Introduction
Osteomyelitis is inflammation of the bone and its marrow, usually caused by infection. It often involves the periosteum (periostitis) and, less commonly, the cartilage covering (perichondritis). This note covers the causes and routes of infection, the main types of osteomyelitis, and the periosteal and perichondral inflammations, with their clinical picture, diagnosis, treatment and complications.
Etiology and routes of infection
Bone infection is most often bacterial. Staphylococcus aureus is the leading pathogen in nearly all forms. Other organisms depend on the setting: streptococci, Enterobacteriaceae, and Pseudomonas aeruginosa after puncture wounds through footwear; anaerobes in diabetic foot and dental or jaw infections; Salmonella in sickle cell disease; and Mycobacterium tuberculosis in tuberculous (Pott) spondylitis.
Infection reaches bone by three main routes:
- Hematogenous spread from a distant focus such as a boil, tonsillitis, urinary tract infection or endocarditis. It is typical of children, who have slow blood flow through the metaphyseal loops of long bones.
- Contiguous spread from adjacent soft tissue infection, ulcers or a septic joint.
- Direct inoculation after open fractures, penetrating trauma, orthopaedic surgery or implant placement.
Predisposing factors include diabetes mellitus, peripheral vascular disease, immunosuppression, sickle cell disease, intravenous drug use, malnutrition and chronic renal failure. Local ischaemia and bone necrosis allow bacteria to survive and form biofilms, which explains why infection tends to become chronic.
Pathogenesis: bacteria lodge in the bone, cause vascular thrombosis and ischaemic necrosis, and pus accumulates in the marrow cavity. Pressure lifts the periosteum and spreads the pus under it. Dead bone separates as a sequestrum, while the periosteum lays down a new bone shell, the involucrum. Pus escapes through openings called cloacae, creating sinus tracts.