Brodie Abscess

Brodie abscess is a localised, subacute form of osteomyelitis in which a contained abscess forms in the metaphysis (or epiphysis or diaphysis) of a long bone, surrounded by sclerotic bone. It usually occurs in children and adolescents aged 2 to 20 years, with a male predominance, and accounts for about 2 to 3% of cases of osteomyelitis.

The distal tibia (the most common site), proximal tibia, distal femur, distal radius, humerus and calcaneus are affected. Staphylococcus aureus is the commonest organism (about 50 to 60% of positive cultures), with coagulase negative staphylococci, Kingella kingae in young children, streptococci, and Gram negative bacilli in others. Cultures are often negative (about 30 to 50%), since the organisms are of low virulence.

Roberts classified the radiological appearances: type I is a punched out lesion; type II is a metaphyseal lesion with a sclerotic margin (the classic type); type III is a cortical lesion with a periosteal reaction; type IV is an onion skin periosteal reaction; type V is an epiphyseal lesion; and type VI is a cortical lesion.

Complications are growth disturbance (if near the physis), pathological fracture, recurrence, chronic osteomyelitis, joint involvement if epiphyseal, and diagnostic confusion with tumour.

Pathophysiology.

Haematogenous bacteria lodge in the slow flowing metaphyseal capillaries. A good host defence and organisms of low virulence wall off the infection, so a pus filled cavity forms with surrounding reactive sclerosis. The cavity may communicate with the physis through transphyseal vessels in young children.

Insidious localised pain (often worse at night and after activity, relieved by aspirin or NSAIDs) lasting weeks to months, mild swelling and tenderness over the metaphysis, mild limp, and little or no fever. Systemic features are unusual. Previous antibiotic use may mask signs.

Examine the site for warmth, tenderness, swelling and joint effusion, range of motion, gait and the neighbouring joints.

• Radiographs: a round or oval metaphyseal lucency with a sclerotic rim, possibly with a track toward the physis. Periosteal new bone may be present.
• MRI shows the abscess with a rim of low signal on T1 (penumbra sign), marrow oedema, a track to the physis, and soft tissue involvement.
• CT for the bone detail and sequestrum. Bone scan is hot in the lesion.
• Blood tests: inflammatory markers are often normal or mildly raised (white cell count normal in most; erythrocyte sedimentation rate and C-reactive protein mildly elevated in about 50%).
• Aspiration or curettage with culture and histology to confirm organism and exclude tumour; samples before antibiotics when the child is stable.

Differential diagnosis.

• Osteoid osteoma and chondroblastoma.
• Langerhans cell histiocytosis and Ewing sarcoma.
• Unicameral bone cyst and non-ossifying fibroma.
• Stress fracture and acute or chronic osteomyelitis.

Antibiotics. Start after bone sampling in a stable patient.

• Children: flucloxacillin 50 mg/kg intravenously every 6 hours (maximum 2 g a dose) for 2 to 7 days, then oral cefalexin 25 mg/kg every 6 hours (maximum 1 g a dose) or flucloxacillin 25 mg/kg orally every 6 hours (maximum 1 g a dose) on an empty stomach, for a total of 4 to 6 weeks. With penicillin allergy, clindamycin 10 mg/kg intravenously or orally every 6 to 8 hours (maximum 600 mg a dose intravenously, 450 mg orally), watching for diarrhoea and Clostridioides difficile infection. Add vancomycin when meticillin resistant organisms are likely.
• Adults: flucloxacillin 2 g intravenously every 6 hours for 1 to 2 weeks, then flucloxacillin 1 g orally every 6 hours or clindamycin 450 mg orally every 6 hours, for a total of 6 weeks (maximum clindamycin 1.8 g orally daily; stop if diarrhoea). Check liver function, and avoid with a history of flucloxacillin cholestatic jaundice.
• Review the response by clinical improvement and C-reactive protein.

Surgery. Curettage (with or without antibiotic loaded bone cement or bone graft) is indicated for large lesions near the physis, failed antibiotic treatment, diagnostic uncertainty, and sequestrum. Small lesions in children may respond to antibiotics alone. Avoid damaging the physis. Send curettings for culture and histology.

Operative antibiotic prophylaxis. Withhold until bone samples are taken. Then cefazolin 2 g intravenously (3 g when weight is 120 kg or over; 30 mg/kg to a maximum of 2 g in children) within 60 minutes before incision as a single dose; with severe penicillin allergy, vancomycin 15 mg/kg intravenously starting 90 to 120 minutes before incision as a single dose.

Analgesia.

• Paracetamol 1 g orally every 6 hours as needed for up to 5 to 7 days (maximum 4 g daily; 3 g daily when weight is under 50 kg, with chronic alcohol use or hepatic impairment).
• Ibuprofen 400 mg orally every 8 hours with food for up to 5 days (maximum 1200 mg daily); avoid with eGFR under 30 mL/min/1.73 m2, peptic ulcer, heart failure or anticoagulant use.
• Children: paracetamol 15 mg/kg orally every 4 to 6 hours (maximum 75 mg/kg daily and not exceeding 4 g daily) and ibuprofen 10 mg/kg orally every 6 to 8 hours with food (maximum 40 mg/kg daily and not exceeding 2400 mg daily), for up to 5 days.

Monitoring. C-reactive protein weekly during intravenous therapy, then every 2 weeks. Clinical review at 2 weeks, 6 weeks, 3 months and 12 months. Radiographs at 6 weeks and 3 months to show healing and growth, and MRI if symptoms persist. Liver function with prolonged flucloxacillin. Limb length and angulation in children until skeletal maturity.

Referral and escalation. Refer every suspected Brodie abscess to orthopaedics (paediatric orthopaedics in children). Urgent referral for fever, sepsis, joint involvement, pathological fracture or failed antibiotic treatment.

Special groups.

• Children: protect the physis; treat Kingella kingae with a beta lactam (it is resistant to clindamycin and vancomycin).
• Sickle cell disease: consider Salmonella.
• Immunocompromised and diabetes: broaden cover and extend treatment.
• Kidney impairment: avoid ibuprofen and adjust antibiotics.