What Are Bone Infections?
Bone infections are called osteomyelitis. They 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.
How Are Bone Infections Diagnosed?
Factors that can increase the risk of osteomyelitis impact its severity are considered during evaluation with an in-depth history (predisposing factors–diabetes, trauma, intravenous drug abuse, vasculitis, etc.) and by a thorough physical exam. A bone probe to investigate the depth of involvement over any suspected soft tissue site of infection is used to identify the need for further testing for bone involvement.
- Laboratory tests can indicate something is occurring: But usually are not any more specific than that. Leukocytosis (white blood cell count elevations as part of the immune response) may not even be present with chronic osteomyelitis.
- The blood test markers for inflammation: Erythrocyte sedimentation rate (ESR) and the C-reactive protein (CRP)–may or may not be elevated.
- Blood cultures are only positive if there is bacteremia: Either the cause of osteomyelitis (hematogenous) or from the osteomyelitis (secondary bacteremia or sepsis from the bone infection).
- X-rays are obtained: If inconclusive or negative, can prompt more involved imaging such as MRI. If a patient has internal metal hardware, CT scans can be used as an alternative.
- Bone biopsy is the standard for diagnosing osteomyelitis: Bacteria can be cultured and the microscopic examination of the bone biopsy tissue can be examined for cellular inflammatory changes and necrosis (tissue death). The biopsy is preferably via an open surgical technique; the alternative is a needle biopsy through the skin, but this often has less reliable findings.
In diagnosing osteomyelitis, other similar clinical presentation should be ruled out, such as soft tissue infection (bone involvement is identified with the bone probe), osteonecrosis from steroids or radiation, gout, fracture, bursitis, malignancy, or synovitis.
How Do I Manage Bone Infections?
Treatment of bone infections involves the three standard approaches to any serious tissue-destructive infection:
- Debridement: surgical removal of necrotic tissue must accompany antibiotic therapy to expect improvement. This may include removal of implanted metal materials previously used for other reasons but now clearly involved in the bone infection.
- Antibiotic therapy: Based on the bone biopsy-directed culture and sensitivity of the infectious organism(s).
- Therapy: Over time sufficient to treat infection based on an individualized assessment of the types of bacteria involved, the anatomic site, and the local environment (devascularized? immunosuppressed?) where the infection resides.
Antibiotic therapy for osteomyelitis is typically lengthy. Parenteral (intravenous) antibiotics can be given in the hospital, nursing facility, or as an outpatient via home health nursing. A signal that the duration has been long enough is when all debrided bone has been covered by well-vascularized soft tissue (≥ 6 weeks). If there is involved hardware that mustn’t be removed, the course of parenteral antibiotics is continued via oral antibiotics.
Other therapies for stubborn infections include hyperbaric oxygen that aids the function of the blood supply and negative pressure wound therapy (“vacuum-assisted closure,” or NPWT). NPWT suctions the healing areas such that the tissue collapses to reapproximate, without the dissection effects of secretions.
Prevention of Bone Infections
Trauma is unpredictable, making it impossible to avoid the inoculation causes of bone infections.
Penetrating wounds of the limbs should be probed for bone exposure or involvement and any open lacerations irrigated copiously.
Contiguous spread from other tissues is prevented by aggressive and scrupulous attention and therapy to infected tissues, especially in diabetics and others with peripheral vascular disease, and also in those who are immunocompromised (HIV, chemotherapy, and steroid patients).
- Strict glycemic control of diabetics: Aggressive management of pressure sores and bedsores in diabetics or patients who are immobilized, especially those in nursing or assisted-living facilities
- Avoid intravenous drug abuse which carries a high risk of bacteria
- Sickle cell anemia vigilance for the need for transfusion: Sickling of red blood cells can occlude the blood supply to bone, especially relevant in hip necrosis in these patients
- Frequent assessments for infection in patients on chemotherapy: immunosuppression (transplant patients), and patients with peripheral vascular disease (arteritis, claudication, atherosclerosis, or venous stasis)
- Smoking cessation if smoking: Nicotine is a powerful vasoconstrictor that can make vascular compromise much worse
- Altering steroid therapy: For rheumatic inflammatory diseases when obvious soft tissue infection is present
Hematogenous spread can be prevented by early identification treatment for bacteremia or sepsis in hospitalized patients and dialysis patients.