Aspiration Before Steroid Injection for Large Knee Effusion
For a large knee effusion, you should aspirate before administering a corticosteroid injection—this serves both diagnostic and therapeutic purposes, particularly to rule out infection and crystal disease, and to improve the effectiveness of the steroid injection. 1, 2
Primary Rationale for Aspiration
Aspiration is rated as "usually appropriate" (9/9) by the American College of Radiology when an effusion is present, and should be performed after reviewing knee radiographs. 1, 3
Diagnostic Benefits
Joint aspiration is essential to exclude infection and crystal disease before injecting corticosteroids, as these conditions require different management and corticosteroid injection could worsen septic arthritis. 1, 2
Synovial fluid analysis should include Gram stain, total and differential cell counts, and aerobic/anaerobic cultures to establish the correct diagnosis. 3
In cases of unexplained effusion, aspiration aids in earlier establishment of diagnosis, particularly for non-traumatic causes. 4
Therapeutic Benefits
Removing excess synovial fluid before steroid injection prevents undue dilution of the corticosteroid, which can compromise its effectiveness. 5
The FDA label for triamcinolone acetonide specifically states: "If an excessive amount of synovial fluid is present in the joint, some, but not all, should be aspirated to aid in the relief of pain and to prevent undue dilution of the steroid." 5
Following aspiration, corticosteroid injections result in a 1 to 2 week reduction in synovitis in approximately two-thirds of patients. 1
Clinical Approach Algorithm
Step 1: Obtain Radiographs First
- Plain radiographs should be obtained prior to joint aspiration to assess for underlying structural abnormalities. 3
Step 2: Perform Aspiration
Use strict aseptic technique to minimize infection risk. 5
For large effusions, aspirate "some, but not all" of the fluid—complete drainage is not necessary. 5
Send fluid for cell count with differential, Gram stain, aerobic/anaerobic cultures, and crystal analysis if clinically indicated. 3, 6
Step 3: Proceed with Steroid Injection
After aspiration and if infection is excluded, inject the corticosteroid into the same site using the same needle entry point. 5, 7
For larger joints like the knee, doses of 5-15 mg of triamcinolone acetonide are typically used, with up to 40 mg for larger areas. 5
Important Caveats and Pitfalls
Timing Considerations
If infection is suspected, withhold antibiotics for at least 2 weeks before aspiration to increase the likelihood of recovering an organism. 3
False-negative aspirations may occur in patients who received preaspiration antibiotic treatment. 8, 2
When Aspiration Alone May Be Sufficient
Aspiration provides only temporary improvement (lasting approximately one week) due to early re-accumulation of effusion, particularly in traumatic cases. 4
The clinical benefit of aspiration alone without corticosteroid injection is limited beyond the first week. 4
Potential Complications
Postinjection flares can occur, ranging from mild joint effusion to severe inflammatory reactions mimicking septic arthritis, though these are rare. 9
The acute crystal-induced inflammatory response from triamcinolone acetonide crystals can produce severe symptoms requiring repeat aspiration. 9
Technical Points
A "dry tap" does not exclude the presence of joint fluid or infection—if initial aspiration is unsuccessful, ultrasound-guided repeat aspiration should be performed. 8
Multiple aspirations may be necessary in some cases, with one study reporting that more than three aspirations were needed to obtain positive culture in 16% of infected patients. 2
Care should be taken to inject into the joint space rather than surrounding tissues to avoid tissue atrophy. 5
Special Populations
Post-Total Knee Arthroplasty
Aspiration is crucial to rule out periprosthetic joint infection in patients with total knee arthroplasty and persistent effusion, with reported sensitivity, specificity, and accuracy of up to 100%. 2, 3
If the first aspiration is negative but clinical suspicion remains high, weekly repeat aspirations are recommended. 2