Dental Material Science Flashcards
What is the composition of Nickel Chromium?
70-80% Nickel
10-25% Chromium
What are the features of nickel in nickel chromium?
High melting point
High casting shrinkage
What are the features of chromium in nickel chromium?
High Young’s modulus
Lowish bond strength
What are six features of cobalt chromium?
High melting point
2-3% casting shrinkage
High tensile strength (850MPa)
Lowish bond strength (50MPa)
High Young’s modulus (220 GPa)
High hardness (360-430VHN)
What is the tensile strength of cobalt chromium?
850MPa
What is the bond strength of cobalt chromium?
50MPa
What is the Young’s modulus of cobalt chromium?
220GPa
What is the hardness of cobalt chromium?
360-430VHN
What is the mechanical bonding mechanism for metal ceramic crowns?
Stressed skin effect
What is the stressed skin effect due to?
Slight difference in thermal contraction compared co-efficient that results in compressive forces that aid bonding
What is the chemical bonding mechanism in metal ceramic crowns?
Electron sharing in oxides
What is the ideal thermal expansion coefficient of the metal in a metal ceramic crown?
0.5ppm/C higher than porcelain (14ppm/C)
What is a wrought alloy?
An alloy which can be manipulated or shaped by cold working
What is the composition of steel?
> 98% Iron
<2% Carbon
0.5-1% Chromium
What % of chromium does cast iron have?
2%
What does allotropic mean?
Undergoes 2 phase changes with temperature
What phase is Iron in when temp >1400C
Body Centered Cubic (BCC)
Low carbon solubility
0.05
What phase is iron when temp 900-1400C?
Face centered cubic
Higher carbon solubility
0.2
What phase is iron when temp <900C?
Body centered cubic
Low carbon solubility
0.05
What is formed from the quenching of austinite?
Martensite
What is formed from the slow cool of martensite?
Pearlite
Ferrite
Cementite
What is austenite?
Interstitial solid solution of iron and carbon
Face centered cubic
What is ferrite?
Very dilute solid solution
Exists at low temperature
What is cementite?
Fe3C
Exists at low temperature
What is pearlite?
Eutecoid mixture of ferrite and cementite
What is a solid solution?
2 metals soluble in each other that form a common lattice structure
What are the two types of substitutional solid solution?
Random
Ordered
What does an interstitial solid solution contain?
Two atoms of different size
What are the features of martensite?
Distorted lattice
Hard
Brittle
What is tempering?
Heating to 450C followed by quenching
What does the addition of chromium in stainless steel provide?
Lowers the temperature of Austenite to martensite
Corrosion resistance from chromium oxide
What does the addition of nickel to stainless steel provide?
Increased strength
Increased corrosion resistance
Decreased austenite to martensite temperature
What is work hardening?
Work done on a metal at a low temperature that causes slip
The dislocations collect at grain boundaries and this results in a stronger metal
What is the difference between decorative and dental ceramics?
The kaolin is replaced with feldspar and silica
(Kaolin causes opacity)
What is feldspar?
Potassium or sodium alumina silicate
What does feldspar do?
Lowers the fusion and softening temperature of the glass
What colour does chromium oxide make?
Green
What colour does cobalt oxide make?
Blue
What colour does manganese oxide make?
Lavender
What colour does copper oxide make?
Green
What colour does iron oxide make?
Brown
What colour does nickel oxide make?
Brown
What size reduction is associated with sintering?
20%
What are the properties of dental ceramics?
Aesthetics
Chemical stability
Biocompatibility
Thermal
Dimensional stability
Mechanical
Stable colour
Smooth surface
Don’t stain
Optical properties
What does the Yttria do in Zirconia?
3-5%
Increases translucency
Decreases physical properties
What shape is a zirconia molecule?
Monocyclic
What shape is a yttria molecule?
Tetragonal
What type of crown is the strongest?
Milled crowns
What type of crown is stronger than lithium disilicate?
Zirconia
What advantage do crowns with layered porcelain have?
Better aesthetics but more likely to chip
What type of crown is preferred in posterior teeth?
Zirconia
What type of crown is preferred in anterior teeth?
Lithium disilicate
What are the two components of glass ionomer cement?
Silica
Polyacrylic acid/tartaric acid
What is the formula for the setting reaction of glass ionomer cement?
MO.SiO2 + H2A —> MA + SiO2 + H2O
What are the stages of glass ionomer cement reactions?
Dissolution
Gelation
Hardening
What happens during dissolution?
Mixing
H+ ions attack glass surface leaving silica gel around unreacted glass
What happens during gelation?
Forms a gel
Bivalent Ca+ ions crosslink with polyacrylic acid by chelation reaction
What happens during hardening?
Over 24 hours
Crystalisation
Trivalent Al+ ions crosslink increasing strength
How do glass ionomer cements bond?
Bonds the calcium ions in the enamel surface
What are the advantages of glass ionomer cements?
No setting contraction
Fluoride release
Bonds tooth
Good aesthetics
Minimal tooth prep
What are the contents of resin modified glass ionomer cement?
Fluoro-aluminio-silicate glass
Barium glass
HEMA
Polyacrylic/Tartaric acid
What are the pros of RMGIC?
Improved mechanical
Dual set
Longer working time
What are the cons of RMGIC?
Setting contraction
Unreacted monomer may be toxic
What type of cure can RMGIC have?
Dual or tri
What is dual cure?
Light
Mix
What is tri cure?
Light
Mix
Redox
What are some used of GIC?
