CIM Case: Brodie's Abscess

Clinical Scenario

Patient: 18-year-old Malaysian international student Presentation: 6-week history of left knee pain and swelling, initially attributed to soccer injury, worsening despite rest Relevant history: Previously healthy, no prior infections or surgery, no immunocompromise, no recent travel to endemic areas, no systemic symptoms (no fever, night sweats, weight loss) Examination findings:

• Well-appearing young man, afebrile
• Warm, tender swelling over medial aspect of proximal tibia
• No overlying skin changes or sinus
• Full range of motion at knee (limited by pain at extremes)
• No effusion
• No regional lymphadenopathy
• Neurovascularly intact distally
• Remainder of examination unremarkable

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Investigations Provided

Laboratory Results

Imaging

Image 1: AP and Lateral Radiographs of Left Knee/Proximal Tibia

Radiological features:

• Well-defined lucent lesion in proximal tibial metaphysis
• Eccentric location (medial)
• Size approximately 3 x 2cm
• Dense sclerotic rim (reactive bone)
• No aggressive periosteal reaction
• No cortical breakthrough
• No soft tissue mass
• Open physis (skeletally immature)

Image 2: MRI of Left Proximal Tibia

Findings:

• Well-defined T2-hyperintense lesion in proximal tibial metaphysis
• Penumbra sign: High T1 signal rim around abscess cavity (granulation tissue)
• Surrounding marrow oedema
• No cortical breach
• No soft tissue abscess
• Thin rim enhancement on post-contrast images

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Questions & Model Answers

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Key Teaching Points

Pattern Recognition

This pattern suggests Brodie's Abscess:

• Young patient with subacute limb pain
• Minimal systemic symptoms despite prolonged course
• Normal or mildly elevated inflammatory markers
• Well-defined metaphyseal lytic lesion
• Sclerotic rim on radiograph
• Penumbra sign on MRI

Distinguish from Acute Osteomyelitis:

Critical Management Points

1. Penumbra sign on MRI is highly specific for Brodie's abscess
2. Biopsy before treatment - rules out tumour, identifies organism
3. Surgical debridement is essential - antibiotics alone insufficient
4. Culture-negative is common (30-50%) - treat empirically
5. 6-12 weeks antibiotics - shorter courses have higher recurrence
6. Consider TB in patients from endemic regions

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Common Examiner Follow-ups

Q: "What is the penumbra sign and what causes it?"

The penumbra sign is a rim of high T1 signal surrounding the abscess cavity on MRI. It is caused by:

• Vascular granulation tissue rich in haemoglobin breakdown products
• The paramagnetic effect of methaemoglobin creates T1 shortening
• Present in pyogenic infections
• Absent in tuberculosis (caseous necrosis lacks vascularity)
• Absent in tumours

Sensitivity ~75%, specificity ~95% for pyogenic osteomyelitis.

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Q: "Why are inflammatory markers often normal in Brodie's abscess?"

Inflammatory markers are normal or mildly elevated because:

• Host immune response has successfully contained the infection
• The abscess cavity is walled off from systemic circulation
• Low-grade, indolent infection with balance between organism and host
• No systemic inflammatory response despite local infection

This differentiates from acute osteomyelitis where WCC, CRP, and ESR are markedly elevated.

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Q: "What is the culture-negative rate and how does this affect management?"

Culture-negative rate in Brodie's abscess is 30-50%, due to:

• Prior antibiotic therapy
• Delayed or inadequate transport
• Low bacterial load
• Fastidious organisms

Management when culture-negative:

• Continue empirical anti-staphylococcal antibiotics
• Histology confirms infection (acute/chronic inflammation, necrosis)
• Consider broadening coverage if no response
• Extended tissue cultures (hold for 14 days for slow-growing organisms)

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Q: "When would you consider bone grafting the cavity?"

Bone grafting considerations:

Options include:

• Autograft (iliac crest)
• Antibiotic-impregnated bone substitute
• Calcium sulphate with antibiotics

Most Brodie's abscesses heal without grafting if adequately debrided.

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Related Topics

• Acute Osteomyelitis
• Chronic Osteomyelitis
• Skeletal Tuberculosis
• Eosinophilic Granuloma
• Metaphyseal Bone Lesions
• Antibiotic Therapy in Bone Infection