What are the possible causes of knee effusion in an adult patient with a history of joint problems or recent injury?

Causes of Knee Effusion

Knee effusion in adults with joint problems or recent injury results from traumatic causes (ligamentous tears, fractures, meniscal injuries), infectious causes (septic arthritis requiring urgent aspiration), inflammatory arthropathies (rheumatoid arthritis, gout, pseudogout), or degenerative joint disease (osteoarthritis). 1, 2

Traumatic Causes

In patients with recent injury, ligamentous injuries, fractures, and meniscal tears are the primary causes:

• ACL ruptures are associated with posterolateral corner injuries in 19.7% of cases and anterolateral ligament injuries, both contributing to joint instability and effusion 1
• Meniscal tears occur frequently with trauma, with lateral meniscal tears predicted by lateral tibial plateau depression >11 mm on imaging 1
• Tibial plateau fractures cause immediate hemarthrosis and are detected with 100% sensitivity on CT versus 83% on radiographs 1, 3
• Patellar fractures and dislocations produce immediate swelling with focal patellar tenderness 1
• Bone marrow contusions are highly predictive of focal osteoarthritis development within 1 year and frequently accompany ligamentous injuries 4, 1
• Lipohemarthrosis (fat-fluid level on cross-table lateral radiograph) indicates intra-articular fracture and requires immediate orthopedic evaluation 1, 3

A critical pitfall is overlooking posterolateral corner injuries that accompany ACL tears, leading to persistent instability despite ACL reconstruction. 1

Infectious Causes

Septic arthritis represents an orthopedic emergency requiring immediate joint aspiration:

• **In children <5 years**, C-reactive protein >2.0 mg/dL predicts >90% probability of septic arthritis 1
• Red flags include fever, refusal to bear weight, inability to move the joint, and bacteremia—all mandate urgent aspiration 1
• Joint effusion can be absent in up to one-third of adult patients with septic arthritis, so clinical suspicion must remain high 4
• Early joint aspiration should include routine cultures, Gram stain, and cell count with differential 4

The most dangerous pitfall is delaying septic arthritis diagnosis in young children with atraumatic effusion—urgent aspiration is mandatory even when radiographs appear normal. 1

Inflammatory Arthropathies

Rheumatoid arthritis, gout, and pseudogout commonly cause knee effusion in adults with joint problems:

• Rheumatoid arthritis rarely presents as monoarticular knee effusion but should be suspected when pleural fluid glucose is <1.6 mmol/L (29 mg/dL) 4, 5
• Gout and pseudogout are among the most common causes of atraumatic knee effusion in elderly patients 5
• Systemic lupus erythematosus causes pleural effusions in up to 50% of patients, though knee involvement is less common 4

Degenerative and Chronic Causes

Osteoarthritis is the most common cause of chronic knee effusion in adults:

• Osteoarthritis develops in nearly one in two people by age 85 years and is the most common cause of disability in adults 4
• Joint effusion >10 mm on lateral radiograph in patients <40 years should prompt MRI consideration to evaluate for early degenerative changes 4, 1
• Chronic effusion induced by osteoarthritis may benefit from intra-articular corticosteroid injection, providing short-term relief especially when accompanied by effusion 6

Diagnostic Approach

Obtain standing AP, lateral, and tangential patellar radiographs as the initial imaging study:

• Radiographs should be the first imaging modality when Ottawa rule criteria are present (focal tenderness, inability to bear weight, age >55 years) 3
• MRI without contrast is indicated when pain persists despite conservative management, to evaluate for meniscal tears, bone marrow lesions, articular cartilage damage, and synovitis 6, 3
• Arthrocentesis should be performed early for diagnosis, with both fluoroscopy and ultrasound available to guide needle placement 4, 6

A critical pitfall is assuming negative radiographs exclude significant injury—occult fractures, bone contusions, and soft tissue injuries require MRI if clinical suspicion persists. 1

Rare Causes

Less common etiologies include:

• Post-traumatic chylous effusion presents as milky fluid with innumerable fat globules, self-limited but requiring arthroscopic confirmation to exclude infection 7
• Pigmented villonodular synovitis and tuberculous synovitis cause recurrent effusion requiring synovectomy 8
• Tumor should be considered in the differential diagnosis of atraumatic effusion 2