Chronic bone infections in adults

by Thomas Hilton, Mohammed Daoub, Len Marais, Nando Ferreira & Luan Nieuwoudt

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Introduction

Infection of the bone or osteitis/osteomyelitis (OM) can be divided into the subtypes - acute, sub-acute and chronic. According to the temporal classification, acute OM has a duration of under two weeks, sub-acute OM one to three months and chronic for more than three months. Clinically, acute osteomyelitis is characterised by the absence of sequestrum, sub-acute osteomyelitis by Brodie’s bone abscess and chronic osteomyelitis by sequestra and involucrum or foreign material (such as orthopaedic implants).

Microbiology

• Chronic osteomyelitis is typically characterised by bacterial biofilms on sequestra (dead bone) or foreign bodies (such as orthopaedic implants).
• A biofilm is a complex aggregation of microorganisms in which cells adhere to each other in a fluid matrix on a solid substrate. The biofilm consists of microorganisms in various states of activity, an extracellular polymeric substance produced by organisms which contain extracellular DNA, proteins, polysaccharides. The structure of the biofilm helps bacteria to evade the host’s immune defence mechanisms and antibiotics.
• Apart from the microorganisms embedded in a biofilm, bacteria also show an adaptive stress response and formation of dormant persister cells that provide a survival advantage during an antimicrobial challenge.
• The bacteria may also hide in the bone microstructure and, in some instances, they (Staphylococcus aureus, for example) may internalise themselves in osteoblasts, thus becoming intracellular organisms.
• These mechanisms enable the causative organisms to persist in an asymptomatic host, and the infection can reactivate and become clinically evident years after the primary infection.
  (See the table in the chapter Bone and Joint Infections - Basics for Specific Organisms.)

Risk factors for poor prognosis

Despite surgical debridement and long-term antibiotics, the recurrence rate of chronic OM in adults can still be as high as 30%. Certain risk factors lead to poor prognosis (see table).

Risk factors poor prognosis
Previous surgery or trauma
Smoking
Corticosteroid use
Diabetes Mellitus
Immunocompromise
IV drug abuse
Poor vascular supply
Peripheral neuropathy
Malnutrition
Chronic renal failure/Dialysis

Classification

Cierny and Mader (1984) classified chronic OM according to the infection’s anatomic distribution and the host’s physiological status. The anatomic areas are:

1. Medullary (bone marrow only)
2. Superficial (cortex)
3. Localised (medulla and cortex, stable)
4. Diffuse (medulla and cortex, unstable)

The host is divided into:
A - Healthy (No comorbidities)
B_L: - Local compromise
B_S - Systemic compromise
C - Poor host status (surgical treatment carries higher morbidity than the disease itself)

Diagnosis

History

The duration and severity of symptoms such as pain, pus drainage or functional impairment that is the inability to walk and so on, must be established. Find out about previous treatment or surgery and comorbidities

Examination

Evaluate vital signs (fever, tachycardia, tachypnoea and hypotension suggest sepsis) and signs of systemic disease.

Local

Erythema, tenderness and oedema are commonly seen. Discharge or a draining sinus is common in chronic OM. The soft tissue condition including scarring or Lipodermatosclerosis, abscess formation or cellulitis must be noted.
The vascular status of the limb must be assessed.
Any skeletal instability, pathological fracture or fracture non-union must be noted. The range of motion, limping or being unable to bear weight because of pain are signs of instability.

Radiographs:

AP and lateral views of the affected limb often show bone resorption with a sclerotic rim around the infected bone, disuse osteopenia, periosteal reaction, lucency (lysis around hardware/ implants) and sequestrum and involucrum formation.

CT scan and MRI:

Valuable in diagnosis and surgical planning by identifying necrotic bone.

Laboratory analysis:

WCC, ESR, CRP may not always be raised in chronic osteomyelitis.

Staging of the host:

• FBC to screen for anaemia
• Tests for renal function, liver function, serum albumin, HIV serology and CD4 count as indicated

Identification of causative organisms:

• Blood culture is often negative but may be used to guide antibiotic therapy in acute haematogenous osteomyelitis.
• Sinus tract culture is not recommended.Culture of bone and soft tissues obtained surgically from the infection site remains the gold standard for guiding antibiotic therapy.

Management

The management of chronic osteomyelitis is complex, and cases should ideally be referred to orthopaedic units specialising in treating chronic bone infections.

Optimisation of the patient:

This should entail the cessation of smoking and alcohol abuse, nutritional support (high protein diet for patients with low albumin level), blood sugar control and anti-retroviral therapy for HIV+ve patients.

In patients with acute flare-ups (cellulitis or abscess formation), systemic antibiotics are required to control the infection. Definitive surgery is typically delayed until the soft tissue condition improves.

Local treatment:

Colostomy bags over the sinus protect the skin from excoriation. Acute abscess formation requires urgent incision and drainage with tissue sampling for MCS, followed by directed intravenous antibiotics.

Surgical

During surgery, a tissue biopsy for culture and microscopy is taken, all necrotic and devitalised tissue is debrided, and implants are removed. Skeletal stability is typically achieved by external fixation. Soft tissue reconstruction is then performed, which may involve plastic surgery.

Following surgery, empiric intravenous antibiotics are given until the culture results become available, after which directed oral antibiotics are given for six weeks. The antibiotic regime should include agents that exhibit appropriate activity against biofilm-based organisms.

In severe cases, surgical reconstruction may not be possible, and amputation may need to be considered

Non-surgical

Chronic suppression with antibiotics may be useful for patients where surgery is not feasible. This requires an opinion from a bone infection unit.