What are the causes of acute knee swelling?

Acute Knee Swelling: Causes and Diagnostic Approach

Acute knee swelling results from traumatic injuries (ligamentous tears, meniscal damage, fractures, bone contusions), inflammatory conditions (septic arthritis, crystal arthropathy), or structural abnormalities, with the specific cause determined by mechanism of injury, timing of swelling onset, and associated symptoms. 1

Traumatic Causes

Ligamentous Injuries

• ACL tears are the most common ligamentous injury causing acute swelling, typically from noncontact pivoting mechanisms with immediate swelling within 2 hours due to hemarthrosis 2
• Collateral ligament sprains (medial and lateral) occur from valgus or varus stress mechanisms 3
• Posterolateral corner injuries are associated with ACL ruptures in 19.7% of cases and cause significant morbidity if missed 4
• Anterolateral ligament injuries frequently accompany ACL tears and contribute to anterolateral instability 4

Meniscal Injuries

• Meniscal tears commonly occur with twisting injuries and may present with mechanical symptoms (locking, catching) in addition to swelling 3
• Lateral meniscal tears are predicted by lateral tibial plateau depression >11 mm on imaging 4
• MRI demonstrates 96% sensitivity and 97% specificity for meniscal tear detection 4

Fractures and Bone Injuries

• Tibial plateau fractures are detected with 100% sensitivity on CT versus 83% on radiographs alone 4
• Patellar fractures and dislocations cause immediate swelling with focal patellar tenderness 4
• Segond avulsion fractures (lateral tibial rim) are associated with ACL tears and anterolateral ligament injuries 4
• Bone marrow contusions are highly predictive of focal osteoarthritis development within 1 year after trauma 4

Patellar Injuries

• Transient lateral patellar dislocation causes acute swelling with characteristic bone contusion patterns on MRI 4
• Patellar subluxation events may present with effusion and instability 3

Infectious Causes

Septic Arthritis

• Age <5 years plus C-reactive protein >2.0 mg/dL predicts >90% probability of septic arthritis in children with painful swollen knee 5
• Fever, refusal to bear weight, inability to move the joint, and bacteremia are red flags requiring urgent aspiration 5
• This represents an orthopedic emergency requiring immediate intervention 5

Inflammatory Causes

• Crystal arthropathy and other inflammatory conditions can cause acute swelling without trauma 1
• Joint aspiration aids diagnosis and provides pain relief in inflammatory conditions 1

Timing of Swelling Onset (Critical Diagnostic Clue)

• Immediate swelling (within 2 hours) indicates hemarthrosis from ACL tear, fracture, or peripheral meniscal tear with vascular supply disruption 2
• Delayed swelling (6-24 hours) suggests meniscal injury or less severe ligamentous sprain with synovial fluid accumulation 6
• Gradual swelling over days may indicate inflammatory or infectious etiology 1

High-Energy Trauma Causes

Knee Dislocation

• Femoro-tibial knee dislocation is the most harmful acute knee injury, with vascular injury in 30% of posterior dislocations 4
• Multi-ligament injuries may represent spontaneously reduced dislocations requiring mandatory neurovascular examination 6
• Rapid CT angiography is crucial for vascular assessment 6

Associated Findings That Narrow Differential

• Lipohemarthrosis (fat-fluid level) on cross-table lateral radiograph indicates intra-articular fracture 4
• Joint effusion >10 mm on lateral radiograph in patients <40 years should prompt MRI consideration 4
• Gross deformity or palpable mass mandates immediate radiographs regardless of other criteria 4, 5

Common Pitfalls

• Missing spontaneously reduced knee dislocations that present as multi-ligament injuries without obvious deformity 6
• Delaying septic arthritis diagnosis in young children with atraumatic effusion—urgent aspiration is mandatory 5
• Assuming negative radiographs exclude significant injury—occult fractures, bone contusions, and soft tissue injuries require MRI if clinical suspicion persists 7
• Overlooking posterolateral corner injuries that accompany ACL tears, leading to persistent instability 4