Clinical meaning
Osteomyelitis is classified as acute (developing over days to weeks) or chronic (persisting beyond 6 weeks or recurring after apparent treatment). The distinction is critical because pathophysiology, imaging findings, and management differ significantly between the two forms. In acute osteomyelitis, bacteria reach bone via hematogenous spread (most common in children and vertebral osteomyelitis in adults), contiguous spread from adjacent soft tissue infection (diabetic foot ulcers, decubitus ulcers), or direct inoculation (open fractures, surgical hardware). The most common pathogen across all forms is Staphylococcus aureus, which possesses surface adhesins (MSCRAMMs -- microbial surface components recognizing adhesive matrix molecules) that bind collagen, fibronectin, and bone sialoprotein in the extracellular bone matrix. Once adherent, S. aureus produces biofilm -- a polysaccharide matrix that shields organisms from immune cells and creates antibiotic concentrations 100-1000x lower than in surrounding tissue. The acute inflammatory response recruits neutrophils and macrophages, which release proteolytic enzymes and reactive oxygen species. Within the rigid bony architecture, purulent exudate accumulates and increases intramedullary pressure, compressing nutrient blood vessels within Haversian canals and Volkmann canals. This vascular compromise causes ischemic necrosis of bone segments, forming a sequestrum -- a devitalized fragment of cortical bone that harbors bacteria within its lacunae but is completely impenetrable to systemic antibiotics and immune cells. Chronic osteomyelitis develops when acute infection is inadequately treated or when biofilm-associated infection persists. The periosteum responds by laying down reactive new bone (involucrum) around the sequestrum, while sinus tracts (cloacae) form to drain purulent material to the skin surface. The chronic biofilm-associated infection can persist for months to years with intermittent flares. The Cierny-Mader classification system stages chronic osteomyelitis by anatomic type (medullary, superficial, localized, diffuse) and physiologic host class (A = normal host, B = compromised host, C = treatment worse than disease). The OVIVA trial (2019) demonstrated that highly bioavailable oral antibiotics (fluoroquinolones, linezolid, co-trimoxazole) are non-inferior to IV therapy after an initial IV course for bone and joint infections, changing the paradigm that all osteomyelitis requires 6 weeks of IV antibiotics.