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Microbiology > Bone and Joint Infections > Flashcards

Bone and Joint Infections Flashcards

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1
Q

What are the complications of an untreated joint infection?

A

Loss of cartilage -> Osteoarthritis later in life

Severe sepsis -> imminent septic shock

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2
Q

What is the triad of symptoms for septic arthritis?

A 
Hot swollen joint
Fever (60-80%)
Intense pain (loss of movement)
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3
Q

Which joints are most commonly affected by septic arthritis?

A

Knee in 50% of adults
Hip in 20% of children
Polyarticular involvement occurs in 10-20% of patients - normally just one joint is affected

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4
Q

What are the key investigations to diagnose septic arthritis?

A

Blood cultures - normally a haematological spread
Joint aspirate
- direct gram stain
- microscopy for crystals and culture
Full blood count - gout raises WCC
CRP
Imaging - used to assess the extent of the damage

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5
Q

What are the most common pathogenic causes of septic arthritis?

A 
MSSA or MRSA - mortality of 50%
Streptococci
- S.Pyogenes
- Group G Strep
- Pneumococcus - more common in children
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6
Q

Name some pathogens that can cause septic arthritis, but are not the most common reason.
- they are gram negative, which is generally less likely to cause septic arthritis

A 
H.influenzae - common in children 
Kingella 
N.meningitidis 
N.gonorrhoeae - common in sexually active young people 
E.coli
P.aeruginosa
Salmonella species
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7
Q

How is septic arthritis treated?

A

At least two weeks on IV antibiotics - the infection is deep seated because there is poor blood supply to joints, so a longer course of treatment is needed
- 3 weeks of IV is often needed
- followed by 3 weeks on oral
- Flucloxacillin and Gentamicin
Treatment is monitored by CRP levels and clinical improvement
- CRP decrease is very reliable

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8
Q

Why are prosthetic joint infections harder to treat then normal joint infections?

A

There is abnormal anatomy and blood supply to the area
Foreign bodies are present
Bacteria find it easier to colonise and stick to the metal joint

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9
Q

What does Arthroplasty mean?

A

Putting in an artificial joint

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10
Q

What does resection arthroplasty mean?

A

When a diseased joint is taken out, and a new artificial one is put in

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11
Q

What does revision arthroplasty mean?

A

Re-operation on an artificial joint, to fix any small problems that don’t require a new joint

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12
Q

What does arthrodesis mean?

A

Fusing two bones together

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13
Q

What does arthrosis mean?

A

A joint

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14
Q

What does pseudo-arthrosis mean?

A

Allowing two bones to articulate against one another, but without a joint
- e.g. Girdlestone - limits activity levels

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15
Q

What are the normal infection rates for hip and knee replacements?

A

Hip - 0.3-2% of operations
Knee - <4% of operations
- this has a higher infection rate because the joint is more complicated and move in complicated ways
Rates higher than this are cause for investigation

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16
Q

What are the risk factors for infection after a primary arthroplasty?

A

Rheumatoid arthritis - joints are diseased anyway
Diabetes - increased risk of infection generally
Poor nutritional status - compromised immune system
Obesity - need large antibiotic doses
Concurrent UTI - bacteria in the body
- patient isn’t operated on unless it is an emergency
Steroids - immunosuppressant
Malignancy

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17
Q

What are the risk factors for infection after a revision arthroplasty?

A

Prior joint surgery
Prolonged time in the operating room
Pre-op infection (teeth, skin or UTI)

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18
Q

What is different about a prosthetic joint that is infected with coagulase-negative staph?

A

The joint appears the loosen and there is no pain

The patient will need to be on longer term antibiotics

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19
Q

What are the most common infections of a prosthetic joint?

A 
S.aureus 
Coagulase-negative Staph 
Polymicrobial 
Streptococci 
Gram-negative bacilli
20
Q

What are the two ways in which bacteria can infection a new joint?

A

Haematogenous spread - through the blood

Local spread - infection from doctors/nurses passed into the wound

21
Q

When is local spread most likely to occur?

A

Manifests in immediate post-operation period

  • acute (less than 4 weeks)
  • delayed (greater than 4 weeks)
22
Q

How does local spread occur and cause a primary joint infection?

A

Mostly organisms from the skin coming into direct communication with the prosthetic joint while the fascial planes heal

23
Q

When does haemtogenous spread occur after a primary arthroplasty?

A

Presents later than infection from local spread - not from skin bacteria touching the joint

24
Q

How does haemtogenous spread infect the prosthetic joint?

A

Prosthetic joints are more easily colonised
The intact surrounding connective tissue limits the infection to the bone/cement interface (local)
- this means the joint isn’t often large, red and painful

25
Q

Which organisms can cause an infection via haemtogenous spread?

A

Any organism

  • oral organisms
  • pyogenic skin sepsis
  • GU or GI instrumentation
26
Q

How does the virulence of an organisms dictate the presentation?

