MICRO: Wound, Bone and Joint Infections

Question 1

What pathogens most commonly cause SSIs (surgical site infections)?

Answer 1

• Staphylococcus aureus (MSSA and MRSA)
• Escherichia coli
• Pseudomonas aeruginosa

Question 2

What is the pathogenesis of SSIs?

Answer 2

• Surgical site is contaminated
  
  • SSI risk increased if surgical site is contaminated with >10⁵ microorganisms / gram of tissue
  • Lower dose required if foreign material present (i.e. silk suture)
• Host immune response also plays a role in pathogenesis as well as the pathogenicity of the pathogen

Question 3

What are the three levels of SSIs?

Answer 3

• Superficial Incisional- skin and subcutaneous tissues
• Deep Incisional - ascial and muscle layers
• Organ/Space Infection - any part of the anatomy other than the incision

Question 4

• Case 1:
  
  • Admitted with SAH and sub-dural haemorrhage after a fall–> decompressive craniectomy
  • 2/12 later had a cranioplasty with titanium plate
  • 6/12 later admitted with large subdural collection and midline shift à abscess evacuation and infection
• What is the organism?
  
  • Ecoli, enterobacter, MRSA, neisseria meningitides?

Answer 4

MRSA

Gram positive (dark purple, neg would be pale pink) and beta haemolytic

Question 5

How can preventing SSIs be divided?

Answer 5

1. Pre
2. Intra
3. Post - operative

Question 6

What are the pre-operative measures to prevent SSIs?

Answer 6

• Age (an independent risk factor) - increasing risk until 65 years
• Treat all remote infection (e.g. pneumonia, UTI) before operation
• Underlying illness risk factors (e.g. ASA score >3, diabetes, smoking)
• Pre-operative Showering
  
  • Either chlorhexidine or normal detergent/bar soap (both same incidence of SSI)
  • Advised to showed with soap on the day of surgery or the day before
• Hair removal
  
  • Shaving increases risk of SSI (micro-abrasions from shaving can multiply bacteria)
  • Electric clipper should be used instead on the day of surgery with a single-use head
• Nasal Decontamination
  
  • Staphylococcus aureus is carried in the nostrils of 20-30%
    
    • SA carriage = MOST POWERFUL risk factor for SSI following cardiothoracic surgery
  • Nasal decontamination should be offered if they are found to be carrying S. aureus
• Antibiotic Prophylaxis – administer at time of induction of anaesthesia:

To ensure bactericidal concentration in serum & tissue at time of incision

Question 7

Which underlying conditions increase risk of SSIs?

Answer 7

• ASA score >3
• Diabetes (2-3x increased risk à control blood glucose, HbA1c <7)
• Malnutrition
• Low serum albumin
• Radiotherapy and steroids
• Rheumatoid arthritis (stoop DMARDs before operation)
• Obesity (adipose poorly vascularised à poor access for immune system à risk of SSIs)
• Smoking (nicotine delays wound healing and leads to PVD – see above poor vascularisation)

Question 8

How does smoking increase risk of SSI?

Answer 8

Question 9

Is shaving necessary to prevent SSIs?

Answer 9

• Shaving with razor increases risk of SSI (micro-abrasions from shaving can multiply bacteria)
• Electric clipper should be used instead on the day of surgery with a single-use head
• Shaving should only be done if hair will interfere with the surgery

Question 10

What kind of antibiotic prophylaxis can be given to prevent SSIs pre-op?

Answer 10

• Antibiotic Prophylaxis – administer at time of induction of anaesthesia:
  
  • To ensure bactericidal concentration in serum & tissue at time of incision
  • Must have some bactericidal activity

Question 11

What intra-op measures can be taken to prevent SSIs ?

Answer 11

• Limit number of people in theatre (people shed skin cells)
• Ventilation of theatre (positive pressure) –> laminar flow for orthopaedics
• Sterilisation of Surgical Instruments
• Skin Preparation:
  
  • Povidine-iodine
  • Chlorhexidine (in 70% alcohol)
• Asepsis and Surgical Technique
  
  • Remove all dead tissue
  • IV devices should follow aseptic procedures
• Normothermia (if <36C, consider warming):
• Oxygenation
  
  • SpO₂ >95%
  • Higher O₂ saturations à reduced SSIs

Question 12

What temperatures should be used intra-op?

