What is the best next step for a 50-year-old man with rheumatoid arthritis (RA) on methotrexate, presenting with a red, swollen, and tender right knee joint, and inability to bear weight?

Management of Acute Knee Swelling in a Patient with Rheumatoid Arthritis

Arthrocentesis is the best next step for this 50-year-old man with rheumatoid arthritis presenting with acute knee swelling, redness, and inability to bear weight. 1

Rationale for Arthrocentesis

The patient's presentation raises significant concern for septic arthritis, which requires immediate diagnosis and treatment:

• Acute onset of worsening joint pain and swelling
• Inability to bear weight
• Redness and tenderness of the affected joint
• Patient on immunosuppressive therapy (methotrexate)

These findings meet several of the Kocher criteria for septic arthritis, including inability to bear weight on the affected side 2. In patients with inflammatory arthritis like RA, the distinction between a disease flare and septic arthritis is critical, as septic arthritis is an orthopedic emergency requiring prompt intervention.

Diagnostic Algorithm

1. Perform arthrocentesis immediately

   • Ultrasound guidance may be helpful for accurate needle placement 3
   • Collect synovial fluid for:
     • Cell count with differential (WBC >50,000 with >90% neutrophils suggests infection)
     • Crystal analysis (to rule out crystal arthropathies)
     • Gram stain and culture
     • Glucose and protein levels

2. Interpret synovial fluid results:

   • If septic arthritis: Urgent orthopedic consultation for joint drainage/irrigation and empiric antibiotics
   • If RA flare: Adjust RA medications according to treat-to-target principles
   • If crystal arthropathy: Treat accordingly with anti-inflammatory medications

Why Other Options Are Inferior

• Observation (Option A): Potentially dangerous if septic arthritis is present; delay in diagnosis increases joint destruction and mortality risk 1

• High-dose ibuprofen and oral glucocorticoids (Option B): May mask infection symptoms while allowing infection to progress; inappropriate without knowing the underlying cause 1

• MRI of the knee (Option C): While sensitive for detecting synovitis and erosions, it delays diagnosis of potential septic arthritis, which requires immediate fluid analysis 1

Important Considerations

• Methotrexate therapy should not be discontinued during the evaluation process, as discontinuation can lead to RA flares without reducing infection risk 4

• Comorbidities like obesity and fibromyalgia can complicate assessment of disease activity in RA patients, making objective evaluation through arthrocentesis even more valuable 2

• Ultrasound-guided arthrocentesis is more successful and less painful than landmark-guided techniques, particularly in difficult cases 5

Follow-up Management

After arthrocentesis results:

• If septic arthritis is confirmed: Immediate orthopedic consultation, joint drainage, and empiric broad-spectrum antibiotics 1

• If RA flare is confirmed: Consider intra-articular glucocorticoid injection and reassess overall RA treatment strategy 2

• If crystal arthropathy is identified: Treat with appropriate anti-inflammatory medications 1

The key principle is to never delay arthrocentesis when septic arthritis is in the differential diagnosis, as this condition represents a true medical emergency with significant morbidity and mortality if not promptly diagnosed and treated 3.