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
Compare fracture toughness of metals vs porcelain
Metals= high fracture toughness, good resistance to cracking
Porcelain= low fracture toughness
Define fracture toughness.
Do you want a material with low or high fracture toughness?
Energy required to resist propagation of a crack or flow.
High fracture toughness = good resistance to cracking.
Define compressive strength, what is it used for?
Used to compare brittle materials that are weak in tension - amalgams, cements and resin.
Describe this diagram, specifically the three compressive stress results.
Sc = compressive stress
Ss = shear stress (cone shaped)
ST = tensile stress (centre part horizontally)
What is flexure strength?
Determines the strength of material and amount of distortion expected.
Identify the name for the 4 following descriptions of flexure strength tests:
- Supported. Two loads applied
- Fixed at both ends, weigh is placed in centre
- Fixed at one end, force applied away from that end.
- Supported. Load applied in centre.
- Four point
- Dual Cantilever
- Single Cantilever
- Three point
What type of flexure strength is depicted in this photo?
Two unit/cantilever bridge = Single cantilever
What type of flexure strength is depicted in this photo?
Stress induced in a three unit bridge = Three point loading
Teeth on either end with force applied in the middle where there is a tooth missing
What is the term for this symbol and definition?
What is the equation for this
Coefficient of Thermal Expansion
Fractional increase in length of a material for each degree (C) increase in temperature
Why do amalgam restorations have good clinic performance?
Compressive strength
Explain what is going on in this diagram and the appropriate term.
Fatigue Strength:
Stress at which a material fails under repeated loading. Stress/strain to the number of loading cycles (mastication)
Image: amalgam filling which is brittle. The compressive load on the occlusal puts tensile stress on the base. The base fails and leads to cracks that progress toward occlusal surface- which can induce secondary caries.
What is fatigue vs fatigue strength?
Fatigue= progressive fracture under repeated loading
Fatigue strength= maximum stress at which a material fails under repeated loading
What does the SN Curve tell us?
Stress (S) to the number of loading cycles (N)- magnitude of the load with the number of times the load is applied
What is the endurance limit?
Stress that can be loaded infinite times without failing
The oral cavity is a hostile environment with fluctuations in: (x4)
- pH
- Saliva
- Mechanical Loading
- Temperature
What two things do the success of restorations depend on?
- Physical qualities of restoration
- Qualities of supporting tissues
Define force and its three characteristics.
What unit is it measured in?
One body interacting with another generates force.
Point of application, magnitude and line of action (through the body)
Newtons
How many newtons is occlusal force?
What is the average increase of force in newtons per year as children grow?
200- 3500N
22N/year
How is facial form and muscle definition good predictors of occlusal force?
Higher mandibular angle = lower occlusal force (longer face)
Lower mandibular angle = higher occlusal force (square face)
Will a person with a square, shorter face have a low or high occlusal force?
High occlusal force. Lower the mandibular angle.
What is stress defined as and how is it calculated?
When an external force is applied, an internal force equal in magnitude but the opposite direction is set up.
What is the name used to describe the force on implant against the bone?
Shearing force
Give an example of torsion force
Twisting an Endo file in canal
What are the three types of force?
- Shearing force
- Torsion force
- Bending force
Define the term strain
Change in size of a material that occurs in response to a force
Original length vs change (compression and elongation) of length.
Time dependent recovery. If recoverable goes back to original length.
What units is strain measured in? And how is it calculated?
No units- ratio
Change in length/ Original Length
What does the stress-strain curve express?
What is the proportional limit?
Load to deformation. Stress vs Strain
Proportional limit: highest point on the curve where stress and strain have a linear relationship. Where force is removed, the object returns to original dimensions. = not permanent
What is the elastic region vs elastic limit?
Elastic region: before proportional limit= reversible strain
Elastic limit: maximum stress a material can withstand without permanent deformation - if load is removed original volumes and shapes are regained.
What is the yield strength vs the plastic region?
- Yield Strength: The stress beyond which a material is permanently deformed when force is applied. Just before plastic region, its begining to function in plastic manner - deformed but working.
- Represented by a parallel line to the linear portion of curve.
- Plastic region: past proportional limit- irreversible strain (permanently deformed)
What is the name of the point where maximum stress is reached, and the material is deformed to the point of not-working?
How do you calculate it?
Ultimate strength = Max limit of force a material can withstand before failure.
Maximum load in tension or compression / cross sectional area of material
Identify A,B,C,D on this diagram
A = proportional limit
B = Yield strength
C = ultimate strength
D= fracture strength
What is fracture strength?
The stress when brittle materials fractures
What is meant by elastic modulus (E)?
How is it measured?
- A measure of the rigidity of a material - its stiffness.
- Ratio of stress to strain within the elastic limit.
- Pascals GPa
What is Hooke’s Law?
The law of elasticity
Object returns to its original shape and size upon removal of the load.
The size of the deformation is proportional to the deforming load.
What does a steep slope of E denote compared to a shallow slope?
Steep = high E = rigid material more stiff (i.e. enamel 84GPa)
Shallow= low E= flexible material (i.e. resin 21GPa)
Example: Dentin has a 17 GPa which means it can go back to its original shape better than enamel 84 GPa.
Abfraction: sharp lesions of missing enamel at cervical area of tooth surface
Explain Abfraction in relation to E
Enamel is more rigid (higher E) so will break off with high force -bruxism (clenching) vs dentin that will remain.
What is Poisson’s Effect (v)?
What unit is it measured in?
What is the formula to calculate it?
Deformation is in a direction perpendicular to the specific direction of loading.
Unitless
v= lateral/ longitudinal (within elastic limit)
In regards to Poisson’s effect, what does a negative V tell us compared to a positive V?
negative v = a compressive deformation (longitudinal strain is greater than lateral- gets fatter laterally when stretched)
positive v= a tensile deformation (lateral strain greater than longitudinal strain- becomes narrower laterally when stretched)
What is ductility, give a dental example.
Ability for a material to be drawn out (made longer) material can deform without breaking under tensile stress.
Ortho wires
True or False:
Metals are ductile and ceramics are brittle?
TRUE
What is malleability?
rolled/hammered into thick sheets without fracture- material can deform without breaking under compressive stress.
What is the difference between resilience and toughness?
resilience = resistance of material to permanent deformation- absorb energy within an elastic range
toughness = can withstand impact loading without fracture - absorb energy within plastic and elastic range.
In regards to toughness- brittle objects can/can’t absorb much energy?
Can’t absorb much energy - little plastic deformation before fracture
What does this stand for?
Fracture Toughness= the ability to plastically deform when a fracture is present.
The flaw in brittle materials, and its ability to cause a fracture under applied stress.
