Micro 4 - Wound, bone and joint infections

Question 1

Define superficial incisional and deep incisional infection

Answer 1

• Superficial incisional – skin + SC tissue

* Deep incisional – fascial + muscle layers

Question 2

Gram +ve, Cocci, yellow colony on blood agar which is β haemolytic - which is the causative organism and how do you treat it?

Answer 2

 MRSA (IV linezolid)

Question 3

When should DMARDs be stopped and when should they be continued after the operation?

Answer 3

4 weeks before and 8 weeks post-op.

Question 4

What is the goal of antibiotic prophylaxis given before induction of anaethesia?

Answer 4

Bactericidal concentration of the drug should be established in serum and tissues at time of incision.

Question 5

Ventilation in an operating theatre

Answer 5

o Maintain positive pressure ventilation
o Filter all air
o Maintain around 20 air changes per hour
o Keep operating room doors closed
o Consider laminar flow for orthopaedic implant surgery

Question 6

How does hypothermia increase the risk of infection?

Answer 6

o Mild hypothermia  increased risk of SSIs  vasoconstriction, decreased delivery of oxygen to wound space + subsequent impairment of neutrophil function

Question 7

Causative organisms in septic arthritis

Answer 7

o Staph aureus – most common
o Streptococcus – second most common

o Most common cause in young sexually active (18-30y/o)
– N. gonorrhoea
– Gram-ve diplococci
– IV cefotaxime 4-6/52

o Most common cause in older patients + overall
– Staph. aureus [gram +ve cocci]
– IV flucloxacillin 4-6/52

Question 8

How do host factors contribute to the destruction of the bone in septic arthritis?

Answer 8

o Leucocyte derived proteases + cytokines  cartilage degeneration + bone loss
o  intra-articular pressure  hamper capillary blood flow  cartilage + bone ischaemia + necrosis

Question 9

Pathophysiology of septic arthritis

Answer 9

o Organisms adhere to the synovial membrane  bacterial proliferation in the synovial fluid  generation of host inflammatory response
o Joint damage  exposure of host derived proteins (e.g. fibronectin) to which bacteria adhere

o S. aureus
 Has receptors e.g. fibronectin binding protein that recognise selected host proteins

Question 10

How is vertebral osteomyelitis caused and which is the most common causative organism?

Answer 10

• Acute haematogenous (bacteraemia that settles in the vertebrae)
• Staph aureus

Question 11

Vertebral osteomyelitis ix

Answer 11

o MRI – 90% sensitive – to confirm vertebral osteomyelitis
o Blood cultures – to identify the causative organism
o Biopsy (CT-guided/open) – 2nd line

Question 12

Brucella forms _ on agar plate

Answer 12

silvery colonies

Question 13

Chronic osteomyelitis presentation

Answer 13

Pain
Brodies abscess
sinus tract

Question 14

Where is the Lautenbach technique used? Describe it

Answer 14

used in the treatment of chronic osteomyelitis

= debridement down to healthy bleeding bone + irrigation system used for 3 weeks

 Make sure debridement has occurred all the way down to healthy bleeding bone – check using osteoscopy

 Double lumen suction irrigation system introduced through a SC tunnel

 Abx (chosen based on culture results) instilled through the central lumen
 1L of Hartmanns solution every week – suction fluid sent for culture

 Irrigation continued for 3 weeks after discharge
 Oral abx continued for 6 weeks after discharge

Question 15

Where is the Papineau technique used? Describe it

Answer 15

used in the treatment of chronic osteomyelitis

complete excision of the infected tissue and necrotic bone
open cancellous bone grafting of the osseous defect

Question 16

Causative organisms in prosthetic joint infection

Answer 16

o	Gram +ve cocci 
	Coagulase -ve staphylococci – more common than staph. aureus
	Staph aureus 
	Streptococci sp
	Enterococci sp

o Aerobic gram -ve bacilli
 Enterobacteriaceae
 Pseudomonas aeruginosa

o Anaerobes
o Polymicrobial
o Culture negative
o Fungi

Question 17

Diagnosis of PJI

Imaging
WCC
CRP
Intraoperative microbiological sampling

Knee
Hip

Answer 17

Imaging 
XR/CT/MRI - loosening 
Knee
WCC >1700/ml
CRP >13.5 

Hip
WCC >4200/ml
CRP >5

Intraoperative microbiological sampling
o Tissue specimens from at least 5 sites around the implant
o Histopathology
 Infection = >5neutrophils per high power field
o If >3 specimens yield identical organisms – highly predictive of infection

patient complains that joint was never quite right

Question 18

Treatment of PJI

Endo Klinik

Answer 18

• Endo Klinik single stage revision

o Excision of infected tissue, synovectomy

o Add antibiotics to bone cement according to culture results

o Implantation of new cemented hip/knee prosthesis using abx impregnated cement

o 7-10 days of IV Abx

Question 19

Treatment of PJI

Two stage revision

Answer 19

o Remove prosthesis – spacer put in the joint to take the place of the prosthesis

o Period of abx – 6 weeks
o Stop abx for 2 weeks

o Re-debride sample at second stage
o Re-implantation with abx impregnated cement

o No further abx if samples clear

Question 20

Soft tissue infections - causative agents

• Wound infection
• Open wound, gas gangrene
• Child red oedematous rash on face, spreads quick
• Most common cause of cellulitis

Answer 20

• Wound infection – Staph. aureus, Flucloxacillin
• Open wound, gas gangrene – Clostridium perfringens
• Child red oedematous rash on face, spreads quick – Erysipelas
• Most common cause of cellulitis – Strep. Pyogenes (Group A strep) (+staph aureus)

Question 21

Aetiology of

osteomyelitis
septic arthritis (+rf)
prosthetic joint infection

spread
organism
ix

Answer 21

spread - local or heam for all
IV abx for all

osteomyelitis
s aureus
MRI, blood culture, bone biopsy for culture/histology

septic arthritis
s aureus
(step, e coli)
RF - abnormal joint e.g. RA, prosthetic joint  
joint aspirate MCS
blood culture

prosthetic joint infection
coag neg staph 
(s aureus, e coli)
XR CT MRI loosening 
joint aspirate