Osteomyelitis (inflammation of bone) can be due to autoimmune rheumatoid conditions, age-related osteoarthritis, or infection.
When the cause is infection, the spread to the bone and/or joints will have occurred via any of 3 methods:
- Hematogenous seeding: Spread of infectious organisms into the bone and joints from the blood. This usually involves only one infectious organism (monomicrobial). Gonococcal arthritis (gonorrhea) is one example. This type of infection occurs more often in children.
- Contiguous spread: Infection that spreads from adjacent tissues into the bones and joints. These bone and joint infections usually involve more than one infectious organism (polymicrobial). Contiguous spread occurs primarily in younger persons with trauma or surgery-related infections; in older persons it is typically spread from decubitus ulcers.
- Introduced directly via trauma: Typically this is a polymicrobial infection of the bones and joints.
If inflammation creates an exudate (pus) that ruptures the outer layer of periosteum, the blood supply to the bone is compromised, leading to necrosis. Additionally, vascular compromise to the bone (vascular insufficiency) occurs frequently in diabetics and in sickle cell anemia. This is not an infection, but the hypoxia and necrosis from it can provide an opportunistic bed for infection to seed.
Acute osteomyelitis is bone infection before there is bone death. It is termed a “septic arthritis,” and pus into the joints adds a secondary septic arthritis. Acute osteomyelitis begins as a dull pain and then progresses to involve the signs of infection, the classic dolor, calor, rubor, and tumor–tenderness, warmth, erythema, and swelling, respectively. Systemic fever may also be present.
Chronic osteomyelitis can present with pain, erythema, or swelling. There may be a draining sinus tract, which establishes the diagnosis of chronic osteomyelitis. Decubitus ulcers that do not heal can indicate the likelihood of chronic osteomyelitis, especially over bony prominences. Fractures that do not heal may persist as chronic osteomyelitis.
Septic arthritis is infection in a joint that can ultimately destroy the joint. The joint can be infected by bacteria (most often), fungi, or mycobacteria. Most infections occur through hematogenous spread from the blood, although direct inoculation via trauma or surgery can cause it.
Other causes include infective endocarditis (Staphylococcus aureus, enterococci, or strep), bacterial meningitis, femoral venipuncture, or sexually transmitted infections (gonorrhea). Bacteria and their toxins inflame the synovial membranes and the joint inflammation quickly creates purulence; the pro-inflammatory cytokines that is part of the innate immune system can degrade the cartilage.
- Age, especially persons > 80 years of age
- Rheumatoid or osteoarthritis–not an infection, but pre-existing arthritis makes bacteria more likely to invade
- Artificial joint
- Joint surgery
- Trauma (including animal bites)
When these risk factors accumulate into a multiple risk pool, the risks rise considerably, especially in those with alcoholism or intravenous drug abuse.