Transcript MICROBIOLOGY OF PROSTHETIC JOINT INFECTIONS

MICROBIOLOGY OF
PROSTHETIC JOINT
INFECTIONS
Dr Robert Nelson
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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
• Knee
= 1%.
= 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
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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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