What could be causing severe burning in front of the left knee cap in an otherwise healthy adult?

Severe Burning Pain in Front of Left Knee Cap

The most likely diagnosis is patellofemoral pain syndrome or prepatellar bursitis, and you should obtain plain radiographs of the knee as the initial imaging study to rule out structural abnormalities, followed by MRI if radiographs are normal and symptoms persist. 1

Initial Diagnostic Approach

Clinical Assessment

• Evaluate for specific pain patterns: Anterior knee pain with a burning quality suggests patellofemoral pathology, prepatellar bursitis, or nerve-related pain (infrapatellar branch of saphenous nerve). 1
• Assess for trauma history: Recent direct trauma to the kneecap, repetitive kneeling, or overuse activities are critical historical features. 1
• Check for swelling and warmth: Localized swelling directly over the patella suggests prepatellar bursitis, while diffuse knee effusion may indicate intra-articular pathology. 1
• Palpate for point tenderness: Tenderness along the patellar tendon suggests tendinopathy, while tenderness over the medial or lateral retinaculum suggests patellofemoral friction syndrome. 1

Rule Out Serious Pathology First

• Exclude infection: If there is erythema, warmth, swelling, and systemic symptoms (fever), consider septic prepatellar bursitis or cellulitis requiring urgent aspiration and antibiotics. 1
• Exclude fracture: Inability to bear weight, severe point tenderness over bone, or history of significant trauma warrants immediate radiographic evaluation. 1

Imaging Strategy

Initial Radiographs

• Obtain standing AP, lateral, and sunrise (Merchant) views of the knee: These views assess for patellofemoral malalignment, patellar subluxation, osteochondral lesions, and early osteoarthritis. 1
• Radiographs may be normal initially: In patients 45-55 years with anterior knee pain, radiographs are often normal despite cartilage abnormalities visible on advanced imaging. 1

MRI When Radiographs Are Normal or Non-Diagnostic

• MRI is superior to radiographs for soft tissue evaluation: MRI detects patellofemoral cartilage loss, bone marrow edema (BMLs), patellar tendinopathy, prepatellar bursitis, Hoffa's disease, fat pad impingement, medial plicae, and nerve entrapment. 1
• 3T MRI with quantitative imaging: Elevated T2 mapping values correlate with cartilage degeneration even when radiographs appear normal. 1
• Bone marrow edema patterns: Classic locations for patellofemoral dislocation/relocation injuries (lateral femoral condyle and medial patella) are identified on MRI. 1

Common Causes of Anterior Burning Knee Pain

Patellofemoral Pain Syndrome

• Most common cause in otherwise healthy adults: Results from abnormal patellar tracking, cartilage softening (chondromalacia patellae), or overuse. 1
• Burning quality suggests nerve irritation: The infrapatellar branch of the saphenous nerve can become entrapped or irritated, causing burning anterior knee pain. 1

Prepatellar Bursitis

• Inflammation of the bursa anterior to the patella: Caused by repetitive kneeling, direct trauma, or infection. 1
• Aspiration indicated if infection suspected: Send fluid for cell count, Gram stain, and culture. 1

Patellar Tendinopathy

• Overuse injury of the patellar tendon: Presents with anterior knee pain, typically at the inferior pole of the patella. 1
• MRI confirms diagnosis: Shows thickening and increased signal within the tendon. 1

Hoffa's Disease and Fat Pad Impingement

• Inflammation of Hoffa's fat pad: Causes anterior knee pain with a burning quality. 1
• Contrast-enhanced MRI is diagnostic: Enhancing synovitis thicker than 2 mm in Hoffa's fat correlates with peripatellar pain. 1

Treatment Algorithm

Conservative Management (First-Line)

• Activity modification: Avoid activities that exacerbate pain (kneeling, squatting, prolonged sitting with knee flexion). 1
• NSAIDs for pain control: Oral NSAIDs after evaluating gastrointestinal, renal, and cardiovascular status. 2
• Physical therapy: Quadriceps strengthening (especially vastus medialis oblique) and patellar taping/bracing to improve tracking. 1, 2
• Ice application: Apply ice for 15-20 minutes after activities to reduce inflammation. 1

When Conservative Management Fails

• Consider MRI if not already obtained: To identify specific structural pathology requiring targeted treatment. 1
• Intra-articular or periarticular corticosteroid injection: For persistent bursitis or synovitis confirmed on imaging. 2
• Referral to orthopedic surgery: For structural abnormalities (severe malalignment, osteochondral lesions, loose bodies) or failure of 3-6 months of conservative therapy. 1

Critical Pitfalls to Avoid

• Do not assume burning pain is always musculoskeletal: Nerve entrapment (infrapatellar branch of saphenous nerve) can mimic other anterior knee pathology and requires specific treatment. 1
• Do not miss septic bursitis: Any prepatellar swelling with systemic symptoms requires aspiration to rule out infection before initiating treatment. 1
• Do not delay imaging in patients with trauma history: Occult fractures or osteochondral injuries may not be clinically apparent initially. 1
• Do not use capsaicin for inflammatory arthritis: If rheumatoid arthritis or other inflammatory arthropathy is suspected, capsaicin is not recommended and disease-modifying therapy is required. 2