Restorative Dental Materials- Chemical & Physical Properties W5 Flashcards
Patient has tobacco stains lingually and tea stains labially.
What cleaning and polishing agents do you use?
Pumice
Good for natural teeth
Won’t damage tooth structure
What are three abrasive dentifrices?
Phosphates, Carbonates and Silicas
What is the difference between polishing and cleaning?
Cleaning agents do not contain abrasive particles, polishing agents do.
What are some symptoms a patient may complain of with a non-smooth tooth surface?
- irritation to to tongue
- gingival inflammation and bleeding
- not aesthetically pleasing
- unhygienic
- plaque trap
What is the difference between finishing and polishing?
Finishing is done before polishing.
Finishing is contouring (by removing materials by abrasion) to remove excess material to create a smooth surface. Must fit occlusion and contour.
Polishing, is done after finishing and minimal removal of material, with no further change to contour. Results in no scratches, smooth and highly reflective.
What is abrasion and an abrasive?
Abrasion: Wearing away of a surface by rubbing, cutting or scrapping.
Abrasive: used to abrade a surface.
Which abrasives can be bonded to paper strips or discs? (Rank in most used order)
Aluminium Oxide (Al2O3)
Emery
Cuttle (SiO2)
What abrasive contains diamond particles and is used for restorations (acrylics and composites)?
Aluminium oxide
What abrasive material is used to polish metallic restorations specifically?
Tin Oxide (Sn02)
What two types of abrasives are used to polish acrylic appliances and amalgam?
Garnet and Pumice
What is pumice?
A glass abrasive with SiO2, used to polish acrylic appliances, amalgam and enamel.
What 8 factors affect finishing and how?
- Hardness = abrasive particles must be harder than the surface being abraded
- Shape = cutting efficiency is enhanced by tool/instrument having sharp edges
- Size = abrasive particles proportional to surface
- Grit = size of abrasive particles
- Gradual reduction in size = larger replaced by shallower scratches
- Pressure = greater force means more rapid removal
- Speed = fast speed, more efficiency cutting
- Lubrication = water - cools and removes debris
(Her Gritty Skin Gradually Softens She Looks Princess)
What is mohs scale?
Ranks materials in terms of being scratched - hardness measurement
How does size and grit affecting finishing? What is the unit grit is measured in?
Grit is the size of the particles (measured in micrometres)
and size is the particles in relation to the surface: the larger the particles the more rapidly the surface abrades.
Why is pressure a factor that can affect finishing?
The greater the force- the more removal of material- the higher the temperature - the more chance of distortion/physical changes- can result in over contouring.
How does speed influence finishing?
Influences cutting efficiency:
Faster speed = fasting cutting = higher speed = higher temp = more cutting
Why do we finish and polish/outcome
- Used for restorations, intraoral appliances and tooth surfaces.
- Removal of excess material and roughened surfaces
- Improves aesthetics
- Improves tissue health
- Minimises trauma to hard and soft tissues
- Increased longevity of restorative materials
What are the two most common abrasive tools/instruments types?
- Diamonds = made of C, hardest substance, don’t lose sharpness, used with water to prevent build up of heat.
- Carbides= Si or B- pressed into discs.
Tungsten carbide (W)- used for burs (high speed) to cut tooth structure
What are the 4 reasons why we polish?
- Decrease adhesion- prevent plaque adhesion
- Feel of a smooth surface
- Increase aesthetics
- Corrosion resistance- reduce metal restorations
Define hardness and why is it important in dentistry (x3)?
Abrasive must be harder than tooth-
Resistance to permanent surface indentation (plastic deformation).
Influences ease of cutting, finishing and polishing, resistance to scratching while in use.
- What is the hardness test, what does it measure?
- What do various tests depend on to measure hardness?
- What are the three traits used to distinguish which test to choose?
- Determines the resistance to penetration. Where a fixed load is applied with a standard symmetrical indenter. Dimensions of the indentation are measured.
- Depends on small object penetrating into surface of material.
- Test differs by: type of material, range of hardness and degree of localisation.
What are the three attributes of higher hardness values?
Difficult to scratch, difficult to polish, more resistant to wear.
Name the macro hardness tests
- Brinell Hardness Test
- Rockwell Hardness Test
What is the result of a Coefficient Thermal Expansion value higher than another specimen’s it comes in contact with?
- Leakage of oral fluids between restoration and tooth
- Irritation of dental pulp- cause decay /patient sensitive to temp changes
What happens when there is different Coefficient Thermal Expansion values
I.e. amalgam vs enamel?
What is an ideal CTE?
Amalgam CTE is higher (22ppm) than enamel (11.4ppm) therefore amalgam contracts faster than enamel and creates gaps between the tooth surface- where oral fluids can leak between restoration and tooth surface.
You want CTE as close to tooth CTE as possible.
Define creep
Give example of dental material where this applies
Time dependent deformation of an object subjected to constant load
= Small changes in shape under continuous compression over time.
Amalgams and composite undergo creep - ditching and marginal breakdown overtime
What materials do not undergo creep?
Most metals and ceramics
What is the difference between toughness and fracture toughness?
Toughness = material to absorb energy so a fracture is delayed
Fracture toughness= material with cracks already, to resist fracture by absorbing energy