Joint Effusion (Swollen Joint): Causes, Diagnostic Approach, and Treatment

Joint effusion—excess synovial fluid in a joint cavity—causes swelling, stiffness, and pain. Identifying the underlying etiology is essential for targeted treatment.

Common Causes

• Trauma: Hemarthrosis (blood) from fracture or ligament tear.
• Inflammatory: Rheumatoid arthritis, gout, pseudogout, psoriatic arthritis, septic arthritis.
• Degenerative: Osteoarthritis (mechanical irritation).
• Infection: Septic arthritis (urgent orthopedic emergency).
• Overuse: Repetitive stress.
• Systemic: Heart failure, cirrhosis, nephrotic syndrome (low oncotic pressure).
• Baker's cyst: Popliteal cyst rupturing or communicating with knee joint.

Diagnostic Approach

1. History and physical exam (warmth, erythema, range of motion).
2. Imaging: X-ray, ultrasound, MRI as indicated.
3. Arthrocentesis with fluid analysis: cell count, crystals, Gram stain, culture, glucose, protein.

Treatment Principles

• Address underlying cause (antibiotics for sepsis, urate-lowering for gout, DMARDs for RA).
• Symptomatic: NSAIDs, rest, compression, elevation.
• Therapeutic aspiration for tense, painful effusions.
• Intra-articular corticosteroids (only after infection excluded).

Recurrent or unexplained effusion warrants rheumatology or orthopedic referral.