial fluid analysis.
Alternatively, a topical vapor coolant, such as ethyl chloride, may be sprayed on the skin before needle aspiration. Infrequently, procedural sedation may be required in young children or uncooperative patients.
Equipment used for elbow arthrocentesis includes the following:
Sterile gloves and towels
Skin cleanser (povidone-iodine, chlorhexidine)
Local anesthetic agent (lidocaine 1%, vapor coolant)
Small (25-gauge) needle and small (5-mL) syringe for anesthetic injection
Needle (18-20 gauge) and syringe (5 or 10 mL) for joint aspiration
Specimen tubes (ethylenediaminetetraacetic acid [EDTA] for cell count and differential, lithium heparin for crystals, and a culture tube)
Elastic wrap, if needed
Place the patient sitting upright on a stretcher. Bend the patient’s elbow to 90 º. Pronate the patient’s forearm and rest it with the palm down on a side table set at the appropriate height for comfort.
Explain the procedure to the patient and obtain informed consent.
With the patient positioned as described (see Positioning), identify the olecranon process, lateral epicondyle, and radial head, and find the depression (or bulge, if the effusion is large) in the soft triangle. This site is used for all approaches.  (See the image below.) Identify the site of entry, and mark the site with a plastic needle sheath or sterile marker. Prepare the skin with a cleansing agent and drape with towels.
Anesthetize the area by injecting 1-2 mL of lidocaine 1% and forming a skin wheal. Insert an 18-gauge needle into the depression perpendicular to both the skin and radial head from the lateral side. This is the lateral approach, which is preferred.
Alternatively, the posterolateral approach can be used. An increased risk of injury to the radial nerve and triceps tendon exists, but this approach is useful if the bulge of an effusion is palpated inferior to the lateral epicondyle. In the posterolateral approach, insert the needle perpendicular to the skin but parallel to the radial shaft. (See the image below.)
Advance the needle slowly while aspirating the syringe until synovial fluid is obtained. If the aspiration is unsuccessful, draw back, reidentify the landmarks, and correct the needle insertion position. If bone is encountered, withdraw the needle slightly and redirect it.
When synovial fluid is obtained, remove the needle Apply a bandage and elastic wrap if a large effusion was present. Place the fluid in specimen tubes and send for analysis.
Do not confuse an olecranon bursitis with a joint effusion. An olecranon bursitis is located posteriorly over the olecranon.
Do not pass the needle through a site with cutaneous signs of infection unless the source is strongly suspected to be from the joint. This should only be done after consultation with an orthopedist.
The landmarks may be easier to find if the arm is first extended to locate the depression and then flexed and pronated for the procedure.
A medial approach should not be used because of risk of injury to the ulnar nerve and the superior ulnar collateral artery.
Arthrocentesis of the elbow or any joint is associated with infrequent complications, including the following:
Infection - Bacteria from the skin may rarely be introduced into the joint (1 per 10,000 aspirations) 
Iatrogenic hemarthrosis - A small amount of blood may be obtained from a small synovial blood vessel, which is a benign occurrence; obtaining larger amounts is rare, except in patients who are taking an anticoagulant or who have a history of bleeding diatheses
Allergic reaction - Hypersensitivity to the anesthetic agent can be found on history if present