Osteomyelitis is an inflammatory process of the bone and bone marrow that is generally due to a bacterial infection. The most common form in childhood is acute hematogenous osteomyelitis (AHO); 50% of cases occur in children younger than 5 years.1,2 The incidence of AHO is approximately 60 in 100,000 children,3 and boys are about twice as likely to be affected as girls.1,2 The majority of cases occur in long bones, with the femur and tibia accounting for almost half of all cases. Most cases are limited to a single site.
Staphylococcus aureus is the most commonly identified organism, accounting for 60% to 89% of cases of AHO,4,5 with group A beta-hemolytic streptococci (GABHS) next in frequency. In the past, Haemophilus influenzae accounted for 5% to 7% of cases,1,4,5 but the advent of effective immunization has decreased this incidence dramatically. Streptococcus pneumoniae is a relatively uncommon organism in patients with AHO. Kingella kingae is a common cause of osteomyelitis in the Middle East and is being recognized increasingly in the United States. Osteomyelitis due to K. kingae tends to occur in young children (<4 years) following an upper respiratory tract infection or stomatitis. Group B Streptococcus and enteric gram-negative organisms such as Escherichia coli may be identified in neonates with osteomyelitis but are rare in older children. Salmonella species are commonly identified in patients with sickle cell disease, and Pseudomonas aeruginosa is often identified in cases of osteochondritis following puncture wounds of the feet. Mycobacteria and fungi are rare causes of osteomyelitis. Bartonella henselae is an atypical cause of osteomyelitis, which occurs in patients with cat scratch disease.
Several routes of infection are hypothesized in the pathogenesis of osteomyelitis. In children, most cases result from hematogenous spread after a transient episode of bacteremia. About one-third of patients report a history of blunt trauma,4 which increases the likelihood of seeding an infection during an episode of bacteremia (due to disruptions in the microcirculation in the medullary bone). Less likely, osteomyelitis may result from direct inoculation of bacteria into bone, which may occur during surgery or as a result of penetrating trauma.
Osteomyelitis most commonly begins in the metaphysis of long bones, which are highly vascular structures. Certain bacteria such as S. aureus adhere to bone by expressing receptors (adhesions) for a component of the bone matrix. Growing colonies of bacteria surround themselves with a protective glycocalyx, shielding them from circulating white blood cells. As infection advances, cortical bone is destroyed, and infection and inflammation may extend into the subperiosteal space.
The periosteum in young infants is thin and is not tightly adherent to the underlying bone. As a result, the periosteum is more likely to perforate, spreading infection into the surrounding tissues. In young infants, blood vessels ...