What is the recommended next step in managing a patient with knee effusion and a small‑to‑moderate superior patellar enthesophyte on a normal radiograph, without fracture, dislocation, arthritis, or other intra‑articular pathology?

Management of Knee Effusion with Patellar Enthesophyte on Normal Radiograph

The next step is to perform arthrocentesis (joint aspiration) under ultrasound or fluoroscopic guidance to analyze synovial fluid for infection or crystal disease, followed by MRI without contrast if the aspiration is non-diagnostic and pain persists. 1

Immediate Diagnostic Priority: Rule Out Infection and Crystal Disease

• Joint aspiration is essential when effusion is present to distinguish between infectious, inflammatory, and mechanical causes of knee effusion that cannot be differentiated by imaging alone. 1
• The ACR guidelines explicitly state that imaging cannot distinguish infected from noninfected joints or fluid collections, and aspiration with culture is needed for definitive diagnosis. 1
• Image-guided aspiration using ultrasound or fluoroscopy is strongly preferred because it confirms proper needle placement, significantly reduces vascular and nerve complications, and allows planning of needle trajectory to avoid contaminating adjacent normal tissues. 1
• Synovial fluid analysis will identify septic arthritis (which can present with normal radiographs in early stages) and crystal deposition diseases (gout, pseudogout) that require specific treatment. 1

The Patellar Enthesophyte Finding

• The small-to-moderate superior patellar enthesophyte is an incidental finding that forms at tendon insertion sites and is distinct from intra-articular osteophytes. 2
• Enthesophytes rarely cause symptoms unless fractured during eccentric loading, which would present with acute trauma history and be visible on radiographs. 2
• This enthesophyte does not explain the knee effusion and should not distract from investigating the underlying cause of fluid accumulation. 2

If Aspiration is Non-Diagnostic: Proceed to MRI

• MRI without IV contrast is the next indicated examination when radiographs show only effusion and aspiration does not reveal infection or crystals. 1, 3, 4
• The ACR recommends MRI to evaluate for soft tissue pathology, bone marrow lesions, and early cartilage damage not visible on plain films. 3
• MRI accurately detects the common causes of knee effusion with normal radiographs, including:
  • Meniscal tears (though equally common in painful and asymptomatic knees in the 45-55 age group) 3
  • Articular cartilage damage 3
  • Bone marrow lesions strongly associated with knee pain 3
  • Subchondral insufficiency fractures (radiographically occult initially) 3
  • Synovitis (though contrast is optimal for visualization) 3
  • Ligament injuries 3
  • Popliteal cysts 3

Critical Pitfall to Avoid: Referred Pain

• Before proceeding to knee MRI, clinically assess for hip and lumbar spine pathology that can refer pain to the knee. 3, 4
• Examine the hip for range of motion limitations, groin pain, or positive impingement signs. 3
• Assess for lumbar radiculopathy or neurogenic claudication patterns. 3
• If hip or spine pathology is suspected, obtain appropriate radiographs of those regions first rather than proceeding directly to knee MRI. 3, 4
• Approximately 20% of patients inappropriately undergo knee MRI without recent radiographs or consideration of referred pain sources. 3

Role of Ultrasound

• Ultrasound can detect synovial pathology, effusions, and guide aspiration but is not useful as a comprehensive screening examination. 1
• Power Doppler ultrasound demonstrates increased synovial blood flow associated with knee pain and can show reduced blood flow correlating with pain reduction after joint injections. 1
• Ultrasound may demonstrate meniscal extrusion (suggesting underlying tear), chondrocalcinosis, and peripheral meniscal tears. 1

What NOT to Do

• Do not order CT scan as it provides no additional diagnostic value when radiographs are normal and would not explain the effusion. 1
• CT is only indicated for evaluating patellofemoral anatomy in repetitive subluxation or confirming prior osseous injuries, neither of which applies here. 1
• Do not order bone scan as it has low specificity and decreased anatomic resolution compared to MRI, and is not indicated for evaluating knee effusion. 1
• Do not proceed directly to MRI without first performing aspiration when effusion is present, as this may delay diagnosis of treatable infection or crystal disease. 1