Equipment
The materials required for knee arthrocentesis include the following:
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Sterile gloves and drapes
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Gauze pads (5), 4 × 4 in.
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Skin preparatory solution
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Lidocaine 1%
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Syringes, 5 mL, 20 mL, 30 mL, 60 mL
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Needles, 18 or 20 gauge and 25 or 27 gauge
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Patients who are morbidly obese might require a 21-gauge spinal needle for arthrocentesis
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Hemostat
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Specimen tubes
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Bandage
Patient Preparation
Anesthesia
Patients who are anxious, in severe pain, or unable to cooperate with the procedure (most commonly children) might require procedural sedation and/or analgesia.
Local anesthesia is always warranted. After skin preparation, draping, and identification of the needle insertion site, use a 25- or 27-gauge needle to inject 2-5 mL of local anesthetic (eg, lidocaine 1%) into the subcutaneous tissue (see the image below). (See Local Anesthetic Agents, Infiltrative Administration.) Deep injections that might enter the joint space are not recommended, because they may alter the synovial fluid analysis results.
Positioning
After obtaining informed consent, place the patient supine on a gurney. Place a rolled towel below the patient’s knee. A small but randomized study demonstrated that more joint fluid was aspirated from patients in the supine position than from patients in the sitting position. [4]
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Anterior view of right knee.
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Anatomic landmarks for knee arthrocentesis.
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Infiltration of local anesthetic via medial parapatellar approach (left knee).
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Left-knee aspiration via medial parapatellar approach.
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Application of bandage after left-knee aspiration.
Tables
|
Appearance |
WBCs, cells/µL |
PMN cells |
Glucose concentration, mg/dL |
Protein concentration, g/dL |
Normal |
Clear |
< 150 |
< 0.25 |
Serum glucose |
1.3-1.8 |
Noninflammatory |
Clear |
< 3000 |
< 0.25 |
Serum glucose |
2-3.5 |
Inflammatory |
Cloudy |
>3000 |
< 0.75 |
< 25 |
>4 |
Purulent |
Cloudy |
>50,000 |
>0.9 |
< 25 |
>4 |
Hemorrhagic |
Bloody |
>2000 |
~0.3 |
Serum glucose |
… |
PMN = polymorphonuclear; WBC = white blood cell. |
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