Osteomyelitis is an infection of bone, usually caused by pyogenic bacteria or mycobacteria. It can be usefully subclassifed on the basis of the causative organism, the route, duration and anatomic location of the infection.
Generally microorganisms may be disseminated to bone hematogenously (i.e., via the blood stream), spread contiguously to bone from local areas of infection, such as cellulitis, or be introduced by penetrating trauma. Leukocytes then enter the infected area, and in their attempt to engulf the infectious organisms, release enzymes that lyse bone. Pus spreads into the bone's blood vessels, impairing the flow, and areas of devitalized infected bone, known as sequestra , form the basis of a chronic infection. On histologic examination, these areas of necrotic bone are the basis for distinguishing between acute osteomyelitis and chronic osteomyelitis.
Osteomyelitis often requires prolonged antibiotic therapy, lasting a matter of weeks or months, and may require surgical debridement. Severe cases may lead to the loss of a limb.
Because of the particulars of their blood supply, the tibia, the femur, the humerus, and the vertebral bodies are especially prone to osteomyelitis.
The vast predominance of hematogenously seeded osteomyelitis is caused by Staphylococcus aureus. Escherichia coli, and streptococci are other common pathogens. In some subpopulations, including intravenous drug users and splenectomized patients, Gram negative bacteria, including enteric bacilli, are significant pathogens.
Staphylococcus aureus is also the most common organism seen in osteomyelitis seeded from areas of contiguous infection, but here Gram negative organisms and anaerobes are somewhat more common, and mixed infections may be seen.
In osteomyelitis involving the vertebral bodies, about half the cases are due to Staphylococcus aureus, and the other half are due to tuberculosis (spread hematogenously from the lungs). Tubercular osteomyelitis of the spine was so common before the initiation of effective antitubercular therapy that it acquired a special name, Pott's disease, by which it is sometimes still known.
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