Answer 12

Hypothermia –> increase risk of SSIs by causing vasoconstriction and decreasing oxygen delivery to wound space with impairment of neutrophil function

Measure the patient’s temperature before inducing anaesthesia

Start warming air if <36 degrees C

Question 13

What oxygenation levels should be aimed for pre-op and why?

Answer 13

• SpO₂ >95%
• Higher O₂ saturations –> reduced SSIs

Question 14

Why is it important to reduce theatre traffic intra-op?

Answer 14

One person sheds 1 billion skin cells per day - 10% of these carry bacteria

Microbial load in threatre is related to the number of people present

Theatre personnel should be kept to a minimum

Question 15

How common is septic arthritis?

Answer 15

• Incidence: 2-10 per 100,000
• More common in patients with RhA; 28-38 per 100,000
• Mortality 7-15% and morbidity 50%

Question 16

Risk factors for septic arthritis?

Answer 16

• Rheumatoid arthritis
• Osteoarthritis
• Crystal arthritis
• Joint prosthesis
• IVDU
• Diabetes, chronic renal disease, chronic liver disease
• Immunosuppression (e.g. steroids)
• Trauma – intra-articular injection, penetrating injury

Question 17

What is the pathophysiology of septic arthritis?

Answer 17

• Organisms adhere to synovium
• Bacterial proliferation in synovial fluid –> host inflammatory response –> joint damage
• Joint damage –> exposure of host derived protein (e.g. fibronectin) to which bacteria can adhere

Question 18

Which bacterial factors are important in septic arthirtis?

Answer 18

• Bacterial Factors:
  
  • S. aureus has receptors such as fibronectin-binding protein that recognise selected host proteins
  • S. aureus (some strains) produce cytotoxin PVL (Panton-Valentine Leucocidin) à fulminant infection
  • Kingella kingae synovial adherence is via bacterial pili

Question 19

Which host factors are important in pathophysiology of septic arthritis?

Answer 19

• Leucocyte derived proteases and cytokines can cause cartilage and bone damage
• Raised intra-articular pressure impedes capillary blood flow –> cartilage and bone ischaemia/necrosis
• Genetic deletion of macrophage-derived cytokines –> reduce host-response in S. aureus sepsis
• Absence of IL-10 increases the severity of staphylococcus joint disease

Question 20

What are the most common organisms causing septic arthritis? Which is the single most common?

Answer 20

• Staphylococcus aureus - 46%
• Streptococci - 22%
  
  • Streptococcus pyogenes
  • Streptococcus pneumoniae
  • Streptococcus agalactiae
• Gram-negative organisms:
  
  • Escherichia coli
  • Haemophilus influenzae
  • Neisseria gonorrhoea
  • Salmonella
• Coagulase-negative staphylococci - 4%
• Lyme disease, Brucellosis, Mycobacteria, Fungi - Rare

Question 21

Which investigations should be done for septic arthritis? What is the WCC in the synovium in this condition? Which imaging is useful?

Answer 21

• Blood cultures (before ABx)
• Synovial fluid aspiration MC&S- synovial count >50,000 WBC/mL is used to suggest septic arthritis
  
  • USS confirms and guides aspiration of fluid
• ESR and CRP
• CT – check for erosive bone change, periarticular soft tissue extension
• MRI – joint effusion, articular cartilage destruction, abscess, contiguous osteomyelitis

Question 22

What is the management of septic arthritis?

Answer 22

• ABx, 4-6 weeks (outpatient setting) - mostly IV
• Drainage of the joint

Question 23

What are the most common causes of vertebral osteomyelitis?

Answer 23

• Causes:
  
  • Acute haematogenous spread (bacteraemia)
  • Exogenous (after disc surgery, implant associated)
• Causative organisms:
  
  • Staphylococcus aureus (48.3%)
  • Coagulase-negative staphylococcus
  • Gram-negative rods
  • Streptococcus

Question 24

Where is vertebral osteomeylitis most commonly located?

Answer 24

• Lumbar (43.1%)
• Cervical (10.6%)
• Cervico-thoracic (0.4%)

Question 25

What are the symptoms of VO? What investigations should be done?

Answer 25

• Symptoms:
  
  • Back pain
  • Fever
  • Neurological impairment
• Investigations:
  
  • MRI (90% sensitive)
  • Blood cultures
  • CT-guided/open biopsy

Question 26

What is the management of vertebral osteomyelitis?

