Osteomyelitis, adult

Description

Osteomyelitis, though derived from Greek roots meaning ‘inflammation of the bone and bone marrow’, is not a inflammatory condition, per se.  The term refers specifically to infection of the bone. Osteomyelitis is distinct from other infectious processes in the body because of bone’s unique response to infection, namely, the formation of new bone around the infected area.

Clinical Manifestations

Typical signs and symptoms of osteomyelitis include local bone pain, erythema, and swelling, as well as systemic symptoms such as fever. A classic syndrome of cyclic pain correlating with the progression of bone necrosis has been described, although this is not a sensitive finding.
On examination, signs of local infection may be seen, possibly including small recurrent sinus tracts draining to the skin which can communicate with underlying infected bone.

Red flags

Osteomyelitis is particularly worrisome in two settings: infection of the vertebra (because of the risk of spread into the epidural space with neurological consequences) and infection of artificial joints (because of the tenacity of adherent bacteria adhere). Thus, particular attention must be paid to patients with potential spinal infections or with patients who have an implant

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Epidemiology

Osteomyelitis in the normal adult host is rare, except in the case of penetrating trauma. Osteomyelitis due to tuberculosis can be seen occasionally and is more common in patients who use intravenous drugs, have sickle cell anemia, diabetic vascular disease or other conditions of that compromise the immune system.

Pathology and pathophysiology

Bone may be infected by either direct inoculation of via spread in the bloodstream. The direct infection may be from overlying cellulitis or penetrating trauma such as open fractures.

The first response to infection is an inflammatory response which has two ramifications: first, the surrounding bone undergoes remodelling (resorption and redeposition); and second, blood flow to the core is impaired leading to ischemia (and ultimately death of the bone). Taken together, there is often new bone built (the involucrum) surrounding the infected core (the sequestrum).  Because the vascularity of bone and function of the periosteum are different in children, pediatric osteomyelitis is a distinct entity, with a presentation and treatment requirements different from the adult form.

Differential diagnosis

Because osteomyelitis deforms bone and causes new bone growth, its radiographic appearance may suggest a tumor. Likewise, because osteomyelitis is more common in patients with diabetes and vascular disease, a Charcot arthropathy may be present as well and obscure the radiographic findings. A history of prior trauma, immune suppression or concurrent infection elsewhere (eg, bacteremia) all increase the likelihood that a given presentation that looks like osteomyelitis actually is osteomyelitis.

Etiology

Osteomyelitis can result from direct trauma, spread in the blood stream from distant infection (such as dental abscess) or spread from nearby cellulitis.
Microbiology most often reveals Staphylococus aureus infection, although Staphylococcus epidermis and gram negative rods are implicated in a minority of cases.  Patients with sickle cell anemia are prone to osteomyelitis.  Salmonella is often the cause.

Radiographic and laboratory findings

The diagnosis of osteomyelitis is established by first identifying the lesion on imaging followed by confirmation via biopsy and culture.
Plain films can show characteristic changes within 10-14 days of inoculation. Plain films also can help delineate the underlying anatomy, identify prior or current fractures, and indicate the presence of surgical hardware or artificial joints. In short, plain films are required.

Three phase bone scan has a high sensitivity for osteomyelitis but is not specific. Indium scans can be used to detect areas of acute infection, whereas gallium scintigraphy is preferable for chronic cases, as gallium binds to even dead neutrophils.  MRI has both high sensitivity and specificity but requires localized imaging. As such, indium and gallium scans may be better when screening for infection over a wide area, with MRI reserved for investigation of a better delineated area of concern.

An elevated serum white blood cell count, sedimentation rate or C-reactive protein levels are all nonspecific markers of inflammation. Accordingly, elevated levels may not reflect infection. On the other hand, normal levels decrease, but do not exclude, the likelihood of infection. 

Risk Factors and Prevention

Trauma is a cause of osteomyelitis, so any mode that decreases the incidence of trauma could decrease the incidence of osteomyelitis. Once trauma has occurred, expeditious care, including debridement of open wounds and initiation of antibiotics can decrease the risk of osteomyelitis.  Similarly, aggressive treatment of nascent infections in patients with elevated risk for osteomyelitis (ie, sickle cell anemia or diabetes) may prevent the establishment of infection within the bone.

Treatment options

Osteomyelitis can be suppressed but rarely eradicated by antibiotics alone.  Surgical debridement of devitalized bone is imperative. Beyond the debridement itself, surgeons must decide how to stabilize the bone and fill the resulting “dead space.” At the time of debridement, antibiotic-containing beads may be implanted in this space for up to 30 days and subsequently removed.

During a second stage surgery, a surgeon may place a bone graft. The overlying tissue is either allowed to heal directly or, more commonly, is aided by the transfer of a free muscle flap to provide well-vascularized tissue to aid in local healing.

An alternative approach to focal osteomyelitis is to completely excise the area (which also shortens the bone). The placement of an Ilizarov external fixator promotes compensatory lengthening within a healthy area by so called “distraction osteogenesis”.  

Amputation is considered if a limb is unsalvageable.  Systemic antibiotics are used as adjuncts to surgery to help prevent infection in the healing of the operative wound and to prevent spread to other tissues.

Outcomes

Heterogeneity among cases of osteomyelitis make it difficult to report unified outcomes data. A case of osteomyelitis can be defined by the causative organism (and its antibiotic sensitivity), mode of infection, duration of infection, bone involved, size of the lesion, proximity to the joint, as well as many host-specific factors. In general, it has been reported that wide surgical debridement and free flap reconstruction as needed usually prevents recurrence of chronic osteomyelitis.

Holistic medicine

Poor nutrition and excessive alcohol intake are thought to predispose at-risk patients to developing post-traumatic osteomyelitis. Additionally, patients with diabetes may develop vascular insufficiency, which would also put them at higher risk for developing osteomyelitis if there is skin breakdown on the feet. As such, attention to foot hygiene and toe nail for these patients is imperative

Miscellany

The world's largest known Tyrannosaurus rex housed at The Field Museum in Chicago had osteomyelitis.
Thomas Eakin’s masterpiece painting, “The Gross Clinic”, depicts treatment of osteomyelitis.

Key terms

sequestrum

involucrum

Skills and competencies

Provide urgent care for open fractures to prevent osteomyelitis; recognize situations where expeditious treatment is needed