Osteomyelitis is a serious bacterial infection of the bone and bone marrow. The dense structure of bone tissue makes it difficult for the body’s immune system and antibiotics to clear the infection once established. Early identification is important to prevent progression and permanent bone damage. The primary pathogen in most cases is Staphylococcus aureus, a bacterium commonly found on the skin.
Understanding the Causes and Entry Points
Infection is introduced into the bone tissue through one of three main pathways.
The first is hematogenous spread, where bacteria travel through the bloodstream from a distant site of infection, such as pneumonia or a urinary tract infection. This route is most common in children, often affecting the long bones, and in adults, where it typically involves the vertebrae of the spine.
A second pathway is contiguous spread, which occurs when an infection from adjacent soft tissue moves into the bone. This is frequently seen in individuals with compromised circulation, such as those with diabetes who develop infected foot ulcers.
The third mechanism is direct inoculation, where bacteria are introduced directly into the bone through trauma, such as an open fracture, or during a surgical procedure.
The Three Stages of Osteomyelitis Progression
The progression of osteomyelitis is typically categorized into three stages based on the duration and pathological changes within the bone: acute, subacute, and chronic. This time-based classification reflects the shift from a highly symptomatic, treatable infection to one that is structurally complicated and persistent.
Acute Osteomyelitis
Acute osteomyelitis represents a new, rapidly developing infection, generally presenting within the first two weeks of onset. Symptoms are often systemic and pronounced, including fever, chills, and localized pain and swelling over the affected bone. The infection causes inflammation and the accumulation of pus within the rigid bone environment, leading to increased internal pressure. This pressure can compromise the blood supply to the bone tissue, initiating bone death, known as necrosis.
Subacute Osteomyelitis
The subacute stage is characterized by a more gradual and often milder presentation, sometimes occurring when a partial immune response or prior antibiotic exposure has contained the infection. Symptoms are typically less severe than the acute stage, often manifesting as a persistent, low-grade ache rather than pain, and may lack systemic signs like high fever. This reduced virulence allows the body to attempt to wall off the infection, leading to the formation of a localized abscess within the bone. This type of localized bone abscess is often referred to as a Brodie’s abscess.
Brodie’s abscess is a focus of pus and necrotic tissue encapsulated by a thick layer of reactive, sclerotic bone. The mild nature of the infection means that inflammatory markers in the blood, such as the white blood cell count, may remain nearly normal, making diagnosis challenging. This stage frequently occurs in the metaphysis—the end-part of a long bone—and can sometimes be mistaken for a bone tumor due to its appearance on imaging.
Chronic Osteomyelitis
Chronic osteomyelitis is defined as an infection that persists for months or years, often resulting from inadequate treatment of the earlier stages. The defining pathological feature of the chronic stage is the presence of sequestrum, which is a fragment of dead, infected bone that has become detached from the surrounding healthy bone. The body attempts to isolate this dead tissue by forming a sheath of new bone around it, called an involucrum.
The presence of the sequestrum makes chronic osteomyelitis difficult to eradicate, as the dead bone fragment is avascular, meaning it has no blood supply. Without blood flow, intravenously delivered antibiotics cannot penetrate the sequestrum to reach the bacteria. The infection may persist indefinitely, sometimes leading to the formation of a draining sinus tract that tunnels through the soft tissue and skin.
Diagnosis and Management Approaches
Diagnosing osteomyelitis relies on a combination of laboratory tests, advanced imaging, and definitive tissue sampling. Blood tests often show elevated markers of inflammation, such as the erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP), which indicate an infection is present. While X-rays are a starting point, they can miss early infections since bone changes may not be visible until a significant portion of the bone matrix is destroyed.
Magnetic resonance imaging (MRI) is the preferred imaging modality for its high sensitivity in detecting early signs of bone marrow edema and localizing the infection. The most definitive diagnostic step is a bone biopsy or aspiration, which allows for culturing the specific organism responsible for the infection. Identifying the pathogen is necessary to select the most effective antibiotic treatment.
Management differs significantly based on the stage of the disease. Acute osteomyelitis is primarily treated with high-dose, long-term intravenous antibiotics, typically administered for four to six weeks to ensure the bacteria are fully eliminated from the bone. If an abscess is present, surgical drainage may be necessary to relieve pressure and remove the collection of pus.
In contrast, chronic osteomyelitis almost always requires surgery in addition to antibiotics due to the presence of dead bone. The primary surgical procedure is debridement, or sequestrectomy, which involves physically removing the necrotic sequestrum and all infected tissue. Following surgical removal of the dead bone, long-term antibiotics are administered, often combined with antibiotic-loaded cement or beads placed directly into the surgical site to maximize local drug concentration.