OSTEOMYELITIS: PATHOPHYSIOLOGY AND MICROBIOLOGY
Osteomyelitis is the inflammation of the bone and marrow, typically caused by infection. Despite much study on adult osteomyelitis, the wide range of presentations and the complex diagnostic and management issues have made this a “hot topic” with many unanswered questions.
Pathophysiology
The origin of infection can be used to categorize osteomyelitis. The route of infection may be (1) hematogenous, (2) contiguous from an adjacent site of infection, or (3) secondary to direct inoculation.
Osteomyelitis can also be described as acute or chronic. Acute osteomyelitis implies a newly recognized infection. Chronic osteomyelitis suggests relapsing or untreated disease, or the presence of inert substrate for bacterial attachment that makes the infection refractory to antibiotics alone. Possible inert substrates include sequestrum, defined as necrotic bone resulting from ischemia caused by suppurative build-up, as well as prosthetic devices and other foreign bodies. Other pathologic findings of chronic osteomyelitis may include draining sinuses and formation of reactive bone, called involcrum.
Microbiology
Familiarity with the common pathogens in different types of osteomyelitis is important to direct empiric antibiotic therapy when culture results are not yet available. In approximately 50% of cases, no organism is isolated, and treatment must be directed against anticipated pathogens.
Among most types of osteomyelitis, Staphylococcus aureus (methicillin-susceptible or -resistant) is the microorganism most frequently isolated. Other types of bacteria, as well as fungi, have been associated with specific patient populations or clinical syndromes. Infections associated with prosthetic joints are typically caused by S. aureus or coagulase-negative staphylococci. Osteomyelitis due to diabetic foot infections is often polymicrobial, with aerobic and anaerobic bacteria. Osteomyelitis in intravenous drug abusers is commonly associated with staphylococci, gram-negative rods, or Candida species. Mycobacterium tuberculosis may cause osteomyelitis and typically affects the axial skeleton in adults (Pott’s disease). Pathogens related to specific exposures such as bites or animal contact, as well as the endemic mycoses (for example, blastomycosis and coccidioidomycosis) may also cause osteomyelitis in exposed individuals.
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