Micro: Wound, Bone and Joint Infections

Question 1

Aetiology of surgical site infection

Answer 1

Wound contamination - dependent on bacterial flora on site of infection. Severity of disease witll depend on pathogenicity of microorganism and host immune response.

Question 2

What threshold of contamination of a surgical site is associated with increased risk of surgical site infections?

Answer 2

More than 10^5 organisms per gram of tissue

Lower dose of microorganisms required if foreign material present (e.g. sutures, prosthesis)

Question 3

Name three major pathogens that cause surgical site infections.

Answer 3

• Staphylococcus aureus
• Escherichia coli
• Pseudomonas aeruginosa

Question 4

What are the three levels of surgical site infections?

Answer 4

• Superficial incisional - skin and subcutaneous tissues
• Deep incisional - fascial and muscle layers
• Organ/space infection - any part of the anatomy that is not the incision

Question 5

Patient Risk factors for surgical site infections

Answer 5

• Older age
• Diabetes
• Smoking
• Obesity
• Steroids

impact wound healing

Question 6

Signs and Symptoms of Surgical site infections

Answer 6

Pain, swelling in incision area
Failure of wound healing

Question 7

Management of surgical site infection

Answer 7

Abx: Fluclox for Staph / Linezolid if MRSA

Question 8

Prevention measures for surgical site infections

Answer 8

Pre-op (showering of pts, nasal decontamination if Staph A carrier, prophylactic abx given at induction of anaesthesia - bacterialcidal levels achieved by incision)

Intra-op (limiting ppl in theatre, ventilation, normothermia)

Question 9

Aetiology for septic arthritis

Answer 9

Local or haematological (bacteriaemia) spread to joint

Organism adheres to synovial membrane resulting in host inflammatory response. Increased intraarticular pressure results in further bone damage which exposes host-derived proteins (e.g. fibronectin) which further bacteria (e.g. Staph A) can bind to

Question 10

Risk factors for septic arthritis

Answer 10

• RA
• IVDU
• Joint prosthesis
• DM
• Immunosuppression
• Trauma

Question 11

List common organisms that can cause septic arthritis

Answer 11

• Staph. Aureus (commonest)
• Strep
• E.coli
• N. gonorrhoea (if young)

Question 12

List some bacterial factors that enable bacteria to cause septic arthritis.

Answer 12

• Staphylococcus aureus has receptors such as fibronectin-binding protein
• Kingella kingae have bacterial pili which adhere to the synovium

Question 13

Clinical features of Septic Arthrits

Answer 13

• ACUTE history of red, swollen, painful joint (monoarticular)
• Fever
• Restricted movement

Commonest joint = knee!

Question 14

Investigations for septic arthrits

Answer 14

• Blood culture (if febrile)
• Joint aspirate + MC&S (WCC synovial count >50k = septic arthritis likely)
• MRI (for joint effusion, articular cartilage destruction, bone abscess, osteomyelitis)

Question 15

Management of septic arthritis

Answer 15

IV Abx - Cephalosporin or Fluclox (good response at 2w = switch to oral)
+
Arthroscopic washout

Question 16

Aetiology of osteomyelitis

Answer 16

• Acute haematogenous spread (bacteriaemia)
• Direct (e.g. septic arthritis)
• Exogenous (from surgery - implant or prosthesis)

Vertebral is most common (lumbar > thoracic-cervical > cervical)

Question 17

Commonest organisms causing vertebral osteomyelitis

Answer 17

• Staphylococcus aureus
• Streptococcus
• Coagulase-negative staphylococcus
• Gram-negative rods

Question 18

What are the symptoms of vertebral osteomyelitis?

Answer 18

• Back pain
• Swelling
• Fever
• Neurological impairment

Question 19

Investigations for osteomyelitis

Answer 19

• MRI
• Blood cultures (if septic)
• CT-guided/open biopsy (if blood culture comes back -ve)

Question 20

Treatment for osteomyelitis

Answer 20

Abx for at least 6 weeks

debridement second line or if chronic osteomyelitis

Question 21

Common organisms causing prosthetic joint infection

Answer 21

• Coagulase-negative staphylococcus
• Others: streptococci, enterococci, enterobacteriaciae, Pseudomonas aeruginosa, anaerobes

Question 22

Clinical Features of prosthetic joint infection

Answer 22

• Red, hot, swollen joint
• Joint failure - “joint never right since operation”
• Sinus tract

Question 23

Diagnosis of prosthetic joint infection

Answer 23

• Radiology (XR/CT/MRI) - shows loosening of the prosthesis
• CRP > 13.5 for prosthetic knees
• CRP > 5 for prosthetic hips
• Joint aspiration WCC (>1700/mL if knee; >4200/mL if hip)

BE CAUTIOUS OF JOINT ASPIRATION AS IF NOT INFECTED BEFORE IT CAN BECOME INFECTED AFTER ASPIRATION ATTEMPT

Question 24

How should specimens be taken intraoperatively?

Answer 24

• Specimens should be taken from at least 5 sites around the implant and sent for histology
• NOTE: if 3 or more specimens yield identical organisms, this is suggestive of prosthetic joint infection

Question 25

Treatment options for prosthetic joint infection

Answer 25

Single stage revision

• Remove all foreign material and dead bone
• Re-implant new prosthesis with antibody-impregnated cement and give IV antibiotics

Two stage revision

• Remove prosthesis and put in a spacer
• Take samples for microbiology and histology
• Period of IV antibiotics for 6 weeks then stop for 2 weeks
• Re-debride and sample at second stage
• Re-implantation with antibody impregnated cement
• If antibiotics are needed, OPAT is used