What are bone infections?
Infection in the bone is called osteomyelitis. It can occur by three possible routes:
- Hematogenous (infection delivered to the bone from the blood stream)
- Contiguous spread (infection extends to bone from infected adjacent tissues or joints)
- Inoculation (infection inserted into the bone via trauma or surgery)
Hematogenous spread to the bone is via a bacteremia or sepsis and is usually from only one infectious organism. Contiguous spread, from ulcers or cellulitis, for example, can be due to one organism, but is usually more than one (polymicrobial). Direct inoculation from trauma is typically polymicrobial.
Bone infections are categorized by either duration (acute vs chronic) and infectious mechanism (hematogenous, contiguous, inoculation); or by location, immunocompetence of the host, and the nature of the infection’s environment (well-vascularized vs devascularized, immunocompetent vs immunosuppressed, etc).
Complications of Osteomyelitis
All infections evoke an inflammatory process due to the innate and humoral immune systems. This results in inflammatory purulent exudates (pus). The bone, as a closed container, can rupture from the pressure of the exudate, which interrupts the blood supply, killing sections of bone (called “sequestrum,” singular; “sequestra,” plural). Sequestra can be identified on X-rays.
Joint septic arthritis can occur if pus is discharged into the joint.
There is a gradual onset of symptoms over a few days, with dull pain. There is tenderness, warmth, erythema, and swelling locally, as well as fever or other systemic signs and symptoms.
Chronic bone infection also has the tenderness, warmth, erythema, and swelling locally, but over time a sinus tract can drain purulent discharge to the skin. Ulcers that do not heal often have an underlying chronic bone infection.