Tips for General Practitioners – Knee Aspiration

 

Knee Aspiration

Knee pain or swelling are some of the most common reasons for seeing a doctor. In fact many patients are particularly concerned about knee effusions even when not associated with pain.

There can be many varied reasons for a patient developing a knee effusion. These include:

  • acute knee injury causing a hemarthrosis
  • degenerative arthritis
  • crystalline arthropathy ie gout or pseudogout
  • infection

Aspirating a knee enables a surgeon to both make a diagnosis and offer a therapeutic solution to patients with effusions.

 

Diagnostic indications

The type of fluid aspirated often provides a clue to the diagnosis. Occasionally laboratory tests are required to confirm the diagnosis. Table 1 offers a simple but not exhaustive guide to some of the more common causes for knee effusions.

Type of aspirate Differential diagnosis
Straw coloured effusion Acute or chronic meniscal inury
Degenerative arthritis
Frank blood Acute ligamentous injury or osteochondral fracutres
Fat globules Osteochondral fractures
Presence of birefringent crystals under polarized light Gout or Pseudogout
Turbid fluid or pus Infection

 

Therapeutic indications

Aspiration of a tense haemarthrosis or very large effusion will quite often bring immediate pain relief to a patient. Supplementary treatment with analgesia and ice is also useful.

 

Technique

Equipment

      • Use a 20 mL syringe and a 21-Gauge needle.
      • A small amount of Lignocaine can be used to make the procedure more comfortable for the patient. Generally, 2mg per kg bodyweight provides adequate analgesia within safe limits for an average adult.
      • Aseptic techniques should be strictly adhered to.
      • Sterile containers for culture and crystal cytology

Location

The most important landmark for aspirating the knee is the lateral edge of the patella.

1. Start off by identifying the superior pole and lateral edge of the patella (Figure 1).

Knee Asp

 

Figure 1.

2. Identify the soft spot approximately 1 cm below the lateral edge of the patella. This is the landmark for aspirating the knee (Figure 2).

Knee Asp 2

 

Figure 2.

3. Lightly hold the patella between the thumb and index finger. The needle should be introduced into the soft spot just under the patella. (Figure 3)

Knee Asp 3

 

Figure 3.

4. There should be no resistance to flow and the patient should be comfortable throughout the procedure. Occasionally the needle may need to be withdrawn or angled slightly to maximize extraction of the effusion or blood.

Frank blood is aspirated from this patients knee suggesting the likelihood of an Anterior Cruciate Ligament tear. (Figure 4).

Knee Asp 4

 

Figure 4.

If there is a clinical suspicion of infection, urgent Gram stain should be requested for and the aspirate should be sent for microscopy and culture.

 

Post op advice for patients

  • Avoid strenuous activity for the following 24 hours.

 

Contraindications

  • Skin abrasions or infections.
  • Known allergy to local anesthetics.