What is the most likely diagnosis for a superficial fluid‑filled swelling over the patella and how should it be managed?

Prepatellar Bursitis: Diagnosis and Management

A superficial fluid-filled swelling over the patella is most likely prepatellar bursitis, and the critical first step is to distinguish septic from non-septic causes through bursal aspiration with fluid analysis, as septic bursitis requires immediate antibiotic therapy while non-septic cases are managed conservatively. 1

Clinical Presentation and Diagnosis

Key Diagnostic Features

• Prepatellar bursitis presents as a localized fluid collection superficial to the patella, easily distinguished from septic arthritis by the characteristic bursal swelling anterior to the knee joint and normal joint examination 2, 3
• The condition is commonly known as "housemaid's knee" and occurs in individuals who spend prolonged periods kneeling, such as clergy, gardeners, tatami craftsmen, or those with occupational knee trauma 4, 3
• Ultrasonography is the preferred initial imaging modality to confirm the diagnosis and distinguish bursitis from cellulitis, showing an anechoic fluid-filled structure in the prepatellar region 1, 5

Etiologic Categories

The underlying cause determines management approach 1:

• Chronic microtrauma (most common): repetitive kneeling or pressure
• Acute trauma/hemorrhage: direct blow to the knee
• Inflammatory disorders: gout or rheumatoid arthritis
• Septic bursitis: bacterial infection, most commonly Staphylococcus aureus

Critical Distinction: Septic vs. Non-Septic Bursitis

When to Suspect Infection

Bursal aspiration with fluid analysis is mandatory when infection is suspected to avoid misdiagnosis, which occurred in 32% of cases in one series despite characteristic findings 3. Perform aspiration if the patient has 1, 3:

• Severe pain with warmth and erythema
• Fever or chills
• History of recent trauma or sustained occupational pressure
• Underlying diabetes or immunocompromise 4

Bursal Fluid Analysis

Send aspirated fluid for 1, 3:

• Gram stain and culture (essential for diagnosis)
• White blood cell count (>1,000 cells/mm³ suggests septic bursitis)
• Crystal analysis (to identify gout or pseudogout)
• Glucose measurement

Important caveat: Septic bursitis was misdiagnosed as non-septic in eight cases despite characteristic bursal fluid leukocytosis and positive cultures, emphasizing the need for complete fluid analysis 3. Additionally, gout can present as prepatellar bursitis even with normal serum uric acid levels, requiring repeated aspiration if initial results are inconclusive 6.

Management Algorithm

Septic Bursitis

When Staphylococcus aureus is identified (88% of cases) or suspected, initiate antibiotics effective against this organism, noting that 76% are penicillin-resistant 3:

• Outpatient oral antibiotics may be considered for patients who are not acutely ill 1
• Inpatient intravenous antibiotics are required for acutely ill patients with extensive infection 1, 3
• Bursal fluid drainage is uniformly necessary in conjunction with antibiotics 3
• Surgery is reserved for cases not responsive to antibiotics or recurrent infections 1

Non-Septic Bursitis Management

Acute traumatic/hemorrhagic bursitis 1:

• Conservative treatment with ice, elevation, rest, and analgesics
• Aspiration may shorten symptom duration

Chronic microtraumatic bursitis 1:

• Treat conservatively and address the underlying cause (occupational modification)
• Bursal aspiration is generally NOT recommended due to risk of iatrogenic septic bursitis
• Intrabursal corticosteroid injections are sometimes used, though high-quality evidence of benefit is lacking 1

Inflammatory bursitis (gout, rheumatoid arthritis) 1:

• Treat the underlying condition
• Intrabursal corticosteroid injections are often used and appropriate

Imaging Considerations

• Ultrasound-guided corticosteroid injection can be performed for non-septic inflammatory cases, with the bursa appearing as an anechoic fluid-filled structure anterior to the patella 5
• MRI is rarely needed but can confirm diagnosis when ultrasound is inconclusive 5
• Blood testing (white blood cell count, inflammatory markers) helps distinguish infectious from non-infectious causes 1

Common Pitfalls to Avoid

1. Failing to aspirate when infection is possible: This leads to delayed diagnosis and inappropriate treatment 3
2. Aspirating chronic microtraumatic bursitis unnecessarily: This increases risk of introducing infection 1
3. Assuming normal serum uric acid excludes gout: Gouty bursitis can occur with normal uric acid levels; repeat aspiration if diagnosis remains elusive 6
4. Confusing prepatellar bursitis with septic arthritis: The bursal swelling is superficial and anterior to the joint, with normal joint examination 3