your hospitals, your health, our priority MICROBIOLOGY OF PROSTHETIC JOINT INFECTIONS Dr Robert Nelson

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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, = 184,113.

your hospitals, your health, our priority WHAT IS BEING REPLACED? 98% are hips and knees. Remainder are mostly shoulders. Ankle replacement remains unusual.

your hospitals, your health, our priority INFECTION RATES Over the lifetime of the joint: Hip = 1%. Knee = 2%.

your hospitals, your health, our priority CLASSIFICATION OF PJI Early onset: less than 3 months Delayed onset: 3 months to years Late onset: >1 - 2 years.

your hospitals, your health, our priority 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.

your hospitals, your health, our priority 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.

your hospitals, your health, our priority LATE ONSET Spread from a distant source of infection. 50% have no apparent source Likely to be acute. Staphylococcus aureus. E. coli. Coliforms.

your hospitals, your health, our priority 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.

your hospitals, your health, our priority 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.

your hospitals, your health, our priority 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?

your hospitals, your health, our priority 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.

your hospitals, your health, our priority 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.

your hospitals, your health, our priority 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.

your hospitals, your health, our priority 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.

your hospitals, your health, our priority DEALING WITH THE RESULT Patients rapidly discharged home. Is the result significant? What do we do when we grow virulent organisms?

your hospitals, your health, our priority OPERATIVE CULTURES How many should we take? How should we handle them?

your hospitals, your health, our priority NUMBER OF SAMPLES “Osiris” Paper 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%

your hospitals, your health, our priority 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.

your hospitals, your health, our priority 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.

your hospitals, your health, our priority 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.

your hospitals, your health, our priority BROTH CULTURES Inspect daily for visible turbidity. Sub culture if turbid. Terminal sub culture at five days.

your hospitals, your health, our priority 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.

your hospitals, your health, our priority 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.

your hospitals, your health, our priority 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.

your hospitals, your health, our priority 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.

your hospitals, your health, our priority 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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