A skilled clinician can usually perform the procedure without an assistant. If the patient is anxious or exceptional circumstances apply, an assistant will make it much easier to perform the procedure.

Patients who are anxious, in pain, or unable to cooperate with the procedure might require an assistant and procedural sedation. It is a strong recommendation to utilize some type of local anesthesia. For most surgeons, lidocaine 1% will work to numb the skin. It is important to avoid deep injections with the local anesthetic because there is a risk that it may alter the synovial fluid analysis.

The clinician or medical care professional must obtain informed consent. A timeout should take place to confirm the patient and correct joint. The most crucial step is having the patient lay in a comfortable position with the affected knee fully extended or flexed at 15 to 20 degrees with a towel roll under the knee. This position helps to facilitate procedure success by ensuring quadriceps muscle relaxation. The clinician should then locate the patella. The clinician may mark it with a marking pen.

The knee is the largest synovial cavity in the body and is easily accessible from either the medial or lateral aspect, and superior, inferior at the midpoint of the patella.

Sterilize area and drape in a typical sterile fashion.

Choose the approach, then use a small syringe and small-bore needle, draw up lidocaine and anesthetize superficial skin and then deeper tissue in the projected trajectory of joint aspiration to anesthetize the track.

• For the midpoint approach, insert 18 g needle with 30 cc to 60 cc syringe one cm lateral or medial to the patella, directing the needle posterior and horizontal toward the intercondylar notch of the femur.
• Make sure to pull back on the syringe while inserting and stop once you aspirate synovial fluid.
• Attempt to aspirate as much fluid as possible.
• The superior approach is performed one cm superior and one cm medial or lateral to the patella, directing towards intercondylar notch of the femur. 
• The infrapatellar approach requires the patient to be sitting upright, with the knee flexed at 90 degrees. Needle insertion is five mm below the inferior border of the patella while also directing posterior to the patellar tendon, making this a less desirable approach.

“Milking” or compressing the joint can help facilitate the aspiration of fluid. 

Transfer fluid to specimen tubes. Remove the needle from the joint and place bandage over the insertion site.

Local trauma, including damage to nearby structures, pain, infection, and reaccumulation of effusion.[5][6][5]

If the needle placement is poor or if the synovium is thickened, it may result in a dry tap.

Hemarthrosis can occur if a large needle damages a blood vessel when performing multiple attempts. In most cases, the hemarthrosis presents within a few hours after the procedure. It is often associated with pain, stiffness, and swelling of the joint. The majority of hemarthrosis is self-limited and resolve within a few weeks. If the patient has a coagulopathy, it may need to be corrected, and consultation with a hematologist may be helpful.

If arthrocentesis is performed through an infected area of skin to look for a septic joint, the patient must receive antibiotics promptly. If the fluid is frankly purulent, admission is recommended.

Arthrocentesis is performed for the identification of the etiology or pain relief, injection of medications, or effusion drainage. To avoid complications, the clinician should be familiar with the anatomy. The risk of complications can be minimized by using an appropriate technique.

Crystal analysis is usually an important result of a successful knee aspiration. Most commonly, the clinician differentiates gout (negatively birefringent urate crystals), pseudogout (weakly positive birefringent crystals), and an infectious process.[7]

Synovial fluid associated with septic or infectious joint effusion is as follows:

• Total white blood cell count higher than 25,000/microliter (approximately 75% sensitive, 75% specific)
• Total white blood cell count higher than 50,000/microliter (approximately 60% sensitive, approximately 90% specific)
• Total white blood count higher than 100,000/microliter (approximately 20% sensitive, approximately 99% specific)
• PMN cell proportion 0.9 or higher (approximately 75% specific, 80% sensitive)
• Lactic dehydrogenase (LDH) concentration higher than 250 U/L (approximately 100% sensitive, 50% specific)
• Synovial glucose or serum glucose concentration lower than 0.5 (approximately 50% sensitive, 85% specific)
• Protein concentration higher than 3 g/dL (approximately 50% sensitive, 50% specific).

Knee joint aspiration is often an outpatient procedure. However, in most cases, the primary care provider or nurse practitioner should always consult with the orthopedic surgeon on management because there are many causes of a swollen knee. To avoid complications, the knee should only be aspirated by clinicians familiar with the anatomy.