Osteomyelitis

Osteomyelitis is infection of bone. It is classified by route of acquisition (haematogenous, contiguous spread from an adjacent soft tissue infection, or direct inoculation through trauma or surgery) and by chronicity (acute versus chronic).

The pattern and typical causative organisms vary with age, the mechanism of acquisition, and any relevant underlying predisposing condition.

Definitive management requires prolonged targeted antibiotic therapy and, frequently, surgical debridement, particularly in chronic or complicated disease.

Classification and pathophysiology

• Acute haematogenous osteomyelitis is predominantly a disease of children. The rich, relatively stagnant vascular supply to the metaphysis of long bones in this age group predisposes to bacterial seeding.
• It typically presents with a single site of infection. In the majority of cases it responds well to antibiotic therapy alone if treated promptly.
• Contiguous spread osteomyelitis typically occurs in adults. It reflects direct extension of infection from an adjacent soft tissue source. Diabetic foot infection is a particularly important and common example.
• Infection is frequently established and chronic by the time of diagnosis, so contiguous spread osteomyelitis generally requires a combined surgical and antibiotic approach.
• Osteomyelitis following direct inoculation occurs after open fracture, surgical fixation, or other direct contamination. The organism profile reflects the mechanism and setting of the original injury or procedure.
• Chronic osteomyelitis is characterised by necrotic, devascularised bone (sequestrum). Sequestrum shields residual infection from both host immune defences and antibiotic penetration.
• Chronic disease therefore frequently requires surgical debridement to achieve cure. This is in clear distinction to the generally antibiotic responsive acute haematogenous disease of children.

• Acute haematogenous osteomyelitis in children typically presents with localised bone pain, reluctance to bear weight or use the affected limb, fever, and localised tenderness, warmth and swelling over the affected site, developing over several days.
• Adult osteomyelitis, whether from contiguous spread or direct inoculation, frequently presents more insidiously, with localised pain and, where there is an overlying wound or sinus tract, persistent or recurrent discharge, sometimes without significant systemic upset, particularly in chronic disease.
• Chronic osteomyelitis characteristically presents with a persistent or intermittently discharging sinus tract, recurrent low grade symptoms punctuated by acute flares, and, on imaging, evidence of bone destruction, sequestrum formation and surrounding new bone formation (involucrum).
• Vertebral osteomyelitis has a distinct presentation and specific management considerations, particularly regarding the risk of neurological compromise.
• A relevant history of preceding trauma, surgery, diabetes, peripheral vascular disease, or an identifiable contiguous or haematogenous source of infection should be sought, together with careful examination for any overlying wound, sinus tract, or evidence of adjacent soft tissue infection.

• Magnetic resonance imaging is the most sensitive imaging modality for early diagnosis. It shows characteristic marrow oedema and, in more established disease, abscess formation, sequestrum and involucrum.
• Plain radiography is frequently normal in early acute disease, with changes typically lagging behind the clinical presentation by one to two weeks. It is useful for identifying more established chronic changes and for baseline comparison.
• Obtain blood cultures in every suspected case. They are particularly relevant in acute haematogenous disease, where they are frequently positive and can obviate the need for a more invasive diagnostic procedure.
• Bone biopsy, obtained surgically or by image guided percutaneous sampling, gives the definitive microbiological diagnosis. Use it where blood cultures are negative or a specific tissue diagnosis is otherwise required. Obtain it before antibiotic administration wherever clinically feasible.
• Superficial wound or sinus tract swabs are generally unreliable for guiding antibiotic therapy. They are frequently contaminated with surface colonising organisms that may not reflect the true deep bone pathogen.
• Do not rely on superficial swabs in isolation to guide definitive treatment where deep tissue or bone sampling is achievable.
• Inflammatory markers, particularly C reactive protein and erythrocyte sedimentation rate, support diagnosis and are useful for monitoring treatment response.
• Where diagnostic uncertainty persists despite these investigations, nuclear medicine imaging may have a role. Options are bone scan or, with greater specificity for infection, labelled white cell scan. This is particularly relevant with overlying metalwork, which can degrade magnetic resonance imaging quality.

• Acute haematogenous osteomyelitis in children is generally managed with intravenous antibiotic therapy alone in the majority of cases where treatment is started promptly.
• Flucloxacillin 50 mg per kg intravenously four times daily (to a maximum adult equivalent dose, adjusted according to local protocols) is a standard first line choice. It covers the predominant causative organism, Staphylococcus aureus.
• Transition to an appropriate oral agent to complete a total course typically of four to six weeks, guided by clinical response and inflammatory marker trend.
• Adult osteomyelitis, particularly chronic or contiguous spread disease, generally requires a combined surgical and antibiotic approach.
• Surgical debridement of all necrotic, infected bone and soft tissue (removal of any sequestrum) is an essential component, given the limited ability of antibiotic therapy alone to eradicate infection within devascularised, necrotic bone.
• Empirical antibiotic therapy pending culture results typically comprises flucloxacillin or vancomycin (where methicillin resistant Staphylococcus aureus is a concern).
• Add broader Gram negative and anaerobic cover where relevant to the clinical context (contiguous spread from a diabetic foot ulcer, or following open trauma with significant contamination). Narrow therapy once culture and sensitivity results are available from bone biopsy or, where positive, blood culture.
• Total antibiotic duration for adult and chronic osteomyelitis is prolonged, typically a minimum of six weeks following adequate surgical debridement.
• Some evidence supports appropriately selected oral regimens with good bioavailability for a substantial portion of this course, rather than prolonged intravenous therapy throughout. Guide this by specialist infectious disease input and individual clinical circumstances.
• Management of any underlying predisposing condition is an essential component of care. It includes optimising glycaemic control in diabetic patients and assessing and, where relevant, improving peripheral vascular supply. This bears directly on treatment success and prevention of recurrence.
• Reconstructive surgery may be required, including management of any soft tissue or bony defect after debridement. In severe, extensive or recurrent disease not amenable to limb salvage, consider amputation.
• Individualise the overall approach through multidisciplinary discussion involving orthopaedic surgery, plastic surgery and infectious disease specialists, according to the pattern and extent of disease.
• Long term follow up is important, given the recognised risk of relapse in chronic osteomyelitis. Relapse sometimes presents after a prolonged apparently disease free interval.
• Monitor clinical response, inflammatory markers and, where relevant, serial imaging. Stay vigilant and keep a low threshold for reassessment if symptoms recur.