7. Dental Pulp & Tissues Microbiology Flashcards
Describe the microbiology of a primary root canal infection.
Majority of the bacteria found in root canal infections are obligate anaerobes and facultative anaerobic micro-organisms.
These bacteria are present as biofilms on dentine walls or in necrotic tissues.
What are the objectives of mechanical instrumentation? What is the main challenge associated with mechanical instrumentation?
Objectives:
• Disturb or detach biofilms that adhere to canal surfaces
• Remove a layer of infected dentine
• Shaping of the main canal helps to create space for needle penetration and irrigant flow
Challenges:
• Mechanical instrumentation usually does not plane the entire root canal surface due to the mismatch between instrument shapes and the complex root canal shapes. This results in residual bacteria in the root canal.
Briefly explain why bacteria in biofilms are harder to kill?
- Slow penetration of antibiotics due to the extracellular martrix of the biofilm
- Resistant bacteria phenotypes due to the transfer of resistance genes, efflux pumps and neutralizing enzymes (such as beta-lactamases)
- Altered micro-environment where the bacteria replicates more slowly, this results in fewer targets for the antibiotics as most antibiotics inhibit the synthesis of cell wall, protein, DNA and cell membranes
What are the desired properties of irrigants? What are the common irrigants used for endodontics?
• Antibacterial, compatible with dentine , tissue-dissolving (for necrotic tissue removal), low surface tension, non-staining, non-cytotoxic, non-corrosive to dental instruments,, cheap, easy to use
Common irrigants: Sodium hypochlorite, antibiotics (MTAD), EDTA
Why is sodium hypochlorite the most commonly used irrigant in endodontics?
It has the following desirable properties:
- Antimicrobial
- Dissolves organic materials
- Minimally irritating to tissues at low concentrations (0.5-1%)
Sodium hypochlorite (NaOCl) is used in concentrations of 0.5-6.0% (0.5% used in clinics, should not exceed 2.5%!).
Sodium hypochlorite reacts with water to form hypochlorus acid, which acts as a strong oxidizing agent that irreversibly oxidises proteins, cells walls and membranes as well as nucleic acid, giving it the antibacterial effect
Note 1: Sodium hypochlorite is quickly inactivated in the presence of dentine debris and organic material, thus it should be replenished throughout the instrumentation phase
Note 2: Sodium hypochlorite has to be agitated and displaced for it to work!
Apart from sodium hypochlorite, name the other 3 irrigants used in endodontics. What are their desirable properties?
Chlorhexidine, Antibiotics (MTAD) and Ethylenediaminetetraacetic acid (EDTA)
Chlorhexidine:
• Antimicrobial, biocompatible and has sustained antimicrobial effects (SUBSTANTIVITY**)
• However, it is NOT tissue dissolving! Residual organic tissue may affect the quality of the permanent root filling seal
MTAD:
• Mixture of doxycycline, citric acid and detergent. Usually used as a final rinse after sodium hypochlorite irrigation
EDTA:
• Acts as a calcium chelator, able to remove the smear layer that forms on the inner root canal walls during instrumentation** (sodium hypochlorite is unable to do this!)
What is the purpose of an inter-appointment dressing?
Name the most common inter-appointment dressing used.
An inter-appointment dressing (AKA intracanal medicament) is used to eliminate micro-organisms that survived mechanical instrumentation and irrigation.
The most common is calcium hydroxide
Why is calcium hydroxide a good intracanal medicament?
- It has an alkaline pH of 12.5. Thus alkalinity elicits antimicrobial effects upon direct contact with the microbes
- It is tissue-dissolving
- It has low solubility in water, which allows it to be applied as a paste to fill the canal space
Note: Calcium hydroxide and sodium hypochlorite have synergistic effects
Why can some bacteria survive in the root canal despite the use of calcium hydroxide?
- Calcium hydroxide kills by direct contact with microbes. However, it is unable to reach the microbes lodges in pulpal remnants, crevices of the canal and the dentinal tubules
- Dentine also has a natural buffering effect which decreases the alkaline pH of calcium hydroxide, allowing pH tolerant organisms to survive
How is calcium hydroxide placed in the canal and how long is it placed for?
- Pure calcium hydroxide powder is mixed with distilled water to produce a paste
- The paste is placed into the canals with the aid of a lentulo spiral or with a hand file after canal instrumentation
- It is placed for at least a week
