Microbiology 12 - Wound, bone and joint infections

Question 1

Important pathogens implicated in wound infections

Answer 1

1. Staph auerus (MSSA or MRSA)
2. E Coli
3. Pseudomonas

Question 2

What infectious dose increases the risk of surgical site infections?

Answer 2

>10^5 g/L

*lower if there is foreign material in the wound

Question 3

What is the major pathogen involved in wound, bone and joint infections?

Answer 3

MRSA/ MSSA

Question 4

What are the 3 levels of wound infection?

Answer 4

Superficial incisional- skin and subcutaneous tissue
Deep incisional- fascial and muscle
Organ/ space infection- any part of anatomy other than incision

Question 5

Recall 2 important risk factors for wound/ bone/ joint infection

Answer 5

Diabetes
Obesity (adipose tissue is poorly-vascularised)

Question 6

What is the most important independent risk factor for SSI following cardiothoracic surgery?

Answer 6

Nasal contamination with Staph Aureus

Question 7

Reducing risk of surgical site infections: pre-operative

Answer 7

pre-operative

1. Treating other infections
2. Underlying illnesses
   - ASA score of 3 or more indicates having systemic disease
   - Diabetes- risk of post operative hypoglycaemia
   - Malnutrition
   - Radiotherapy and steroids: need to taper steroids preoperatively
   - Rheumatoid arthritis: stop DMARDS 4 weeks before and 8 weeks postoperatively
   - Obesity: due to poor vascularisation + oxygenation of adipose tissue
3. Smoking cessation: increased vascular resitance = delayed immune response
4. Pre-operative Showering:
5. Avoiding hair removal or using electric clippers:
6. Nasal Decontamination for staph aureus
7. Antibiotic Prophylaxis: bactericidal concentration of drug should be present at time of incision. with additional doses if significant blood loss or delayed procedure

Question 8

What can be done during orthpaedic procedures to reduce risk of infection?

Answer 8

Laminar flow

Question 9

Why do patients need to be kept at normothermia in theatre?

Answer 9

Mild hypothermia increases the risk of surgical site infections because of vasoconstriction - reduced oxygenation/wound healing

Question 10

Reducing risk of surgical site infections: intra-operative

Answer 10

ventilation = positive pressure and 20 air changes/hour

skin preparation with povidone-iodine or chlorhexidine.

• Trust recommendation: Chlorhexidine in 70% alcohol is used

prevent hypothermia: warm IV fluids and warm irrigation fluids and start forced airwarming if temperature <36

Question 11

Which patient group is particularly at risk of septic arthritis?

Answer 11

Those who already have rheumatoid arthritis

Question 12

What organism most commonly causes septic arthritis?

Answer 12

Staph aureus - most common in adults- and also most likely to cause joint destruction

Question 13

Which organism causes septic arthritis in young sexually active people?

Answer 13

N. Gonorrhoea

Spreads haematogneously from an initial infection

Question 14

Other causatvie organisms of septic arthritis

Answer 14

1. Staph aureus (most common)
2. Strep - strep pyogenes, strep pneumoniae, strep aglagatiae
3. gram negatives - N gonorrhoea, E. Coli, Salmonella, Haemophilus influenzae
   * Coagulase negative staphylococci- 4% incidence

Question 15

Example of coagulase negative staph

Answer 15

Staph epidermidis

(Staphylococcus saprophyticus - UTIs not wound)

Question 16

Where in the joint do organisms adhere to in septic arthritis?

Answer 16

Synovial membrane

Question 17

What are the symptoms of septic arthritis?

Answer 17

Red, painful and swollen joint with *restricted movement*

\

*mostly monoarticular, and most affect the knee

*rhematoid arthritis patients- septic arthritis signs may not be as clear cut

Question 18

What investigations for septic arthritis should be done before starting antibiotics?

Answer 18

1. Blood cultures
2. Synovial fluid aspiration and culture (>50,000 WBC/mm^3)

both the above should be done before starting antibiotics

3. Bloods: CRP, ESP, WCC
4. USS to confirm effusion and guide aspiration

*however a negative culture does not negate septic arthritis

Question 19

When is MRI indicated for septic arthritis patients?

Answer 19

When osteomyelitis is suspected

Question 20

How is septic arthritis managed?

Answer 20

4-6 weeks IV antibiotics

joint aspiration: either arthroscopically or clsoed needle aspiration

Surgical washout/drainage may be needed in more complicated/severe cases

Question 21

What are the symptoms of vertebral osteomyelitis?

Answer 21

Back pain and fever

often localised tenderness on percussion of the vertebrae

neurological symptoms

Question 22

Routes of infection for vertebral osteomyelitis + risk factors

Answer 22

Routes of infection: exogenous (eg disc surgery or spinal injection) or haematogenous from bacteraemia

Question 23

What is the most common pathogen implicated in vertebral osteomyelitis?

