microbiology of prosthetic joint infections
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MICROBIOLOGY OF PROSTHETIC JOINT INFECTIONS. Dr Robert Nelson. SIR JOHN CHARNLEY FRCS FRS. Pioneer of low friction arthroplasty. Established a Unit at Wrightington in 1961. PROSTHETIC JOINT INFECTION. Early realisation of the risks of infection. Airborne contamination suspected

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sir john charnley frcs frs
SIR JOHN CHARNLEY FRCS FRS
  • Pioneer of low friction arthroplasty.
  • Established a Unit at Wrightington in 1961.
prosthetic joint infection
PROSTHETIC JOINT INFECTION
  • Early realisation of the risks of infection.
  • Airborne contamination suspected
  • Pioneer in ultra-clean ventilation for operating theatres.
joint replacements
JOINT REPLACEMENTS

IN ENGLAND AND WALES:

  • 1995 = 75,000.
  • 2012 = 184,113.
what is being replaced
WHAT IS BEING REPLACED?
  • 98% are hips and knees.
  • Remainder are mostly shoulders.
  • Ankle replacement remains unusual.
infection rates
INFECTION RATES

Over the lifetime of the joint:

  • Hip = 1%.
  • Knee = 2%.
classification of pji
CLASSIFICATION OF PJI
  • Early onset: less than 3 months
  • Delayed onset: 3 months to 1 - 2 years
  • Late onset: >1 - 2 years.
early onset
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.
delayed onset
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.
late onset
LATE ONSET
  • Spread from a distant source of infection.
  • 50% have no apparent source
  • Likely to be acute.
  • Staphylococcus aureus.
  • E. coli.
  • Coliforms.
features of pji
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.
small colony variants
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.
biofilm and pji
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.
biofilm and pji1
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.
the dilemma
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?
definition of pji
DEFINITION OF PJI
  • Presence of a sinus track that communicates with joint.
  • Presence of acute inflammation on histopathology.
  • Presence of pus surrounding the prosthesis.
culture is still required
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.
preoperative precautions
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.
preoperative aspirate
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.
dealing with the result
DEALING WITH THE RESULT
  • Patients rapidly discharged home.
  • Is the result significant?
  • What do we do when we grow virulent organisms?
operative cultures
OPERATIVE CULTURES
  • How many should we take?
  • How should we handle them?
number of samples
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%
taking samples
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.
laboratory processing
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.
cultures
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.
broth cultures
BROTH CULTURES
  • Inspect daily for visible turbidity.
  • Sub culture if turbid.
  • Terminal sub culture at five days.
should we be incubating for longer
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.
what about the prosthesis
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.
bacterial isolates
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.
sensitivity testing
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.
treatment
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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