Knee Joint Aspiration and Injection OSCE, Technique and Fluid Analysis

Why This Station Is Tested

Knee joint aspiration is one of the most commonly performed joint procedures in clinical practice. OSCE stations test your ability to obtain consent, apply sterile technique, select the correct approach, interpret the aspirated fluid, and manage post-procedure care. The interpretation of synovial fluid, distinguishing septic arthritis from crystal arthropathy, is a particularly high-yield examiner question.

Indications for Knee Aspiration

Diagnostic:

• Suspected septic arthritis (must aspirate before treating)
• Crystal arthropathy (gout or pseudogout)
• Unexplained acute monoarthritis
• Tense haemarthrosis after trauma

Therapeutic:

• Tense, painful effusion causing discomfort
• Followed by injection of corticosteroid for inflammatory arthritis

Contraindications

Before the Procedure: Consent

Explain the procedure fully. Key points to cover:

• What the procedure involves: inserting a needle into the knee to remove fluid
• Why you are doing it: to diagnose the cause and/or relieve pain
• Local anaesthetic will be used to numb the skin
• Risks: infection (rare, approximately 1 in 50,000), bleeding, pain, failure to obtain fluid, damage to cartilage (minimised by technique)
• Confirm no allergies to local anaesthetic
• Obtain verbal consent

Equipment

• Sterile gloves and dressing pack
• Chlorhexidine skin preparation
• Local anaesthetic: lidocaine 1% with 25G needle for skin
• Aspiration: 18-21G needle with 20-50 ml syringe
• Specimen pots: a plain pot for crystals and culture, a pot with anticoagulant (EDTA) for cell count
• Sterile dressing

Knee Aspiration Technique: Medial Patellofemoral Approach

This is the most common approach in OSCE.

1. 1Position the patient: supine, knee extended and relaxed (or slightly flexed with a pillow underneath if the patient is more comfortable)
2. 2Identify the landmarks: the medial border of the patella and the medial joint line
3. 3Clean the skin with chlorhexidine in a circular motion; allow to dry for 30 seconds
4. 4Raise a skin bleb of local anaesthetic (lidocaine 1%) at the entry point using a 25G needle
5. 5Wait 1-2 minutes for the anaesthetic to work
6. 6Warn the patient: "You'll feel some pressure now"
7. 7Insert the 18-21G needle at the midpoint of the medial border of the patella, angled 45 degrees posteriorly and slightly inferiorly, directing toward the joint space behind the patella
8. 8Advance until a give or loss of resistance is felt (entering the synovial space)
9. 9Aspirate: pull back the plunger gently; fluid should flow freely
10. 10If no fluid returns, reposition 1-2 mm; do not redirect dramatically
11. 11Send fluid to laboratory; apply gentle pressure and a dressing

Synovial Fluid Analysis: The Key Examiner Topic

This is where the marks are earned in follow-up questions.

Crystal Analysis: Gout vs Pseudogout

This is a classic examiner question:

Post-Procedure Instructions

• Apply a sterile dressing; the patient can remove it after 24 hours
• Relative rest for 24 hours; avoid strenuous activity
• Ice packs for comfort
• The joint may feel temporarily more uncomfortable after aspiration
• If injecting with corticosteroid: expect improvement over 1-2 weeks, possible post-injection flare in first 48 hours
• Return urgently if: increasing redness, warmth, swelling, fever (signs of infection)

Frequently Asked Questions

"How do you distinguish septic arthritis from gout on history and investigation?"

Clinically, both present with an acutely hot, swollen, painful joint. Features favouring septic arthritis: systemic features of infection (fever, rigors, raised CRP and WCC), a clear source of bacteraemia (recent skin infection, UTI, IV drug use, joint injection, or surgery), immunosuppression, and no prior history of gout. Features favouring gout: prior episodes of acute monoarthritis, classic first MTP joint involvement (podagra), alcohol history, diuretic use, high purine diet, and known hyperuricaemia. Crucially, the two can coexist, and septic arthritis can occur in a joint affected by gout. Therefore, always aspirate the joint and send for microscopy and culture before diagnosing crystal arthropathy. Crystals in the fluid do not exclude infection.

"What does the synovial fluid look like in haemarthrosis and why does it occur?"

Haemarthrosis (blood in the joint) presents as a tense, painful effusion appearing very rapidly after injury, typically within 2 hours. The aspirated fluid is frank blood or blood-stained and does not clear with continued aspiration (distinguishing true haemarthrosis from a traumatic tap, where the fluid clears). Causes include: anterior cruciate ligament (ACL) rupture (most common cause of acute haemarthrosis in a young patient after a pivoting injury), tibial plateau fracture, patellar dislocation, and peripheral meniscal tear with vascular involvement. Fat globules floating in the blood (lipaemia haemarthrosis) indicate an intra-articular fracture, as bone marrow fat has entered the joint space. If fat globules are visible, urgent X-ray and CT are required to identify the fracture.

"A patient with a hot swollen knee has fluid aspirated showing 80,000 WBC per mm3 with 90% neutrophils and glucose 1.2 mmol/L (serum 6.0 mmol/L). What is the diagnosis and management?"

This synovial fluid analysis is consistent with septic arthritis. A WBC count above 50,000 per mm3 with a predominance of neutrophils and synovial glucose below 50% of the simultaneous serum glucose (1.2 versus 6.0 represents a ratio of 0.2, well below 0.5) are diagnostic criteria for joint infection. Management: send the aspirated fluid for urgent Gram stain and culture (already done), blood cultures, FBC and CRP. Start IV antibiotics empirically after cultures are taken, do not delay waiting for results if the patient is systemically unwell. The choice of antibiotic depends on local protocol and Gram stain results; flucloxacillin covers Staphylococcus aureus (most common organism). Refer urgently to orthopaedics for consideration of surgical joint washout, which is required in most cases to debride infected tissue and prevent cartilage destruction.

"What is chondrocalcinosis and what does it look like on X-ray?"

Chondrocalcinosis is the deposition of calcium pyrophosphate dihydrate (CPPD) crystals within articular cartilage and fibrocartilage. On X-ray, it appears as a dense white line running parallel to the joint surface within the cartilage (not the bone). In the knee, it most commonly affects the fibrocartilage of the medial and lateral menisci and the hyaline cartilage, producing the characteristic "tram-track" or double-line calcification visible on plain films. It can also occur in the triangular fibrocartilage of the wrist, the symphysis pubis, and the articular cartilage of the hip. Not all patients with chondrocalcinosis develop pseudogout attacks, and the condition may be asymptomatic. Associated conditions include hyperparathyroidism, haemochromatosis, hypomagnesaemia, and hypothyroidism, which should be screened for in younger patients with chondrocalcinosis.