Osteomyelitis

Osteomyelitis is infection of bone and marrow caused by microorganisms reaching bone through the bloodstream, contiguous tissues or direct inoculation.

The infection can produce:

• Marrow inflammation
• Cortical destruction
• Subperiosteal abscess
• Bone necrosis
• Sequestration
• Sinus formation
• Mechanical instability

Staphylococcus aureus is the most important pathogen across many osteomyelitis syndromes, but the likely organisms vary substantially with age, mechanism and host.

Important microbiological associations include:

• S. aureus in most haematogenous disease
• Gram negative bacilli after open trauma and healthcare exposure
• Pseudomonas after puncture through footwear and some water related exposures
• Salmonella in sickle cell disease
• Polymicrobial infection in diabetic foot and severe contiguous disease
• Mycobacterium tuberculosis in chronic granulomatous bone infection

Classification by pathogenesis

The Waldvogel system separates osteomyelitis into:

Haematogenous osteomyelitis

Microorganisms reach bone via bloodstream.

Common in children, vertebrae in adults.

Contiguous focus osteomyelitis without vascular insufficiency

Examples:

• Open fracture
• Orthopaedic surgery
• Penetrating injury

Contiguous focus osteomyelitis with vascular insufficiency

Common in:

• Diabetic foot infection
• Severe peripheral arterial disease

Classification by duration

Acute osteomyelitis

Early infection without established necrotic sequestrum or chronic sinus.

Subacute osteomyelitis

More indolent infection.

A localised metaphyseal abscess is termed a Brodie abscess.

Chronic osteomyelitis

Established necrotic bone, sequestration, sinus formation or prolonged recurrent infection.

The distinction is biological rather than simply based on a fixed number of weeks.

Acute osteomyelitis can present with:

• Local bone pain
• Tenderness
• Fever
• Swelling
• Reduced limb function
• Refusal to bear weight in children

Systemic illness may be prominent in haematogenous disease but relatively mild in chronic contiguous infection.

Look specifically for:

• Recent surgery
• Open fracture
• Penetrating injury
• Orthopaedic implant
• Diabetes
• Peripheral vascular disease
• Immunosuppression
• Intravenous drug use
• Sickle cell disease
• Previous osteomyelitis

A draining sinus over bone strongly suggests chronic osteomyelitis.

Infection adjacent to a joint can spread into the joint and produce simultaneous septic arthritis, particularly around the hip in children.

Obtain:

• Full blood count
• C reactive protein
• ESR
• Renal function before prolonged antimicrobial therapy

Obtain blood cultures before antibiotics when haematogenous infection is suspected and this does not delay emergency treatment.

Plain radiographs: initial imaging. Early films may be normal.

MRI with contrast where appropriate: best imaging investigation for defining marrow involvement, abscess, sinus tracts and soft tissue extension.

MRI can detect osteomyelitis before plain radiographic changes become apparent.

CT is particularly useful for:

• Cortical destruction
• Sequestrum
• Surgical planning
• Chronic osteomyelitis

Microbiological diagnosis

Bone or deep tissue culture is the definitive microbiological investigation.

Superficial wound or sinus swabs frequently reflect colonising organisms and should not determine definitive therapy.

When a stable patient requires operative debridement, obtain deep tissue and bone specimens before starting new antibiotics where clinically safe.

A septic patient should receive antibiotics immediately after appropriate cultures rather than waiting for elective biopsy.

Management depends on:

• Acute versus chronic disease
• Presence of abscess
• Necrotic bone
• Mechanical stability
• Implant involvement
• Vascularity
• Organism

Empirical antibiotics

In an acutely unwell adult with suspected haematogenous osteomyelitis and no major resistant organism risk, a practical anti staphylococcal regimen is:

Cefazolin 2 g IV every 8 hours.

If MRSA risk is significant or severe healthcare associated infection is present:

Vancomycin IV, using weight based loading in severe infection followed by renal and AUC guided dosing.

When Gram negative infection is also plausible, add for example:

Ceftriaxone 2 g IV every 24 hours

or use a broader antipseudomonal agent when epidemiologically appropriate.

Once cultures identify the pathogen, narrow therapy promptly.

MSSA

A practical regimen is:

Cefazolin 2 g IV every 8 hours

or

Flucloxacillin 2 g IV every 6 hours, depending on local availability.

MRSA

Use:

Vancomycin IV with AUC guided monitoring

or an appropriate alternative such as daptomycin when vancomycin is unsuitable.

Surgical indications

Operate when there is:

• Abscess
• Necrotic bone
• Failure of antimicrobial treatment
• Significant implant infection
• Open fracture contamination
• Pathological fracture or instability
• Persistent bacteraemia from bone source
• Associated septic joint requiring drainage
• Compression of neural structures

Acute haematogenous disease

Early disease without abscess or necrotic bone can sometimes be treated medically.

Clinical improvement should include:

• Reduced pain
• Resolution of fever
• Declining CRP
• Return of function

Failure to improve should prompt reassessment for an undrained focus rather than simply changing antibiotics repeatedly.

Duration

Treatment is usually measured in weeks rather than days.

Approximately 4 to 6 weeks of effective antimicrobial therapy is commonly used for uncomplicated bacterial osteomyelitis after adequate source control, although duration should be individualised according to pathogen, location, residual necrotic bone, implants and response.

Highly bioavailable oral therapy can replace prolonged IV therapy once:

• Organism and susceptibility are known
• Source control is adequate
• Gastrointestinal absorption is reliable
• The selected oral drug achieves appropriate bone exposure

The route is less important than achieving a reliably active regimen.