Osteomyelitis is a bacterial infection of bone that primarily affects infants and young children. 1/2 of all cases occur before 5 years of age with 1/3 of these cases occurring before age 2. Patients may present clinically with pain, fever, and elevated WBC and/or ESR. Many patients will have a history positive for a recent respiratory tract infection or otitis media (most cases of osteomyelitis in the pediatric population have a hematogenous etiology).

Osteomyelitis usually occurs in the metaphyses or metaphyseal equivalents in children. Roughly 75% of all infections will involve the long bones (femur > tibia > humerus), while the remaining 25% will involve the flat bones, in particular the hip.

Radiographic findings of osteomyelitis:

• plain films: deep soft-tissue swelling => displacement or obliteration of the the fat planes adjacent to the metaphysis (early finding); initial bony changes (i.e. loss of cortical white line with poorly-defined lucencies of the metaphysis) are not apparent until 7-10 days after the onset of symptoms; evidence of periosteal reaction becomes evident at about ten days; subacute and chronic changes include mixed bone lysis and sclerosis.
• MRI (most specific): cortical destruction of the metaphysis with bone marrow edema (high signal on T2WI) as well as surrounding soft tissue edema.
• bone scan: focal area of increased uptake in all three phases (angiographic/blood flow, soft tissue/blood pool, and skeletal phases); differs from cellulitis where there is increased activity in the blood flow and blood pool phases but a normal late phase.

* evidence of osteomyelitis is seen on MRI and bone scans early after the onset of symptoms.