Osteomyelitis and Septic Arthritis

Osteomyelitis is infection of the bone, most commonly caused by Staphylococcus aureus (including MRSA) reaching bone through hematogenous spread (children), contiguous spread from adjacent soft tissue infection (adults, diabetic foot), or direct inoculation (open fractures, surgical hardware). Once established in the medullary cavity, bacteria form biofilms on bone surfaces—complex polysaccharide matrices that shield organisms from antibiotics and immune cells. The infection triggers an intense inflammatory response: PMN infiltration, abscess formation, increased intraosseous pressure compromising blood supply, and bone necrosis. Necrotic bone fragments (sequestra) become avascular refuges for bacteria, perpetuating infection. The periosteum responds by laying down new reactive bone (involucrum) around the infected area. Septic arthritis is infection of the joint space, also most commonly caused by S. aureus (or N. gonorrhoeae in sexually active young adults). Bacteria in the synovial fluid trigger cytokine release, neutrophil infiltration, and release of proteolytic enzymes that destroy articular cartilage within hours. Untreated septic arthritis causes permanent joint destruction within 24-48 hours, making it an orthopedic emergency.