Knee Arthrocentesis

Article Author:
Halleh Akbarnia

Article Editor:
Elise Zahn

Editors In Chief:
Sisira Reddy
Joseph Nahas
Chokkalingam Siva

Managing Editors:
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Orawan Chaigasame
Khalid Alsayouri
Kyle Blair
Radia Jamil
Erin Hughes
Patrick Le
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Matthew Varacallo
Heba Mahdy
Ahmad Malik
Sarosh Vaqar
Mark Pellegrini
James Hughes
Beenish Sohail
Hajira Basit
Phillip Hynes
Sandeep Sekhon

12/16/2018 9:31:48 AM


Arthrocentesis is a procedure performed to aspirate synovial fluid from a joint cavity. It is used both diagnostically and therapeutically. While arthrocentesis is deemed to be a minor surgical procedure, there is always the potential to injure blood vessels, nerves, and tendons. Thus, the procedure should only be performed by healthcare workers who do have knowledge of the anatomy of joints. To minimize the risk of injury, the extensor surface of the joint should be in extension with minimal flexion.[1][2]


Anatomical considerations include:

  • Surgeons can access the knee medially or laterally to the patella, superior or inferior to the patella.
  • The patient should extend their knee with the quadriceps muscle relaxed at 20 degrees flexion.
  • The physician should place their thumb on the patella and push it aside as they insert the needle.
  • During the suprapatellar approach, they should place it 1 cm lateral and 1 cm superior.
  • Upon identifying the midpoint of the patella, they should insert needle either 1 cm lateral or medial.
  • They should aim the needle posterior to the patella and up toward the joint space, then squeeze or "milk" both sides of the joint space to facilitate aspiration.


Therapeutic: Large and painful joint effusions can be aspirated for pain relief. Also, can be used for intra-articular injection of certain medications (primarily steroids). Evaluate response to treatment.[2][3][4]

Diagnostic: Synovial fluid analysis can provide physicians with an unequivocal etiology of acute arthritis, specifically to differentiate between septic arthritis and an inflammatory cause of bloody mono-arthritis.

The procedure can be done in the clinic or the minor operating room.


No absolute contraindications. Relative contraindications include overlying cellulitis (potential intra-articular seeding of bacteria), coagulopathy/bleeding disorders, joint prosthesis (preferably performed by orthopedic surgery), acute fracture, adjacent osteomyelitis, and uncooperative patient.


Skin cleansing solution (betadine, chlorhexidine, etc.), skin marking pen, sterile gloves, sterile gauze, 1% lidocaine, 10 cc syringe, 30cc to 60 cc syringe, 18 g and 27 g needle, specimen tubes (cell count, gram stain, and culture, and crystal analysis).


The procedure may usually be performed without an assistant. If the patient is anxious or special circumstances are predicted, an existent 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 highly recommended that some type of local anesthesia be utilized. For most surgeons, lidocaine 1% is used 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 be performed to confirm patient and correct joint. The most important 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 can easily be accessed from either the medial or lateral aspect, and superior, inferior at the midpoint of the patella.

Sterilize area and drape in 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 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 patella while also directing posterior to the patellar tendon, making this a less desirable approach.

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

Transfer fluid to specimen tubes. Remove the needle from 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 multiple attempts are made. 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 are 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 be given antibiotics promptly. If the fluid is frankly purulent, admission is recommended.

Clinical Significance

Arthrocentesis is performed for 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).

Enhancing Healthcare Team Outcomes

The knee joint is often aspirated as an outpatient. 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 healthcare workers familiar with the anatomy.

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    Contributed by Halleh Akbarnia, MD
Attributed To: Contributed by Halleh Akbarnia, MD

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Knee Arthrocentesis - Questions

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A 65-year-old male with history of atrial fibrillation on warfarin presents with an erythematous, warm, swollen and painful knee, on the leg that he had a total knee replacement 3 years prior. Which of the following is an absolute contraindication to performing arthrocentesis of the knee?

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A 32-year-old male presents to the emergency department complaining of a painful, swollen left knee. He states that the symptoms have been present for four days. He also says he has a fever and diffuse body aches. He denies trauma or an inciting injury but adds that ambulation exacerbates the pain. Vitals include a blood pressure of 105/82 mmHg, a pulse of 87 beats per minute, a respiratory rate of 15 breaths per minute and a temperature of 101.7°F. Physical examination reveals an erythematous, warm left knee, diffusely tender to touch, with a large joint effusion present. The range of motion testing is limited by severe pain. Which of the following synovial fluid findings suggests the diagnosis in this patient?

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Just like any procedure, doing a knee arthrocentesis can have complications. What is the most probable complication of a knee arthrocentesis performed with multiple attempts?

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Knee Arthrocentesis - References


Situ-LaCasse E,Grieger RW,Crabbe S,Waterbrook AL,Friedman L,Adhikari S, Utility of point-of-care musculoskeletal ultrasound in the evaluation of emergency department musculoskeletal pathology. World journal of emergency medicine. 2018     [PubMed]
Partridge DG,Winnard C,Townsend R,Cooper R,Stockley I, Joint aspiration, including culture of reaspirated saline after a 'dry tap', is sensitive and specific for the diagnosis of hip and knee prosthetic joint infection. The bone     [PubMed]
Jennings JM,Dennis DA,Kim RH,Miner TM,Yang CC,McNabb DC, False-positive Cultures After Native Knee Aspiration: True or False. Clinical orthopaedics and related research. 2017 Jul     [PubMed]
Bhavsar TB,Sibbitt WL Jr,Band PA,Cabacungan RJ,Moore TS,Salayandia LC,Fields RA,Kettwich SK,Roldan LP,Suzanne Emil N,Fangtham M,Bankhurst AD, Improvement in diagnostic and therapeutic arthrocentesis via constant compression. Clinical rheumatology. 2018 Aug     [PubMed]
Battistone MJ,Barker AM,Grotzke MP,Beck JP,Berdan JT,Butler JM,Milne CK,Huhtala T,Cannon GW, Effectiveness of an Interprofessional and Multidisciplinary Musculoskeletal Training Program. Journal of graduate medical education. 2016 Jul     [PubMed]
Kontovazenitis PI,Starantzis KA,Soucacos PN, Major complication following minor outpatient procedure: osteonecrosis of the knee after intraarticular injection of cortisone for treatment of knee arthritis. Journal of surgical orthopaedic advances. 2009 Spring     [PubMed]
Guyver PM,Arthur CH,Hand CJ, The acutely swollen knee. Part 1: Management of atraumatic pathology. Journal of the Royal Naval Medical Service. 2014     [PubMed]


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