Arthocentesis of the Knee

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Review by Jose Vega, MD
Column Organized by Evan Schwarz, MD
Division of Emergency Medicine
Washington University
Thomsen TW et al.  Arthocentesis of the Knee.  The New England Journal of Medicine 2006; 354:e19.

It is the first shift of your senior year and your attending asks if you can assist the 3rd year medical student with a “knee tap.”  Of course, you reply with confidence and a hint of arrogance.  You then find the medical student, who is frantically looking up the procedure on his Palm Pilot, and assure him that no pimping will occur today.  Both of you sit down and you began to educate a future physician on the art of the arthocentesis. 
The purpose of performing arthrocentesis of the knee or any other joint is twofold: therapeutic procedure to drain large effusions, hemarthroses and/or instill steroids or anesthetics;  and to diagnose crystal arthropathies or septic arthritis.  Arthrocentesis is contraindicated in patients with cellulitis overlying the site of needle entry.  Suspected bacteremia is a relative contraindication.  The largest synovial cavity in the body resides within the knee joint.  The knee may be tapped 1cm medial or lateral to the superior third of the patella and is directed toward the intracondylar notch.
Prior to performing the procedure, explain the procedure to the patient including risks, benefits and obtain informed consent.  Obtain your equipment and position the patient supine with the knee extended or flexed 15-20 degrees.  Identify the landmarks and mark the entry site with a skin marking pen.  When performing an arthrocentesis, maintain sterile conditions as joint fluid is prone to infection if bacteria is introduced.  Cleanse the skin with povidone-iodine or chlorhexidine and use a sterile drape.  Anesthetize the entry site using a 25-gauge needle, placing a wheal of lidocaine and then anesthetize the deeper tissues.  It is important to intermittenly pull back on the plunger during this step to exclude intravascular placement.  Obtain a large syringe with an 18-gauge needle and direct the needle behind the patella.  Do not “walk” the needle along the inferior surface of the patella as this may damage the delicate articulate cartilage.  Remove as much fluid as possible. “Milking” the effusion by applying gentle compression to the suprapatellar region with the opposite hand may aid in removal.  Once the procedure is complete, remove the needle and apply a bandage.  Crystal analysis can occur with one drop of synovial fluid, while cell counts generally require about 1 mL o ffluid.
The collected synovial fluid is then sent for cell count and differential, gram staining, culture, and crystal analysis.  A cutoff of 2000 white blood cells per milliliter and 75 percent polymorphonuclear cells is used to differentiate between non-inflammatory (OA and trauma) and inflammatory conditions.  Cell count and differential count cannot reliably differentiate among inflammatory conditions.  Gram staining and culture provide the most definitive evidence of septic arthritis.  The sensitivity is much higher for nongonococcal infections (50-75% gram stain and 75-95% culture) than disseminated gonococcal disease (<10% and 10-50% , respectively).  Consider blood, urethral, rectal or oropharyngeal cultures if gonococcus is suspected.  The sensitivity of crystal analysis is 80-95% for gout and 65-80% for pseudogout.  Needle-shaped, negative-birefringent, monosodium urate crystals seen in gout and rhomboid-shaped, positive-birefringent, calcium pyrophosphate crystals seen in pseudogout are visualized under a polarizing light microscope.  The presence of crystals does not exclude septic arthritis, although a St. Luke’s-Roosevelt Hospital retrospective review of crystal-positive synovial fluid aspirates suggests concomitant septic arthritis is rare (1.5% prevalence).  
Potential complications include a “dry tap”, failure to aspirate synovial fluid.  Obesity, hypertrophy of the synovium, obstruction of the needle lumen or misdiagnosis of the knee effusion may result in a dry tap.  A different approach (lateral or medial) should be attempted.  Localized trauma, pain, reaccumulation of the effusion or iatrogenic infection are other potential complications.  Arthrocentesis of the knee is a relatively benign procedure if properly performed.          

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