Infection

• Two stage revision with limited fracture fixation

• Two stage revision with limited fracture fixation

• ESR, CRP, hip aspiration for cell count C and culture sensitivity and alpha defensin
• One or two stage revision

The painful total joint needs to be worked up for infection.

Infecting microorganisms should be identified, their antibiotic sensitivities defined, and appropriate specific antibiotics administered as an adjunct to surgical treatment.

Early identification of an infection is the first step towards the prompt intensive treatment necessary for optimal results. Recognition of those factors that predispose to infection should increase the surgeon’s alertness to the possibility of infection. Early signs of infection are not specific and may easily be misinterpreted, as an inflammatory response is normally present in the region of a fracture, even without infection. Often, the first sign of wound infection is that the inflammation fails to resolve as quickly as expected. Certainly, increasing signs of inflammation (wound drainage, redness, swelling, pain and tenderness, fever) must be regarded as indicators of likely infection.
If the surgeon is concerned about infection, all steps to diagnose, or exclude, this possibility must be taken, as a matter of urgency.

The presence of bacteria in an inflammatory wound exudate is proof of infection. Often this is most readily obtained by sterile aspiration of the joint. Microscopic examination (Gram’s stain) of the exudate, and appropriate microbiological cultures may provide evidence of bacterial presence. It has to be borne in mind, however, that there may be few planktonic bacteria in the wound and that most bacteria may be trapped in biofilms.

Prior antibiotic treatment may interfere with microbiological studies. Occasionally, increasing inflammation is evident without recoverable bacteria, but infection must be assumed if the clinical indicators are strong.

Systemic signs of inflammation, often associated with infection, are provided by several laboratory studies (see below). By themselves, none of these proves, or excludes, infection. The diagnosis of such infections is dynamic and based on serial clinical and laboratory observations.

Rapid advances in the use of molecular detection techniques have recently become available to clinical laboratories, using real-time polymerase chain reactions (PCR) and appropriately selected microarrays. This technology can rapidly identify selected bacteria, based on their DNA, as well as any antibiotic resistance. Microbiological culture is not necessary to use these techniques. As examples, it is possible to discover methicillin-resistance of S. aureus within a few hours, as well as to identify fastidious microbes, and those suppressed by antibiotics. Working with microbiologists using PCR, surgeons may well improve adjunctive antibiotic treatment, as well as improve diagnosis and classification of skeletal infections.

Relevant observations include:

• Serial wound examinations under aseptic conditions
• Maximal daily temperature reading
• Full blood count (FBC) with differential white cell count
• Erythrocyte sedimentation rate (ESR)
• C-reactive protein (CRP)
• Bacteriology (gram’s stain, culture and sensitivity)
• Imaging (plain x-rays, PET, CT scan, MRI, Indium111-labelled white blood cell scan)
• Technetium 99m labeled phosphate bone scan

An important guide to treatment is provided by the discipline of classifying an infection according to several parameters:

• duration of infection
• anatomical location
• host resistance to infection
• infecting organism(s)
• status of fixation
• viability of bone and soft tissues

Serial observations of clinical symptoms and signs are most important for identifying the presence of infection. Increasing pain, seepage (either purulent or serosanguinous), swelling, redness, warmth and tenderness all suggest the possibility of an infection.

In the diagnosis of early infections, x-ray imaging plays a negligible role. Radiographic findings of infection are usually not evident until at least 2 weeks after the onset of infection, even though bone involvement has already occurred.

Imaging becomes important in the later stages of infection. It is helpful to examine serial x-rays for progressive changes that could suggest infection.
Radiographic signs are neither sensitive nor specific for infection, e.g., radiographic evidence of implant loosening may be present as a result of instability, infection, or both.

Ultrasound is useful to identify any accumulation of fluid (hidden abscess). The method is non-invasive and may image deeper layers, especially in the thigh. Ultrasound can be helpful for guiding diagnostic needle aspiration.

Computed tomography offers a cross-sectional guide for exploration and excision, particularly of bone fragments(sequestrum).

As with plain radiographs, CT scans offer no specific diagnostic signs for, or against, infection, and is rarely useful in early infections.

MRI offers improved imaging of soft-tissue abnormalities and can show greater anatomical detail than other imaging methods. Again, MRI signs of infection are non-specific. A disadvantage of MRI is the problem of artifact related to ferrous metallic implants. Titanium implants produce less interference.

Uptake of technetium 99m-labelled phosphate compounds (TCN-MDP) by bone is increased in areas of higher vascularity, including infections and bone healing. With infection, a 3-phase bone scan shows increased uptake of labelling in all 3 scan phases. 
Absence of uptake suggests impaired vascularity, or bone necrosis. Bone scanning detects increased bone remodeling that is present around all fractures for 12-24 months. Bone scanning cannot differentiate aseptic hardware loosening from infection. Indium111-labelled white blood cell scans are more specific for inflammation and infection. The illustration shows a larger area of uptake, surrounding hyperemia and inflammation with TCN-MDP (b) than with the indium111 white blood cell technique (a). However, false positives and false negatives still occur.

If an infected area is to be explored, preoperative aspirate of fluid accumulation and especially intraoperative tissue sampling from several potentially infected sites, prior to the start of any antibiotics, provide the best material for detecting the infecting microorganism(s). At least 5 specimens should be taken for microbiology testing. One sample should be sent for histopathology. Both aerobic and anaerobic cultures should be undertaken. PCR, if available will speed up, and make more reliable, the characterization of the organisms. If hardware is removed, this can be cultured as well preferably after sonication. 
Histological investigation can reveal a bacterial etiology, even if the microbiological tests are negative. Superficial wound swabs should be avoided because of low sensitivity and frequent contamination by surface organisms. Ideally, prior to tissue sampling for culture, it is important to discontinue any antibiotic therapy for at least a week.

If the infection is suspected to involve a joint cavity, there is a need for surgical intervention.

Depending on the acuity of infection, surgical intervention can range from single stage revisions to two stage revisions.