Chapter 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?

What % are polymicrobial? What % anaerobes?

Answer 2

Staph (60%)
E. Coli
Strep

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

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

Question 3

Describe the pathogenesis of osteomyelitis:

Answer 3

Post-trauma or direct inoculation is most common, but hematogenous in young animals happens too.

With infection osteoclastic cytokines are increased -> contribute to necrosis and resorption of the bone matrix -> Leads to ischemia due to collapse of the vascular channels (Haversian, Volkmann and canaliculi)

• Segments of bone lacking an adequate blood supply can form sequestra and offer a protected environment for bacterial organisms

Question 4

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

Answer 4

More aggressive = more periostitis
- Less aggressive infection, slowly separates 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 MRSP were how many times more likely to develop a surgical site infection caused by MRSP within 30 days of TPLO?

Answer 5

13-14 times

Question 6

How does a biofilm form on a surgical implant?

Answer 6

bacteria produce a extracellular matrix of polymeric substances that allow for altered phenotypes
These phenotypes become irreversibly attached to each other and can signal within the film

Question 7

What is quorum sensing?

Answer 7

The ability of bacteria to coordinate gene expression with secreted signal molecules (autoinducers) if there are sufficient numbers.

Question 8

How do biofilm communities have improved survival and oppose antimicrobial therapy (6 ways)?

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 sensitivity 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 (bone)?

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 determining the bacterial cause of osteomyelitis?

Answer 14

Multilocus PCR electrospray ionization mass spectrometer (PCR/EXI-MS)

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

Question 15

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

Answer 15

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

Question 16

What animals are predisposed to hematogenous 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, minimize further disruption to vascular supply
• Biofilm removal/disruption

Question 18

What are the recommendations regarding systemic and local treatment with antibiotics for osteomyelitis?

Answer 18

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

Local
- Antibiotic 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

Question 21

What are the four anatomic types of osteomyelitis?

The Three Physiologic classes of hosts?

Answer 21

I: Medullary
II: Superficial
III:Localized
IV: Diffuse

Class A: Good immune system and delivery
B: Compromized locally or systemically
C: Requires suppressive or no treatment, minimal disability, treatment is WORSE than disease (not a surgical case)