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General Features and Pathology of Hematogenous Osteomyelitis


- Discussion:
    - bone provides and environment that significantly alters interaction between bacterial and host defenses in favor of the invader;
    - bone contains numerous microscopic channels, the walls of which are impermeable to such host defenses as the large molecules of  immunoglobulins;
    - bacteria, once gaining a foothold in these minute channels, may be isolated from both the cellular and chemical elements of host defenses and free to proliferate unimpeded;
    - microcirculation of bone within these channels is highly vulnerable to bacterial toxins, which may produce local thrombosis of these small vessels leading to the death of bone in the vicinity of infection, which further impedes access by host defenses;
    - bone resorption, bone formation, and necrosis are tissue responses seen in varying degrees in all bone infections;
    - tortuous course of nutrient vessels in bone causes bacteria to be trapped in the metaphysis (see pediatric bone circulation);
    - epiphyseal plate prevents infection from entering the joint space in older children but not in neonates;
    - joint infection secondary to osteomyelitis may occur in shoulder and hip as a result of synovial membrane inserting distally to epiphysis, allowing bacteria to spread directly from metaphysis to the joint space;

- Pathology:
    - in classical hematogenous osteomyelitis of childhood, organisms in bloodstream gain access to bone by way of the nutrient artery and pass through branches of this vessel to the metaphysis;
    - terminal vessels enter small, bone-lined channels on metaphyseal side of the epiphyseal plate;
           - these channels have a blind end, and each terminates in the last hypertrophic cartilage cell of epiphyseal plate;
           - on reaching the end of the tunnel, capillary inside this channel must turn 180 deg and exit through the same passage by which it entered;
           - it is in this area of sluggish circulation, in tunnels whose walls are essentially impermeable, that bacteria lodge  & hematogenous  osteomyelitis begins;
           - once the process starts, thrombosis of small terminal vessel and exudate seal the entrance to channel;
    - in depths of this bone tunnel the organisms begin to proliferate, unimpeded by the host defenses that are unable to reach them;
    - after the infection is established in one channel, it extends into metaphysis and involves other channels;
    - hordes of PMNs are mobilized in metaphysis, and cellular destruction occurs at a rapid rate;
    - in adolescents, infection usually begins in metaphysis because of tendency for sluggish & turbulent blood flow in capillary-sinusoid loops adjacent to growth plate & absent or nonfunctioning phagocytes;
    - bacteria that localize to this area proliferate and easily obstruct blood flow, resulting in infarction and creation of environment conducive to further bacterial proliferation;
    - pus in metaphysial area expands under pressure and travels up marrow cavity or passes out of cortex through many small metaphyseal openings;
    - in certain areas such as the hip, where epiphyseal plate is situated w/in joint capsule, early joint involvement by infection is common;
    - at knee, the joint is usually spared, and infections beginning in distal femoral or proximal tibial metaphysis usually spread along shaft of the bone moving away from the joint;
    - stripping of the periosteum from the shaft by pus stimulates intense osteoblastic response;
    - new immature bone is formed as response to periosteal stripping, and, in severe cases, the entire shaft may be encased in a sheath of new bone referred to as an involucrum;
    - where a major portion of the shaft has been deprived of blood supply, resulting sequestrum lies within the involucrum;
           - openings in the involucrum, called cloacae, may permit escape of pus from bone may be fulminant, w/ overwhelming sepsis & death;
           - septicemia can cause bacterial seeding to other bones, leading to multiple sites of osteomyelitis;
           - if disease is treated early w/ appropriate antibiotics, septicemia is controlled, and there may be only limited bone damage