Osteomyelitis

Definition

Osteomyelitis is an infection of bone and bone marrow caused by bacteria or, less commonly, fungi, leading to inflammation, bone necrosis, and potential chronic infection.

Epidemiology

• Can occur at any age; more common in children (hematogenous spread) and older adults (contiguous spread).
• Males are affected more often than females.
• Commonly involves long bones in children (femur, tibia, humerus) and vertebrae in adults.
• Risk factors: diabetes mellitus, peripheral vascular disease, trauma, recent surgery, prosthetic implants, intravenous drug use, and immunosuppression.
• Staphylococcus aureus is the most frequent causative organism across all age groups.

Etiology

• Hematogenous spread: most common in children — bacterial seeding of bone via bloodstream.
• Contiguous spread: from adjacent soft-tissue infection, ulcers, or trauma (common in adults).
• Direct inoculation: open fractures, surgery, or penetrating injuries.
• Common pathogens:
• — Staphylococcus aureus (including MRSA): most common overall.
• — Streptococci and Enterobacteriaceae: in post-traumatic and diabetic foot infections.
• — Pseudomonas aeruginosa: common in puncture wounds, IV drug users, and chronic ulcers.
• — Salmonella species: associated with sickle cell disease.
• — Mycobacterium tuberculosis: chronic, indolent infection (Pott’s disease).

Pathophysiology

• Pathogens reach bone via hematogenous, contiguous, or direct routes.
• Bacterial multiplication triggers inflammatory response with increased intramedullary pressure and vascular compromise.
• Necrosis of bone tissue forms a sequestrum (dead bone) surrounded by new bone formation (involucrum).
• Chronic infection may develop with persistent drainage and sinus tract formation.
• Biofilm formation on prosthetic material complicates eradication and leads to chronicity.