Michael P. D'Alessandro, M.D.
Peer Review Status: Internally Peer Reviewed
Usually spread hematogenously. Local trauma with skin penetration and seeding of organisms is another pathway. Staphylococcus aureus is the causative agent in 70-90% of pediatric cases. Bacteria pass through nutrient vessels to the metaphyses where they lodge and proliferate. The physeal plate acts as barrier to epiphyseal extension of infection because it is avascular. Metaphyseal inflammation leads to exudation, increased intraosseous pressure, vascular stasis, thrombosis, bone necrosis, and bone resorption. Sometimes infection can extend into the adjacent joint. Tubular bones have most rapid growth and largest metaphyses and therefore are a common site of infection in up to 75% of children such as distal and proximal femur and tibia, distal humerus and fibula.
On plain film soft tissue swelling can be seen by 1-3 days after infection. Destructive bone changes don't occur on plain film until 10-14 days after infection starts. Initially see a lucent moth eaten appearance to bone. There is extension of infection through the metaphyseal cortex leading to periosteal new bone formation which if untreated may completely encircle the bone becoming an involucrum which can envelope the non viable infected bone which is called a sequestrum.
A bone scan is usually positive 24 hours after infection and demonstrates a well defined focus of tracer activity 1 - 2 hours post injection that is correlated with radiotracer in same area on dynamic scans.
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