A

Low virulence organisms (e.g. Coagulase negative staph)
- result in low grade infections which are tenacious
High virulence organisms (e.g. MRSA, MSSA, group A or G beta haemolytic strep)
- result in fulminant infection or septic shock

27
Q

Describe the pathogenesis of how bacteria colonise a prosthetic joint?

A

Fewer bacteria are required to establish sepsis than in soft tissue (no immune system - can’t prevent biofilm formation)
The avascular surface protects bacteria from circulating immunological defences and antibiotics
Cement can inhibit phagocytosis and lymphocyte/complement function

28
Q

What is the clinical presentation of an septic prosthetic joint?

A 
Pain
Effusion
Warm joint
Fever and systemic symptoms 
Mechanical dysfunction
Discharging sinus
Loosening on the X-Ray
29
Q

Which antibiotic is given as a single pre-op dose to reduce deep wound infection by 1.8%?

A

Cephalosporin - given 30-60 mins before the first incision

- vancomycin is used if the organism is MRSA

30
Q

When are patients given higher antibiotic doses pre/post-op?

A

When they weigh more - more blood circulating means more antibiotic is needed for the same perfusion
When they lost a lot of blood during the operation - also lost a lot of the antibiotic they were given

31
Q

What are the three surgical operation options when someone has a septic prosthetic joint?

A

1) DAIR - leave infected joint in
2) Take the infected joint out
3) In out and shake it all about

32
Q

How is a DAIR operation performed?

A

DAIR - debride, antibiotics, implant retained
If the infection is acute (Less than 30 days since insertion), then the joint is still mechanically functional and should be kept in
- infected tissues are debrided and joint washed out to reduce burden of infection
- IV antibiotics started for 4-6 weeks
Not done is the infection is S.Aureus

33
Q

When would a septic prosthetic be taken out after an operation?

A

If the infection occurs over 30 days since the surgery then it may no longer be fully functional and may need to be removed
- more common
Prosthesis and cement are all taken out (can’t heal if the foreign body is retained)

34
Q

Once an infection prosthetic joint has been taken out, what are the options for replacement?

A

Girdlestone procedure
One stage revision - put a new one in during the same operation as removing the infection one
Two stage revision - delay putting in a new one until treated the existing infection for 4-6 weeks

35
Q

How is the pathogen that caused the joint infection identified?

A

Depends on macroscopic appearance, histopathology and microbiology
- 5 samples at the time of surgery
- if the pathogen appear in more than once sample then it’s probably a contaminant
Watch out for skin contaminants

36
Q

Is a gram stain alone a sensitive indicator of which pathogen is infecting a joint?

A

No - 12% sensitivity

However - it is 98% specificity

37
Q

Which antibiotics are capable of penetrating bone?

A 
Cephalosporins 
Tazocin 
Carbapenems 
Fusidic acid 
Doxycycline 
Rifampicin
Linezolid 
Trimethoprim
Ciprofloxacin
Clindamycin
38
Q

What are the most important things to remember when managing a prosthetic joint infection?

A

No pre-op antibiotics (unless the infection if life threatening)
Send intra-op specimens to microbiology from at least five different sites
Send debrided tissue to a histopathologist
Start antibiotics rationally based on antibiograms and using bone-penetrating drugs
Measure CRP regularly

39
Q

What is osteomyelitis?

A

Progressive infection of the bone, characterised by death of the bone and the formation of sequestra (a piece of dead bone tissue)
Infection causes destruction of the bone
Painful and disfiguring

40
Q

What are the two ways in which bacteria can spread and cause osteomyelitis?

A

Haemtogenous spread

Contiguous spread

41
Q

Describe contiguous spread can occur and causes osteomyelitis.

A

Overlying infection (e.g. Cellulitic ulcer)
Trauma (compound fracture - dirt from the outside enters the bone)
Surgical inoculation

42
Q

Why is acute osteomyelitis often undertreated?

A

Children more commonly present with this than adults, and the GP thinks the child has a simple infection and prescribed oral antibiotics
- often allows recurrence

43
Q

What are the causative organisms of osteomyelitis?

A

Similar to septic arthritis but includes anaerobes (so treated with metronidazole)

44
Q

How is osteomyelitis treated?

A

Surgery to debulk the infection back to healthy bone and managed dead spaces that remain (with muscle flaps)
Stabilise infected fractures (external fixation), debride sinuses and close wounds
Antibiotic choice is determined by by what grows from debrided bone
- may require long term treatment , 4-6 weeks IV

45
Q

Briefly describe a diabetic foot infection.

A

More complex than septic arthritis - ulcers are normally present - can contain lots of organisms
Usually involves bone (osteomyelitis) but can also involve joints
- penetration deep into the bone
Need to optimised diabetes control as well as treat the infection

46
Q

Briefly describe vertebral discitis.

A

Infection of a disk space and adjacent vertebral end plates
- common in IVDU with S.Auerus
Can be very destructive with deformity, spinal instability, risking cord compression, paraplegia and disability
- presents with cord compression syndrome
Similar organisms to septic arthritis - increased risk of TB

Microbiology (21 decks)

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