What are the causes of a knee joint effusion?

Causes of Knee Joint Effusion

Knee joint effusion results from traumatic injury (most common), infection, inflammatory arthritis, crystal deposition, osteoarthritis, or malignancy, with the specific etiology determined by patient age, trauma history, and systemic symptoms. 1, 2

Traumatic Causes

Acute hemarthrosis from trauma is the most common cause of knee effusion in all age groups. 1

• Ligamentous injuries (ACL, PCL, collateral ligaments) produce immediate effusion and are strongly associated with recent trauma 3, 4
• Meniscal tears occur frequently, though horizontal tears may be pre-existing rather than acute traumatic injuries 4
• Intra-articular fractures including tibial plateau, femoral condyle, and patellar fractures cause lipohemarthrosis 3
• Bone contusions from impaction injuries produce marrow edema and associated effusion 3
• Tendon ruptures (quadriceps, patellar) can cause effusion with associated soft tissue swelling 3

A critical distinction: radial, longitudinal, and complex meniscal tears are strongly related to trauma (odds ratio 8.6 for old trauma, 3.2 for recent trauma), whereas horizontal meniscal tears are often pre-existing and unrelated to the acute presentation. 4

Infectious Causes

Septic arthritis is an orthopedic emergency requiring immediate joint aspiration and empiric antibiotics. 5

• Bacterial septic arthritis presents with fever, inability to bear weight, and refusal to move the joint 5, 6
• In children under 5 years with CRP >2.0 mg/dL, the probability of septic arthritis exceeds 90% 6
• Lyme arthritis causes large knee effusions and requires 28 days of oral antibiotic therapy initially 5
• Tuberculous synovitis is a rare cause requiring synovial biopsy for diagnosis 7

Inflammatory and Crystal-Induced Causes

• Rheumatoid arthritis produces chronic effusions with synovial proliferation 7
• Pseudogout (calcium pyrophosphate deposition) causes acute inflammatory effusions, particularly in patients with spinal cord injury 8
• Gout produces monoarticular effusions with urate crystal deposition 2
• Reactive arthritis follows genitourinary or gastrointestinal infections 1, 2

Degenerative Causes

Osteoarthritis-related effusions are associated with synovitis and bone marrow lesions on MRI. 3

• Osteoarthritis produces effusions related to cartilage breakdown and synovial inflammation (odds ratio 4.7 for effusion presence) 4
• Subchondral insufficiency fractures in the medial femoral condyle cause effusion in middle-aged to elderly females 3
• Bone marrow lesions correlate strongly with knee pain and effusion, especially in males with family history of osteoarthritis 3

Pediatric-Specific Causes

• Transient synovitis is the most common atraumatic cause in children, requiring differentiation from septic arthritis 5, 6
• Osteochondritis dissecans produces effusion with loose body formation 3
• Juvenile idiopathic arthritis causes chronic inflammatory effusions 1

Other Causes

• Pigmented villonodular synovitis produces bloody or serosanguineous effusions 7
• Synovial tumors including malignancies can present with effusion 2
• Popliteal cyst rupture causes posterior knee effusion and calf swelling 3
• Hemophilia and bleeding disorders produce recurrent hemarthrosis 1
• Spasticity in neurologically impaired patients can cause chronic effusions 8

Critical Diagnostic Pitfalls

Never delay aspiration when septic arthritis is suspected—bacterial proliferation rapidly causes irreversible cartilage damage. 5

• Negative joint aspirate culture does not exclude infection if antibiotics were given prior to aspiration 5
• In patients over 70 years, meniscal tears are present in the majority of asymptomatic knees, making MRI findings potentially misleading 3, 9
• Effusion presence on ultrasound in acute trauma has 91% positive predictive value for internal derangement 3
• Initial clinical diagnosis differs from final diagnosis in complex patients with multiple comorbidities, requiring synovial fluid analysis and imaging 8