OSCE Orthopedics: Knee Arthrocentesis

Guide to Knee Arthrocentesis for OSCE

Purpose and Context

Knee arthrocentesis is a diagnostic and therapeutic procedure used to:

• Diagnose: Assess joint effusion for infection, crystal-induced arthritis (e.g., gout), or hemorrhage.

• Treat: Relieve pain and swelling by draining fluid or administering medication, such as corticosteroids, for inflammatory conditions.
  

Step-by-Step Procedure and Technique

1. Patient Positioning

• Position the patient supine with the knee slightly flexed.

• Place a small rolled towel under the knee to maintain slight flexion, which opens the joint space and facilitates needle entry.
  

2. Landmark Identification

• Identify the patella (kneecap) and lateral and medial joint lines.

• There are four possible insertion points for knee arthrocentesis, each with distinct advantages:

  • Inferolateral Approach: Locate the inferior lateral border of the patella (just below and lateral to the kneecap). Insert the needle here to access the joint space without risk to major structures.

  • Inferomedial Approach: Locate the inferior medial border of the patella (just below and medial to the kneecap). This approach also provides a safer entry to the joint space.

  • Superolateral Approach: Locate the superior lateral border of the patella (above and lateral to the kneecap). Insert the needle here if the inferior approach is unsuitable.

  • Superomedial Approach: Locate the superior medial border of the patella (above and medial to the kneecap). This provides an alternative option, avoiding medial neurovascular structures.
    

3. Skin Preparation

• Prepare the skin thoroughly using an antiseptic solution (e.g., povidone-iodine or chlorhexidine).

• Sterilize a sufficiently large area to allow flexibility in needle positioning.
  

4. Local Anesthesia

• Administer 1% lidocaine to numb the puncture site.

• Use a small-gauge needle (e.g., 25-gauge) to anesthetize the skin, subcutaneous tissue, and joint capsule for patient comfort.
  

5. Needle Insertion by Approach

• Use a 20-22 gauge needle for the procedure, adjusting the angle and insertion depth according to the approach:

  • Inferolateral Approach:

    • Insert the needle at a 45° angle below the patella on the lateral side, directing it toward the joint center.

    • Slowly advance and aspirate to confirm joint entry, feeling for a change in resistance.

  • Inferomedial Approach:

    • Similar to the inferolateral approach, but insert below the patella on the medial side.

    • Angle the needle towards the joint space, taking care to avoid medial structures.

  • Superolateral Approach:

    • Insert the needle at a 45° angle above the patella on the lateral side, directing it toward the center of the joint.

  • Superomedial Approach:

    • Similar to the superolateral approach, but insert above the patella on the medial side, avoiding structures like the saphenous nerve and vein.
      

6. Aspiration or Injection

• Aspiration:

  • Attach a syringe to withdraw synovial fluid.

  • Note the color and clarity for diagnostic purposes:

    • Clear/Straw-Colored: Suggests normal or inflammatory effusion.

    • Cloudy/Purulent: Indicates possible infection.

    • Bloody: Suggests hemarthrosis or trauma.

• Injection:

  • For therapeutic injection (e.g., corticosteroids), ensure there is no air in the syringe before administration.

  • Inject medication slowly to avoid causing discomfort.
    

7. Post-Procedure Care

• After removing the needle, apply gentle pressure to the site to prevent bleeding, then cover with a sterile bandage.

• Advise the patient to rest the knee, avoid strenuous activities for 24 hours, and watch for signs of infection.
  

Pain and Recovery

• Mild discomfort and minor swelling are expected post-procedure.

• Patients can use ice on the knee and should avoid excessive knee strain for 24-48 hours.
  

Risks and Complications

• Injury to nerves, blood vessels, or tendons is rare when using correct anatomical landmarks.

• Infection risk is minimized with sterile technique, and ultrasound guidance may further reduce complications.

• Post-Injection Flare: Some patients may experience temporary increased pain after injection.

  
  

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For knee arthrocentesis, when fluid is aspirated, the standard tests sent to the lab typically include:

Recommended Laboratory Tests for Synovial Fluid Analysis

1. Cell Count and Differential:

   • Purpose: To evaluate the white blood cell (WBC) count, which helps distinguish between inflammatory and non-inflammatory processes.

   • Interpretation:

     • < 2000 WBC/µL: Typically indicates a non-inflammatory condition (e.g., osteoarthritis).

     • 2000-75,000 WBC/µL: Suggests an inflammatory condition (e.g., rheumatoid arthritis, gout).

     • > 75,000 WBC/µL: Strongly suggests infection or septic arthritis.
       

2. Gram Stain and Culture:

   • Purpose: To identify potential bacterial pathogens in cases where septic arthritis is suspected.

   • Interpretation:

     • Positive Gram Stain or Culture: Indicates bacterial infection, usually requiring urgent treatment with antibiotics.

     • Negative Results: Reduces the likelihood of bacterial infection but does not fully exclude it, especially in cases of partially treated infections.
       

3. Crystal Analysis:

   • Purpose: To detect crystals associated with crystal-induced arthritis.

   • Interpretation:

     • Monosodium Urate Crystals: Indicates gout.

     • Calcium Pyrophosphate Crystals: Indicates pseudogout.
       

Arthrocentesis Synovial Fluid Profile Interpretation

Synovial fluid analysis can provide essential information to aid in diagnosing the underlying knee joint pathology. Here’s a breakdown based on the findings:

Parameter	Non-inflammatory (e.g., OA)	Inflammatory (e.g., RA, gout)	Septic Arthritis
Appearance	Clear, yellow	Cloudy, yellow	Turbid, purulent
WBC Count	< 2000 cells/µL	2000-75,000 cells/µL	> 75,000 cells/µL
Neutrophil %	< 25%	> 50%	> 75%
Gram Stain	Negative	Negative	Often Positive
Crystal Analysis	Negative	Positive for gout or pseudogout	Negative

Each parameter offers diagnostic insights:

• Appearance and Clarity: Clear fluid is generally non-inflammatory; turbid or purulent fluid suggests infection.

• Cell Count and Differential: Elevated WBC with a high percentage of neutrophils points to infection.

• Gram Stain and Culture: Essential for confirming or ruling out bacterial arthritis.

• Crystal Analysis: Confirms or excludes crystal-induced arthritis.

These parameters combined allow clinicians to diagnose and differentiate between various causes of knee effusion, ensuring appropriate management and treatment.