Answer 23

Staph Auerus

Followed by:

• coagulase negative staph eg staph epidermidis
• gram negative rods
• streptococcus
• rare: brucella (causes less well defined granulomas than TB)

Question 24

How is osteomyelitis diagnosed?

Answer 24

MRI

*to identify causative organism

• blood culture if haematogenous route suspected
• CT guided or open biopsy for culture

Question 25

How is vertebral osteomyelitis treated?

Answer 25

At least 6w of antibiotics

Question 26

Which part of the spine is most commonly affected in vertebral ostemyelitis?

Answer 26

* **LUMBAR SPINE** (43.1%) * Cervical (10.6%)

Question 27

Features of brucella + treatment

Answer 27

**Histology**: vague granuloma (not as well defined as TB) **Culture**: silvery colonies on agar **Treatment**: rifampicin, ciprofloxacin, doxycycline

Question 28

How does prosthetic joint infection present?

Answer 28

Pain Patient complain that the joint was 'never right' after the operation Early failure Sinus tract: * abnormal channel through the soft tissues that allows communication between a joint prosthesis and the outside environment * known or presumed to be colonised by bacteria

Question 29

What is the most common pathogen implicated in prosthetic joint infections?

Answer 29

Coagulase negative staphylococcus eg staph epidermidis Followed by staph aureus, enterococcus

Question 30

What would an x ray show in prosthetic joint infections?

Answer 30

lysis and loosening

Question 31

How many tissue samples are required for the lab when a patient goes to theatre for a suspected joint infection?

Answer 31

5 (If \>3 yield same organism = infection)

Question 32

What are the 2 options for management of prosthetic joint infection?

Answer 32

1. Single stage revision (removal of manky prosthesis and replacement with antibiotic-impregnated material prosthesis) 2. Two stage revision (firstly remove prosthesis and send off samples, put in a spacer, give antibiotics, wait a few weeks, stop antibiotics, rebride and sample, then put new antibiotic impregnanted prosthesis in)

Question 34

How does chronic osteomyelitis present?

Answer 34

Prolonged pain and fever * Pain * Brodie’s abscess * Sinus tract

Question 35

Chronic osteomyelitis: pathophysiology

Answer 35

Primary infection not cleared Formation of: a) sequestrum- necrotic bone that separates off from normal bone- and b) involucrum - new bone formed by osteoblasts that surrounds the sequestrum

Question 36

IVX and MX of chronic osteomyelitis

Answer 36

IVX: bone biopsy + culture is diagnostic * **MRI usually** * **Biopsy and culture for histology to find causative organism** MX: surgical debridement to remove sequestra of affected bone + antibiotics

Question 37

Which pathogen is commonly implicated in wounds in diabetic patients?

Answer 37

Pseudmonas- usually infects opportunistically

Question 38

risk factors for septic arthritis

Answer 38

* Rheumatoid arthritis, osteoarthritis, crystal induced arthritis * **Joint prostheses** * **IV drug abuse** * **Diabetes** * Chronic renal disease, chronic liver disease * **Immunosuppression- steroids** * Trauma- intra-articular injection, penetrating injury

Question 39

_Pathophysiology of septic arthritis_

Answer 39

* Organisms adhere to the synovial membrane * Bacterial proliferation occurs in the synovial fluid with generation of a host inflammatory response * Joint damage Þ exposure of host-derived proteins such as _fibronectin_, which bacteria adhere to

Question 40

bacterial factors causing septic arthritis

Answer 40

* ***S. aureus*** has receptors such as **_fibronectin binding protein_** that recognise selected host proteins * ***Kingella kingae*** have bacterial **_pili_** which adhere to the **synovium**

Question 41

host factors causing septic arthritis

Answer 41

* **Leucocyte derived proteases** and cytokines can lead to **_cartilage degradation**_ and _**bone loss_** **_​_** * **Raised intra-articular pressure** can hamper **_capillary blood flow**_ and lead to _**cartilage and bone ischemia_ and _necrosis_** genetic variation in cytokines = different susceptibility to response

Question 42

**TWO methods used to treat Chronic Osteomyelitis**

Answer 42

_Modified Lautenbach Technique]_ - debrididing and removing necrotic bone. introduce antibiotics to area - every week **1L of Hartmann’s solution is infused through each drain**. Suction fluid is sent for culture + tailor antibiotics to culture - PO antibiotics continued for 6 weeks after discharge _Papineau Technique- MORE RECENT​_ - Complete excision of infected tissue and necrotic bone + bone grafting