Ch 47 - Osteomyelitis and Implant-associated Infections

Question 1

Successful treatment of osteomyelitis is heavily influenced by what three factors?

Answer 1

• The viability and stability of the bone
• The virulence and antimicrobial susceptibility of the organism
• The state or condition of the soft tissue envelope

Question 2

What are the most common bacteria isolated in osteomyelitis?

Answer 2

• Staph
• E.Coli
• Strep

In some studies, methicillin resistant strains represent close to 50%

Polymicorbial in up to 42% and anaerobic organisms may be present in up to 64%

Question 3

Describe the pathogenesis of osteomyelitis

Answer 3

• Cytokines and growth factors are found in altered local concentrations during infection
• Contribute to necrosis and resorption of the bone
• Leads to ischaemia due to collapse of vascular channels (Haversian, volksman and canaliculi)
• Segments of bone lacking an adequate blood supply have a propensity to form sequestra and offer a protected environment for bacterial organisms

Question 4

How is the degree of periosteitis correlated with the aggressiveness of the infection?

Answer 4

positively correlated
- Less aggressive infection, slowly seperates the periosteum from the bone, resulting in thickening of the cortex
- More aggressive infection causes lamellar changes where layers of bone are laid down next to each other

Question 5

Dogs with pre-op nasal/rectal swabs which isolated methicillin resistant sS.Pseud were….

Answer 5

13-14 times more likely to develop a SSI cause by the rganism within 30 days

Question 6

How does a biofilm form on a surgical implant?

Answer 6

• Host lays down a layer of proteins and polysaccharides which are adsorbed to the implant and contain fibronectin
• Bacteria adhesion can then occur via the fibronectin and bacterial adhesins

Question 7

What is quorum sensing?

Answer 7

The ability of bacteria to coordinate gene expression based on population density and the role of secreted signal molecules

Question 8

How do biofilm communities have improved survival and appose antimicrobial therapy?

Answer 8

• Extracellular matrix can capture and concentrate nutrients
• The matrix provided protection from shear stresses, host phagocytic activity and from protease and oxygen radical defenses
• Undergo genotypic and phenotypic alteration (quorum sensing)
• Altered or quiescent growth
• Extracellular matrix alters antimicrobial activity
• Changes to microenvironment (hydration, CO2, decreased O2, lower pH)

Question 9

What predisposed metaphyseal bone to haematogenous osteomyelitis?

Answer 9

• Incomplete basement membrane and endothelial gaps
• Relatively inaccessible to the host inflammatory cells
• Sluggish blood flow

Question 10

In which form of posttraumatic osteomyelitis are dogs systemically unwell?

Answer 10

Acute

Question 11

What is the sensitivty and specificity of radiographs in the diagnosis of osteomyelitis?

Answer 11

• Sensitivity 62.5%
• Specificity 57%

Cortical resorption, periosteal proliferation, loss of trabecular markings, lucency around implants, involucrum

Question 12

What is an involucrum?

Answer 12

An area of live, encasing bone surrounding dead bone (sequestrum) within a compromised soft tissue envelope

Question 13

What is a cloaca?

Answer 13

Draining tract from the radiolucent area of necrosis that surrounds the sequestrum and extends to the skin surface

Question 14

What is a rapid molecular technique for the diagnosis of osteomyelitis?

Answer 14

multilocus PCR electrospray ionisation mass spectrometre (PCR/EXI-MS)

• 91% correctly identified to genus level
• 74% correctly identified to species level

Question 15

How can you improve the culture and sensitivty results from surgical implants?

Answer 15

• Sonication of the implants after retrieval and culturing the sonication fluid

Question 16

What animals are predisposed to haematogenous osteomyelitis?
What are the radiographic signs?

Answer 16

• Very young or immunocompromised
• Polyostotic lesions in the metaphyseal regions of long bones, bone resorption, lysis or periosteal new bone, increased medullary opacity

Question 17

What are the primary objectives of posttraumatic osteomyelitis treatment?

Answer 17

• Removal of necrotic bone, sequestra and foreign material
• Meticulous debridement, minimise durther disruption to vascular supply
• Biofilm removal/disruption

Question 18

What are the recommendations regarding systemic and local treatment with ABx?

Answer 18

Systemic
- IV for 3-5 days
- Followed by oral for at least 4-6 weeks

Local
- ABx impregnated beads
- Initial concentration is 500x greater than typical serum concentrations
- heat-stable, hydrophilic and active against methicillin-resistant Staph (aminoglycosides, vancomycin)

Question 19

What is the prognosis with osteomyelitis?

Answer 19

Variable - relapse or reinfection is possible weeks, months ot years later
Fungal infections are particular problematic and expensive to treat

Question 20

List some implant surface coatings which may help to reduce the incidence of SSI

Answer 20

• bioabsorbable polymeric coatings containing antimicrobials
• resorbale calcium-based matrices with antimicrobials added
• Silver-coated implants