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MICROBIOLOGY OF PROSTHETIC JOINT

INFECTIONS

Dr Robert Nelson

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your hospitals, your health, our priority

your hospitals, your health, our priority

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SIR JOHN CHARNLEY FRCS FRS

• Pioneer of low friction arthroplasty.

• Established a Unit at Wrightington in 1961.

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PROSTHETIC JOINT INFECTION

• Early realisation of the risks of infection.

• Airborne contamination suspected

• Pioneer in ultra-clean ventilation for operating theatres.

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JOINT REPLACEMENTS

IN ENGLAND AND WALES:

• 1995 = 75,000.

• 2012 = 184,113.

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WHAT IS BEING REPLACED?

• 98% are hips and knees.

• Remainder are mostly shoulders.

• Ankle replacement remains unusual.

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INFECTION RATES

Over the lifetime of the joint:

• Hip = 1%.

• Knee = 2%.

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CLASSIFICATION OF PJI

• Early onset: less than 3 months

• Delayed onset: 3 months to 1 - 2 years

• Late onset: >1 - 2 years.

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EARLY ONSET

• Organisms gain entry at the time of operation.

• Generally a virulent infection.

• Wound drainage, erythema, oedema, pain.

• Staphylococcus aureus / MRSA.

• Coliforms.

• Mixed infections.

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DELAYED ONSET

• Also gain entry around the time of operation.

• Take much longer to manifest.• Symptoms are less severe.• Pain in the joint.• Sinus formation may occur.• Coagulase-negative Staphylococcus spp.• Propionibacterium spp.

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LATE ONSET

• Spread from a distant source of infection.

• 50% have no apparent source

• Likely to be acute.

• Staphylococcus aureus.

• E. coli.

• Coliforms.

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FEATURES OF PJI

• Bulk of infections are caused by Staphylococcal species (approximately 50%).

• Propionibacterium may be more common in shoulder joint infections.

• Staphylococcus aureus has a higher incidence in patients with rheumatoid arthritis.

• Small colony variants may be an issue.

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SMALL COLONY VARIANTS

• Formed by S.aureus.

• Non-pigmented and non-haemolytic colonies one-tenth of normal size on culture.

• Auxotrophs for haemin or menadione.

• May persist intracellularly.

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BIOFILM AND PJI

• Presence of a foreign body significantly reduces inoculum required to establish infection.

• Bacteria elaborate an exopolysaccharide which encases them and adheres to the prosthesis. This is a biofilm.

• Organisms embedded in the biofilm are metabolically inert and more resistant to antibiotics.

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BIOFILM AND PJI

• Delayed onset of symptoms following surgery.

• Difficulty in demonstrating organisms in aspirates of delayed onset infection.

• Antibiotic treatment may initially result in response and then relapse.

• Long term suppression may be successful.

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MICROBIOLOGICAL DIAGNOSIS

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THE DILEMMA

Skin flora is the predominant cause of PJI.

• Is the culture clinically significant?

• Did it come instead from the patient’s skin?

• Did it arise from Theatre staff?

• Did the Laboratory contaminate it?

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DEFINITION OF PJI

1. Presence of a sinus track that communicates with joint.

2. Presence of acute inflammation on histopathology.

3. Presence of pus surrounding the prosthesis.

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CULTURE IS STILL REQUIRED

• Scans are unhelpful.

• Molecular methods have not been helpful to date.

• ID and sensitivity results from cultures greatly assist in patient management.

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PREOPERATIVE PRECAUTIONS

• Stop all concurrent antibiotic therapy for at least two weeks prior to aspirate or surgery.

• Obtain all prior culture results from your own and other hospitals.

• Consider a preoperative joint aspirate.

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PREOPERATIVE ASPIRATE

• Should be done under strict aseptic conditions.

• Usually arrives in blood culture bottles.

• Gram and cell count may be helpful.

• Essential that any isolate has full identification and sensitivity testing.

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DEALING WITH THE RESULT

• Patients rapidly discharged home.

• Is the result significant?

• What do we do when we grow virulent organisms?

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OPERATIVE CULTURES

• How many should we take?

• How should we handle them?

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NUMBER OF SAMPLES

• “Osiris” Paper 1995.• Send at least 5-6 samples.• Single positive sample is unlikely to be

significant.• Isolation of indistinguishable microorganisms

from three or more independent specimens is highly predictive of infection.

• Sensitivity 65% specificity 99.6%.• Gram staining sensitivity 12% specificity 98%

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TAKING SAMPLES

• Separate scalpel / container for each specimen.

• Take prior to prophylactic antibiotics

• Aim for abnormal areas, particularly membranes between bone cement interfaces.

• Transport promptly to the Laboratory.

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LABORATORY PROCESSING

• Vortexing with Ballotini sterile glass beads is simple with a low risk of contamination.

• Beads are superior to shaking in broth alone.

• Use homogenate to inoculate cultures.

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CULTURES

• Broth culture is essential given the low numbers of organisms present in samples.

• RCM, FAA or equivalent are suitable.

• Direct culture on plates is optional.

• SCV’s require chocolate agar to grow.

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BROTH CULTURES

• Inspect daily for visible turbidity.

• Sub culture if turbid.

• Terminal sub culture at five days.

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SHOULD WE BE INCUBATING FOR LONGER?

• Evidence suggests a 7 day culture only isolates 73% of pathogens.

• Extending incubation to 14 days increases yield.

• Predominantly Propionibacterium spp, Peptostreptococcus and diphtheroids.

• Increases isolation of contaminants.

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WHAT ABOUT THE PROSTHESIS?• Prosthesis will have many organisms

adherent in biofilm.

• Large and heavy piece of metal.

• Difficult to transport and process aseptically.

• Leakage a significant problem.

• Enlarged specimen containers may be the answer.

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BACTERIAL ISOLATES

• Regard every isolate as potentially significant.

• Identify every isolate.

• Full sensitivity panel.

• MIC for relevant glycopeptides.

• Preserve isolates until all culture work is complete.

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SENSITIVITY TESTING

• Guides initial choice of agents.

• IV and oral options are required.

• Alternatives for intolerant patients.

• Valuable information for determining significance.

• Monitoring of resistance trends.

• Information for future cement choices.

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TREATMENT

• Stop antibiotics if infection is excluded.

• Narrow coverage based on sensitivities.

• Provide treatment plan for IV followed by oral course.

• Antibiotic cement in future procedures.

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