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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