Flashcards in Bone and Joint Infections Deck (46):
What are the complications of an untreated joint infection?
Loss of cartilage -> Osteoarthritis later in life
Severe sepsis -> imminent septic shock
What is the triad of symptoms for septic arthritis?
Hot swollen joint
Intense pain (loss of movement)
Which joints are most commonly affected by septic arthritis?
Knee in 50% of adults
Hip in 20% of children
Polyarticular involvement occurs in 10-20% of patients - normally just one joint is affected
What are the key investigations to diagnose septic arthritis?
Blood cultures - normally a haematological spread
- direct gram stain
- microscopy for crystals and culture
Full blood count - gout raises WCC
Imaging - used to assess the extent of the damage
What are the most common pathogenic causes of septic arthritis?
MSSA or MRSA - mortality of 50%
- Group G Strep
- Pneumococcus - more common in children
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
H.influenzae - common in children
N.gonorrhoeae - common in sexually active young people
How is septic arthritis treated?
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
Why are prosthetic joint infections harder to treat then normal joint infections?
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
What does Arthroplasty mean?
Putting in an artificial joint
What does resection arthroplasty mean?
When a diseased joint is taken out, and a new artificial one is put in
What does revision arthroplasty mean?
Re-operation on an artificial joint, to fix any small problems that don't require a new joint
What does arthrodesis mean?
Fusing two bones together
What does arthrosis mean?
What does pseudo-arthrosis mean?
Allowing two bones to articulate against one another, but without a joint
- e.g. Girdlestone - limits activity levels
What are the normal infection rates for hip and knee replacements?
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
What are the risk factors for infection after a primary arthroplasty?
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
What are the risk factors for infection after a revision arthroplasty?
Prior joint surgery
Prolonged time in the operating room
Pre-op infection (teeth, skin or UTI)
What is different about a prosthetic joint that is infected with coagulase-negative staph?
The joint appears the loosen and there is no pain
The patient will need to be on longer term antibiotics
What are the most common infections of a prosthetic joint?
What are the two ways in which bacteria can infection a new joint?
Haematogenous spread - through the blood
Local spread - infection from doctors/nurses passed into the wound
When is local spread most likely to occur?
Manifests in immediate post-operation period
- acute (less than 4 weeks)
- delayed (greater than 4 weeks)
How does local spread occur and cause a primary joint infection?
Mostly organisms from the skin coming into direct communication with the prosthetic joint while the fascial planes heal
When does haemtogenous spread occur after a primary arthroplasty?
Presents later than infection from local spread - not from skin bacteria touching the joint
How does haemtogenous spread infect the prosthetic joint?
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
Which organisms can cause an infection via haemtogenous spread?
- oral organisms
- pyogenic skin sepsis
- GU or GI instrumentation
How does the virulence of an organisms dictate the presentation?
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
Describe the pathogenesis of how bacteria colonise a prosthetic joint?
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
What is the clinical presentation of an septic prosthetic joint?
Fever and systemic symptoms
Loosening on the X-Ray
Which antibiotic is given as a single pre-op dose to reduce deep wound infection by 1.8%?
Cephalosporin - given 30-60 mins before the first incision
- vancomycin is used if the organism is MRSA
When are patients given higher antibiotic doses pre/post-op?
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
What are the three surgical operation options when someone has a septic prosthetic joint?
1) DAIR - leave infected joint in
2) Take the infected joint out
3) In out and shake it all about
How is a DAIR operation performed?
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
When would a septic prosthetic be taken out after an operation?
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)
Once an infection prosthetic joint has been taken out, what are the options for replacement?
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
How is the pathogen that caused the joint infection identified?
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
Is a gram stain alone a sensitive indicator of which pathogen is infecting a joint?
No - 12% sensitivity
However - it is 98% specificity
Which antibiotics are capable of penetrating bone?
What are the most important things to remember when managing a prosthetic joint infection?
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
What is osteomyelitis?
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
What are the two ways in which bacteria can spread and cause osteomyelitis?
Describe contiguous spread can occur and causes osteomyelitis.
Overlying infection (e.g. Cellulitic ulcer)
Trauma (compound fracture - dirt from the outside enters the bone)
Why is acute osteomyelitis often undertreated?
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
What are the causative organisms of osteomyelitis?
Similar to septic arthritis but includes anaerobes (so treated with metronidazole)
How is osteomyelitis treated?
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
Briefly describe a diabetic foot infection.
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