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.

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

Recommended Laboratory Tests for Synovial Fluid Analysis

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.
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.
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.