Osteomyelitis — SAQ

Stem

A 12-year-old boy presents to the emergency department with 3 days of fever (39.1 C), increasing pain in the right distal thigh just above the knee, and a limp that has progressed to refusal to bear weight. He is otherwise previously well. On examination he is febrile, tachycardic, holds the right leg still, and has marked, well-localised tenderness, warmth and swelling over the distal femoral metaphysis. The knee joint itself is not significantly tender on passive movement. Bloods show WCC 17.4 (neutrophils 14.9), CRP 128, ESR 64. A plain X-ray of the right femur and knee is reported as normal.

Questions

a) State the most likely diagnosis and explain why a normal X-ray does not exclude it. (2 marks)

b) Name the imaging modality of choice and describe its typical findings in this condition. (2 marks)

c) Outline the microbiological diagnosis: name the gold-standard sample, the organism to cover empirically, and the expected duration of antibiotic therapy. (3 marks)

d) Describe the pathophysiology that explains why, in chronic disease, surgical debridement is essential and antibiotics alone often fail. Include the sequestrum, involucrum and biofilm. (3 marks)

Model answer

a) Diagnosis and the normal X-ray (2 marks):

• Acute haematogenous osteomyelitis of the distal femoral metaphysis — febrile child with very localised metaphyseal tenderness, refusal to bear weight, and a markedly raised CRP/ESR.
• A normal early X-ray does not exclude it because radiographic changes lag behind the clinical picture by 1 to 2 weeks. The earliest changes are soft-tissue swelling and periosteal reaction (7 to 14 days); lytic lesions require 30 to 50% bone loss to become visible. Therefore, treat on clinical grounds and image with MRI.

b) Imaging modality of choice and findings (2 marks):

• MRI of the femur (with gadolinium) is the modality of choice — sensitivity and specificity about 90%.
• Findings: marrow oedema — low signal on T1, high signal on T2 / STIR; periosteal reaction; subperiosteal or intraosseous abscess (rim enhancement with gadolinium); definition of the extent and any joint involvement.

c) Microbiological diagnosis and duration (3 marks):

• Gold-standard sample: deep bone biopsy culture (percutaneous CT-guided or intraoperative) — sinus tract culture is contaminated with skin flora and is unreliable. Also send blood cultures before antibiotics (positive in ~40% of acute haematogenous).
• Empirical organism to cover: Staphylococcus aureus — flucloxacillin 2 g IV 6-hourly (or cefazolin/cefuroxime); add vancomycin if MRSA risk factors or severe beta-lactam allergy.
• Duration: 4 to 6 weeks total — typically 2 weeks IV then oral step-down once afebrile, improving, CRP falling and a bioavailable oral agent is available (per Peltola / Paakkonen, an even shorter IV course of 3 to 4 days with early oral switch is acceptable in uncomplicated paediatric disease).

d) Pathophysiology of chronic refractory disease (3 marks):

• Infection within the rigid cortical envelope raises intraosseous pressure, compressing vessels and producing venous stasis, thrombosis and ischaemia; the cortex dies and separates as a sequestrum (dead, avascular bone, dense on X-ray).
• Antibiotics and immune cells cannot reach the avascular sequestrum, and bacteria form a biofilm (extracellular polymeric slime) on it that is 10 to 1000 times more resistant — so chronic disease cannot be cured by antibiotics alone.
• The periosteum lays down new bone (involucrum) around the sequestrum; pus escapes through a cortical cloaca and drains via a sinus tract. Surgical debridement of all necrotic bone to bleeding healthy tissue (the paprika sign) removes the source — the rationale for surgery and the reason antibiotics alone fail.