What are the differentials and management options for anterior knee swelling?

Differential Diagnosis for Anterior Knee Swelling

Begin with plain radiographs (AP, lateral, sunrise/Merchant, and tunnel views) as the mandatory first imaging study, followed by focused history and physical examination to differentiate between traumatic, infectious, inflammatory, and mechanical etiologies. 1

Critical History Elements to Obtain

• Mechanism of injury: Acute trauma or twisting injury indicates ligamentous injury (ACL tear), fracture, or hemarthrosis 1, 2
• Timing of swelling: Rapid onset within 2-4 hours suggests hemarthrosis from ACL tear or fracture; delayed onset (12-24 hours) suggests meniscal tear or inflammatory process 2
• Presence of "pop" sound: Classic for ACL rupture with rapid swelling 2
• Systemic symptoms: Fever, chills, or recent puncture wound raises urgent concern for septic arthritis requiring immediate intervention 1
• Mechanical symptoms: Locking or catching suggests meniscal tear or loose body 3
• Age and activity level: Patients over 40 with usage-related pain and morning stiffness suggest osteoarthritis 4

Essential Physical Examination Findings

• Warmth and erythema with systemic signs: Requires urgent consideration of septic arthritis 1
• Focal bony tenderness: Mandates radiography per Ottawa criteria to exclude fracture 1
• Range of motion limitation: Inability to flex to 90° or fully extend suggests significant intra-articular pathology 1
• Joint line tenderness: Particularly medial joint line tenderness suggests meniscal tear or osteoarthritis 4
• Coarse crepitus: Present in approximately 81% of patients with established osteoarthritis 4
• Lachman test: Assesses ACL integrity; positive test with effusion strongly suggests ACL tear 1

Primary Differential Diagnoses

Traumatic Causes

• ACL tear with hemarthrosis: Rapid swelling (within 2-4 hours), positive Lachman test, history of twisting injury with audible pop 1, 2
• Meniscal tear: Delayed swelling, joint line tenderness, mechanical symptoms (locking/catching) 3, 1
• Fracture: Focal bony tenderness, inability to bear weight, visible on plain radiographs 1
• Patellar dislocation/subluxation: Bone marrow edema in classic location on MRI, history of lateral patellar instability 3

Inflammatory/Infectious Causes

• Septic arthritis: Warmth, erythema, fever, inability to bear weight; requires urgent arthrocentesis with synovial fluid cell count, Gram stain, and culture 1
• Crystal arthropathy (gout/pseudogout): Acute onset, warmth, erythema; synovial fluid analysis shows crystals 5

Mechanical/Degenerative Causes

• Osteoarthritis with effusion: Age >40, usage-related pain, short morning stiffness, coarse crepitus, joint line tenderness 4
• Patellar tendinopathy: Activity-related anterior knee pain, pain at inferior pole of patella with resisted leg extension, pain with decline squat test 3
• Prepatellar bursitis: Localized anterior swelling over patella, history of repetitive kneeling 3
• Deep infrapatellar bursitis: Anterior knee swelling, diagnosed on MRI with contrast showing enhancing synovitis 3
• Hoffa's disease (infrapatellar fat pad impingement): Anterior knee pain, enhancing synovitis thicker than 2mm in Hoffa's fat on contrast MRI correlates with peripatellar pain 3

Other Causes

• Baker's cyst (popliteal cyst): Posterior knee mass with anterior effusion; ultrasound confirms diagnosis and excludes DVT 5
• Pigmented villonodular synovitis: Chronic swelling, requires contrast-enhanced MRI for diagnosis 3
• Loose bodies: Mechanical symptoms, visible on radiographs or MRI 3, 1
• Osteochondritis dissecans: Visible on plain radiographs, requires MRI for staging 3

Diagnostic Algorithm

Step 1: Risk Stratification

• High-risk (urgent intervention required): Suspected septic arthritis, gross deformity, neurovascular compromise 1
• Moderate-risk (advanced imaging needed): Suspected ACL tear, meniscal tear, osteochondral injury 1
• Low-risk (conservative management): Mild effusion without mechanical symptoms or instability 1

Step 2: Mandatory Initial Imaging

• Plain radiographs (four views): AP, lateral, sunrise/Merchant, and tunnel views to exclude fracture, loose bodies, osteoarthritis, osteochondritis dissecans, or tumor 1, 5

Step 3: Diagnostic Arthrocentesis (When Indicated)

• Perform when septic arthritis cannot be excluded clinically: Synovial fluid analysis must include cell count with differential, Gram stain, and culture 1
• Never administer intra-articular corticosteroids before excluding infection: This can worsen septic arthritis and lead to joint destruction 5

Step 4: Advanced Imaging (Based on Initial Findings)

• MRI without contrast: Indicated if radiographs are normal/non-diagnostic and symptoms persist, or if mechanical symptoms suggest meniscal pathology 3, 1
• MRI with IV contrast: More accurate for diagnosing Hoffa's disease, deep infrapatellar bursitis, pigmented villonodular synovitis, and adhesive capsulitis 3
• Ultrasound: Confirms effusion, guides aspiration, diagnoses Baker's cyst, evaluates medial plicae 3, 5

Critical Pitfalls to Avoid

• Never delay arthrocentesis when infection is suspected: Septic arthritis requires urgent intervention to prevent joint destruction 1
• Never perform MRI before plain radiographs: Approximately 20% of patients with chronic knee pain undergo unnecessary MRI without recent radiographs 5
• Do not assume all anterior knee pain is patellofemoral syndrome: This is a symptom, not a diagnosis; specific pathology must be identified 6, 7
• Do not overlook Baker's cyst with DVT: Always perform compression ultrasound to exclude popliteal vein thrombosis when posterior swelling is present 5