Osteomyelitis

Osteomyelitis is an infection of the bone that can involve the cortex, medullary canal, and periosteum. The most common causative organism is Staphylococcus aureus, which accounts for approximately 70-80% of all cases. The infection reaches bone through three primary routes: hematogenous spread (bloodstream seeding from a distant infection site, most common in children and older adults), contiguous spread (direct extension from an adjacent soft tissue infection such as a diabetic foot ulcer or surgical wound), and direct inoculation (penetrating trauma, open fracture, or surgical contamination). Once bacteria reach the bone, they adhere to the bone matrix using surface adhesion molecules and produce biofilm, a protective polysaccharide layer that shields the organisms from both the immune system and antibiotics. The inflammatory response triggers vascular congestion, thrombosis of local blood vessels, and subsequent ischemia of the surrounding bone tissue. As the infection progresses, pus accumulates within the rigid bony structure, increasing intramedullary pressure and further compromising blood flow. Devitalized bone separates from living tissue and forms a sequestrum -- a segment of dead, infected bone that acts as a persistent reservoir for bacteria. The periosteum responds by laying down new bone around the infected area, forming an involucrum (a sleeve of reactive new bone surrounding the sequestrum). The formation of a sequestrum is clinically significant because antibiotics cannot penetrate dead bone, making surgical debridement necessary in chronic osteomyelitis. Acute osteomyelitis develops over days to weeks and is characterized by local pain, warmth, swelling, and systemic signs of infection including fever and elevated inflammatory markers. If not adequately treated, acute osteomyelitis can progress to chronic osteomyelitis, which is defined as infection persisting for more than six weeks or the presence of sequestrum on imaging. Chronic osteomyelitis may feature draining sinus tracts that connect the bone to the skin surface, intermittent flares of infection, and progressive bone destruction. In children, hematogenous osteomyelitis typically affects the metaphysis of long bones (the highly vascular growth plate region) because the sluggish blood flow in metaphyseal sinusoidal veins allows bacterial seeding. In adults, the vertebral bodies are the most common site for hematogenous osteomyelitis, while the feet are the most common site for contiguous osteomyelitis, particularly in patients with diabetes mellitus and peripheral neuropathy.