78 - Osteomyelitis

Acute hematogenous osteomyelitis (AHO) is primarily a disease of young children, presumably because of the rich vascular supply of their rapidly growing bones. Infecting organisms enter the bone through the nutrient artery and then travel to the metaphyseal capillary loops, where they are deposited, replicate, and initiate an inflammatory response (Figure 78-1). Metaphyseal localization results from sluggish blood flow, the presence of endothelial gaps in the tips of growing metaphyseal vessels, and lack of phagocytic cells lining the capillaries. Bacteria proliferate, spread through vascular tunnels, and are anchored to areas of exposed cartilaginous matrix. Large colonies of bacteria surrounded by glycocalyx obstruct capillary lumens, impairing phagocytosis and antibiotic penetration. Age-related differences in the anatomy of the bone and its blood supply influence the clinical manifestations of osteomyelitis. Transphyseal vessels are present in most children younger than 18 months, providing a vascular connection between the metaphysis and the epiphysis. As a result, in infants, infection originating in the metaphysis can spread to the epiphysis and joint space. The risk of ischemic damage to the growth plate is greater in the young infant with osteomyelitis. Before puberty, the periosteum is not firmly anchored to underlying bone. Infection in the metaphysis of a bone can spread to perforate the bony cortex, causing subperiosteal elevation and extension into surrounding soft tissue. Bony destruction can spread to the diaphysis. By age of 2 years, the cartilaginous growth plate usually prevents extension of infection to the epiphysis and into the joint space. When the metaphysis of the proximal femur or humerus is involved, however, infection can extend into the hip or shoulder joint at any age, because at these sites, the metaphysis is intracapsular. Histologic features of acute osteomyelitis include localized suppuration and abscess formation, with subsequent infarction and necrosis of bone. Segments of bone that lose blood supply and become separated from viable bone are called sequestra. An involucrum is a layer of living bone surrounding dead bone. A Brodie abscess is a subacute, well-demarcated focal infection, usually in the metaphysis but sometimes in the diaphysis of bone.