Answer 26

• Treatment:
  
  • ABx, 6 weeks

Question 27

• Case 1:
  
  • 76yo man, 4/12 hx of back pain with left leg radiation
  • WL 25kg over last 6/12
  • PMHx of fracture with metal plate insertion, arthritis R-knee, HTN, lived overseas until 1993
  • MRI: discitis of L2/L3–> spinal biopsy –> Coag -ve staph (staph epidermidis) grown –> histology shows vague granuloma
  • Anti-TB treatment started, empirical IV ceftriaxone started
  • Surgery debridement and stabilisation
  • No growth on agar of standard organisms and further PCR and IgG showed brucella

What is the causative organism? Staph aureus, salmonella, TB, brucella?

Answer 27

Brucella - commonly from unpasterised milk, cheese and eating meat.

• Brucella IgG >1:2560
• Brucella DNA using PCR was positive
• Started on rifampicin, ciprofloxacin and doxycycline

Question 28

How does chronic osteomyelitis present?

Answer 28

• Presentation:
  
  • Pain
  • Brodies abscess
  • Sinus tract

MRI of child shown

Question 29

How is chronic osteomyelitis diagnosed? How is it treated birefly?

Answer 29

• Diagnosis
  
  • XR (often first line to screen; early changes take ~10 days)
  • MRI (much more sensitive for changes)
  • Bone biopsy (culture and histology)
• Treatment = radical debridement to living bone and oral ABx (up to 6 weeks after discharge)

Question 30

What is the Papineau technique for chronic osteomyelitis?

Answer 30

• Complete excision of infected tissue and necrotic bone
• Followed by open cancellous bone grafting of the osseous defect
• Split skin grafting is used to close the wound
• Success rate of 89-93%

Question 31

What is the presentation of prosthetic joint infections?

Answer 31

• Pain
• Patient complain joint was ‘never right’ after the operation à early failure
• Sinus tract releasing pus may be present

Question 32

What are the causative organisms of PJI?

Answer 32

• Gram-positive cocci:
  
  • Coagulase-negative staphylococci > S. aureus
  • Streptococci
  • Enterococci
• Aerobic Gram-negative bacilli:
  
  • Enterobacteriaceae
  • Pseudomonas aeruginosa
• Anaerobes
• Polymicrobials
• Culture-negative
• Fungi

Question 33

How are PJI diagnosed?

Answer 33

• Radiology – loosening (bone loss along the cement-bone interface)
• Raised CRP:
  
  • CRP >13.5 for prosthetic knee joint infection
  • CRP >5 for prosthetic hip joint infection
• Joint Aspiration:
  
  • If >1,700 WCC/mL = knee PJI
  • If >4,200 WCC/mL = hip PJI
• Intraoperative Microbiological Sampling:
  
  • Tissue specimens taken from at least 5 sites around the implant
  • Histopathology (if >3 specimens yield identical organisms –> suggestive of PJI)

Question 34

What is the management of PJI?

Answer 34

• Single Stage Revision (i.e. Endo-Klinik):
  
  • Remove all foreign material and dead bone
  • Change gloves and drapes etc.
  • Re-implant new prosthesis with antibiotic impregnated cement and give IV antibiotics
  • ~89% success rate

OR…

• Two Stage Revision:
  
  • 1. Remove prosthesis and put in a spacer (to take up the space of the prosthesis)
       * Take samples for microbiology and histology
       * Period of IV antibiotics (for 6 weeks) then stop antibiotics for 2 weeks
  • 2. Re-debride and sample at second stage
       * Re-implantation with antibiotic impregnated cement
       * NO further antibiotics needed if the samples are clear
       
       • If antibiotics are required, OPAT is used

Question 35

How many tissue samples should be sent from PJI? What is infection defined as on hitology in PJI? What about cultures?

Answer 35

Question 36

Case 3:

• 70yo diabetic, 1994 right THR –> 1998 revision of acetabulum
• X-ray = lysis around distal femoral component

Likely pathogen? CNS, H. influenzae, E coli or pseudomonas?

Answer 36

Coagulase negative staphylococci

IMPORTANT to send multiple specimens as this is commonly a skin contaminant

Question 37

Answer 37

Silvery white cultures = Brucella