Tips For General Practitioners – Knee Aspiration
In Singapore, 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.
Our doctors at OrthoSports Centre have put together a guide to the best practices of knee aspiration for practitioners. Patients will also get a greater understanding of how it works.
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 i.e. gout or pseudogout
- Aspirating a knee enables a surgeon to both make a diagnosis and offer a therapeutic solution to patients with effusions.
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 effusions from knee aspirations in Singapore.
|Type of Aspirate||Differential Diagnosis|
|Straw coloured effusion||Acute or chronic meniscal injury|
|Frank blood||Acute ligamentous injury or osteochondral fractures|
|Fat globules||Osteochondral fractures|
|Presence of birefringent crystals under polarized light||Gout or Pseudogout|
|Turbid fluid or pus||Infection|
Knee 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
Knee Aspiration Technique
- 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
The most important landmark for knee aspiration is the lateral edge of the patella.
1. Start off by identifying the superior pole and lateral edge of the patella (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).
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)
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).
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 Knee Aspiration Advice For Patients
- Avoid strenuous activity for the following 24 hours.
- Skin abrasions or infections.
- Known allergy to local anaesthetics.