Osteomyelitis is infection of bone, usually bacterial in origin. Microbes can reach bone by three mechanisms: hematogenous spread, extension from a contiguous site of infection, and direct introduction of organisms into bone by trauma and surgery.

Acute hematogenous osteomyelitis involves bone with red marrow. In children, most commonly affected are the long bones with relatively slow flow in metaphyseal sinusoidal veins and a paucity of phagocytes. Infection is often secondary to staphylococcal skin infections. In adults, acute osteo rarely involves the long bones because adipose tissue has replaced red marrow. Instead, it most commonly occurs in vertebral bodies, where the marrow is cellular and has abundant vascular supply. Septicemia is usually the inciting event.

Extension from a contiguous site of infection is a common cause for osteomyelitis. Infection following trauma, radiation therapy, burns or pressure sores. In patients with vascular insufficiency, organisms can enter the soft tissues through a cutaneous ulcer, often in the foot, and cause cellulitis, then osteomyelitis.

Direct introduction of organisms into the bone may occur during open fractures, open surgical reduction of closed fractures, or penetrating trauma by foreign bodies such as bullets. May also arise from perioperative contamination of bone during laminectomy, diskectomy, or joint prosthesis. The causitive organism is typically normal flora, such as Staph Epidermidis.