Ovid: 5-Minute Orthopaedic Consult

Editors: Frassica, Frank J.; Sponseller, Paul D.; Wilckens, John H.
Title: 5-Minute Orthopaedic Consult, 2nd Edition
> Table of Contents > Arthrocentesis

Michelle Cameron MD
Timothy S. Johnson MD
Aspiration of synovial joints is performed for diagnostic or therapeutic purposes.
  • Causes of joint effusions:
    • Infection
    • Crystalline arthropathies
    • Hemophilia
    • Autoimmune arthropathies
    • Trauma
    • PVNS
Signs and Symptoms
  • Synovial joints (Table 1) are aspirated for myriad reasons, the most common of which include:
    • To rule out infection
    • To diagnose arthropathies
    • To relieve pain
  • Joints with enough fluid to perform arthrocentesis generally have a palpable effusion.
  • Infectious, autoimmune, and crystalline
    arthropathies often are warm to the touch and may display overlying
    erythema or cellulitis.
Physical Exam
  • An effusion usually is palpable.
  • The patient with infectious and crystalline arthropathies or trauma may have difficulty with ROM of the affected joint.
  • There may be outward signs of trauma or inflammation such as abrasions, erythema, or cellulitis.
Table 1 Best Locations for Aspirating Joint Effusions
Joint Site of Aspiration
Elbow Posterolaterally
Shoulder Anteriorly (ultrasound, CT, fluoroscopy are helpful)
Hip Anteriorly or laterally (CT, fluoroscopy are helpful)
Knee Laterally or medially from suprapatellar pouch
Ankle Anterolaterally
Table 2 Joint Fluid Analysis
Type Appearance White Blood Cell Count Polymorphonuclear Cells (%) Comment
Noninflammatory Straw-colored 200 25 Osteoarthritis: Serum glucose and protein equivalent to aspirate values
Inflammatory Cloudy yellow 2,000–75,000 50 Gout: Strongly, negatively birefringent crystals
        Calcium pyrophosphate deposition disease (pseudogout): Weakly, positively birefringent crystals
        Rheumatic disease: Serum studies help elucidate type.
Infection Purulent/ opaque >80,000 >75 +Gram stain and culture
        Serum glucose >aspirate glucose
        Serum protein <aspirate protein
        Polymerase chain reaction: May be helpful for detecting Lyme disease
Traumatic Bloody Few <25 Consider fracture if a fat-fluid separation develops in syringe after a few minutes.
Radiographs often are helpful to rule out trauma and to evaluate degenerative changes within the joint.
Pathological Findings (Table 2)
  • Crystalline arthropathies show crystals when specimens are examined with polarized light:
    • Gout: Monosodium urate crystals appear
      sharp (needle-like) by normal light microscopy and are brightly
      birefringent on compensated polarized microscopy.
    • The calcium pyrophosphate crystals of pseudogout have blunt ends and are not birefringent.
  • Aspirates from septic arthritis often
    have cell counts >100,000 with >80–90% polymorphonuclear cells,
    and they may have organisms present on Gram staining.
  • Crystalline and inflammatory arthropathies also can have high white cell counts in the 50,000/mm3 range.
Differential Diagnosis
  • Septic arthritis
  • Gout
  • Pseudogout
  • Autoimmune disorders, such as rheumatoid arthritis or systemic lupus erythematosus
  • Trauma
  • Hemophilia
General Measures
  • Patients with traumatic effusions should
    be treated appropriately for their underlying traumatic injury, but
    arthrocentesis of the affected joint often makes these patients more
  • Patients with septic arthritis require joint irrigation and débridement; the type of débridement depends on the joint involved.
  • Appropriate antibiotics also should be administered after all cultures are obtained.
  • Patients with inflammatory and
    crystalline arthropathies generally respond well to anti-inflammatory
    medications and should be referred to a rheumatologist for evaluation.


  • Sterile skin preparation is required before aspiration.
  • Injection of local anesthetic with a
    small-gauge needle into the skin may lessen the pain of aspiration
    (especially if more than 1 attempt is necessary).
  • The needle gauge should be large enough to withdraw the viscous joint fluid (usually 18 gauge or larger) (Fig. 1).
  • Risk of iatrogenic infection:
    • Care should be taken, especially when aspirating a potentially infected joint.
    • If cellulitis is present, care should be
      taken to aspirate through uninvolved skin to avoid infecting a
      previously aseptic joint.
  • Aspiration may be predisposing in as many as 23% of all cases of septic arthritis (1,2).
    Fig. 1. Arthrocentesis of the knee is most easily done from the lateral side, under the patella.
1. Esterhai JL, Jr, Gelb I. Adult septic arthritis. Orthop Clin North Am 1991;22:503–514.
2. Tsumura
H, Ikeda S, Torisu T. Debridement and continuous irrigation for the
treatment of pyogenic arthritis caused by the use of intra-articular
injection in the osteoarthritic knee: indications and outcomes. J Orthop Surg (Hong Kong) 2005;13:52–57.
Additional Reading
Park AL, Diabach JA. Infectious arthritis. In: Canale ST, ed. Campbell’s Operative Orthopaedics, 10th ed. St. Louis: Mosby, 2003:685–711.
Q: What is the appearance of the joint aspirate under light microscopy in the setting of gout?
A: Sharp, needle-like crystals.
Q: A joint aspirate with a white blood cell count >100,000 per high-powered field is consistent with which type of arthritis?
A: Septic arthritis.

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