MICROBIOLOGY OF PROSTHETIC JOINT INFECTIONS

1. MICROBIOLOGY OF PROSTHETIC JOINT INFECTIONS Dr Robert Nelson

6. SIR JOHN CHARNLEY FRCS FRS • Pioneer of low friction arthroplasty. • Established a Unit at Wrightington in 1961.

9. PROSTHETIC JOINT INFECTION • Early realisation of the risks of infection. • Airborne contamination suspected • Pioneer in ultra-clean ventilation for operating theatres.

13. JOINT REPLACEMENTS IN ENGLAND AND WALES: • 1995 = 75,000. • 2012 = 184,113.

14. WHAT IS BEING REPLACED? • 98% are hips and knees. • Remainder are mostly shoulders. • Ankle replacement remains unusual.

15. INFECTION RATES Over the lifetime of the joint: • Hip = 1%. • Knee = 2%.

16. CLASSIFICATION OF PJI • Early onset: less than 3 months • Delayed onset: 3 months to 1 - 2 years • Late onset: >1 - 2 years.

17. 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.

18. 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.

19. LATE ONSET • Spread from a distant source of infection. • 50% have no apparent source • Likely to be acute. • Staphylococcus aureus. • E. coli. • Coliforms.

20. 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.

22. 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.

25. 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.

26. 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.

27. MICROBIOLOGICAL DIAGNOSIS

29. 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?

30. DEFINITION OF PJI • Presence of a sinus track that communicates with joint. • Presence of acute inflammation on histopathology. • Presence of pus surrounding the prosthesis.

31. 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.

32. 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.

33. 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.

34. DEALING WITH THE RESULT • Patients rapidly discharged home. • Is the result significant? • What do we do when we grow virulent organisms?

35. OPERATIVE CULTURES • How many should we take? • How should we handle them?

36. 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%

37. 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.

38. 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.

39. 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.

40. BROTH CULTURES • Inspect daily for visible turbidity. • Sub culture if turbid. • Terminal sub culture at five days.

41. 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.

42. 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.

43. 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.

44. 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.

45. 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.