Luting cement
Liner
Temporary restoration
What is the calculation for stress?
Force/Area
What are the four types of stress?
Tensile
Compressive
Shear
Torsional
What is a stress concentration point?
Abrupt changes in the shape of a file that results to a higher stress at that point
What is strain?
Response of a material to stress
What is elastic limit?
Maximal stress applied that allows material to return to original dimensions
What is elastic deformation?
Reversible deformation that doesn’t exceed elastic limit
What is plastic deformation?
Permanent bond displacement when elastic limit is exceeded
What happens when a material reaches its plastic limit?
Fracture
What is cyclic fatigue?
Generation of tension/compression angles
When does torsional fatigue occur?
When a file binds dentinal walls and doesnt rotate
What are the pros PMMA?
High softening temp
Insoluble in oral fluid
What are the cons of PMMA?
Poor thermal conduction
What is the cause of gaseous porosity?
Monomer bubbling
What is the cause of contraction porosity?
Poor pressure/too much monomer
What is the cause of granular porosity?
Rough surface
Too much monomer
What is the cause of residual monomer?
Insufficient terminal boil
What is crazing?
Cracks at high stress areas (quenching and different thermal contractions)
What are the properties of a luting cement?
Viscosity and film thickness
Ease of use
Radiopacity
Marginal seal
Aesthetics
Solubility
Cariostatic
Biocompatibility
Mechnanical properties
What are the ideal properties of a luting cement in reference to viscosity and film thickness?
Must be low to allow seating without interference
Viscosity must increase as material seats
Ideal film thickness <25um
What is viscosity and film thickness dependent on?
Size of powder/filler particles
What are the ideal properties of a luting cement in reference to ease of use?
Easy to mix
Long working time, short setting time
What are the ideal properties of a luting cement in reference to radiopacity?
Higher the better, easier to see marginal breakdown
What are the ideal properties of a luting cement in reference to marginal seal?
Should be impenetrable to tooth and restoration
What are the ideal properties of a luting cement in reference to aesthetics?
Tooth coloured
Non-staining
What are the ideal properties of a luting cement in reference to solubility?
Low
What are the ideal properties of a luting cement in reference to cariostaticity?
Fluoride-releasing
Anti-bacterial
What are the ideal properties of a luting cement in reference to biocompatibility?
Non-toxic
No damage to pulp
Low thermal conductivity
What are the ideal properties of a luting cement in reference to mechanical properties?
High compressive strength (similar to dentine 275MPa)
High tensile strength (similar to dentine 50MPa)
High hardness (dentine 70K, enamel 400K)
Young’s modulus (dentine 15GPa)
What are examples of luting materials?
Dental cements- zinc phosphate, zinc polycarboxylate
GIC: conventional, resin modified
Composite resin
How are composite resin luting agents applied?
Total etch for use with DBA
Self etch (still requires etch but has own bonding agent incorporated)
What are the features of zinc phosphate luting agent?
Acid-base reaction
Powder and liquid
Excellent clinical service
Easy to use
Cheap
What does the powder component of zinc phosphate luting agent contain?
Zinc oxide (90%)
Magnesium Dioxide (<10%)
Other oxides- alumina and silica
What does the presence of magnesium dioxide in zinc phosphate luting cement provide?
White colour
Increases compressive strength
What does the presence of other oxides in zinc phosphate luting cement provide?
Improve physical properties
Alters shade
What does the liquid component of zinc phosphate cement contain?
Aqueous solution of phosphoric acid (50%)
Oxides; aluminium and zinc
What does the presence of aluminium oxide in zinc phosphate luting cement provide?
Even consistency of set
What does the presence of zinc oxide in zinc phosphate luting cement provide?
Slows reaction for better working time
What is the acid base reaction for zinc phosphate luting cement?
ZnO + 2H3PO4 –> Zn(H2PO4)2 + H2O
What is the hydration reaction for zinc phosphate luting cement?
ZnO + Zn(H2PO4) + 2H2O –> Zn3(H2PO4)2 .H2O
What is the chemical formula of hopiete?
Zn3(H2PO4)2.H2O
(hydrated zinc phosphate)
Discuss the reaction in zinc phosphate luting cement:
The aluminium oxide prevents crystallisation leading to an amorphous glossy matrix of the acid salt surrounding unreacted ZnO
This matrix is almost insoluble, but it is porous and contains free water from the setting material
The cement subsequently matures binding this water, leading to a stronger less porous cement
What are the problems with zinc phosphate as a luting cement?
Initial low pH (pulpal irritation)
Exothermic setting reaction
Non-adhesive to tooth surface or restoration
Non-cariostauc
Final set takes 24 hours
Brittle
Opaque
What is the difference between zinc phosphate and zinc polycarboxlate cement?
Zinc polycarboxylate uses polyacrylic acid instead of phosphoric acid
What are the features of zinc polycarboxylate cement?
Bonds to tooth
Less exothermic
Low initial pH, returns to neutral more quickly
Cheap
What are the benefits of glass ionomer cement as a luting cement?
Fairly strong
Durable
Low shrinkage
Long term stability
Relatively insoluble
Better aesthetics than zinc phosphates
Self adhesive to tooth
Fluoride release
Cheap
What are the downsides of glass ionomer cement as a luting cement?
No chemicals bond to restoration surface
Surface must be sandblasted to allow mechanical adhesion
Easy to use clinically
What is the hydrophilic monomer in resin modified glass ionomer cement?
HEMA (hydroxethyl methacylate)
What does a secondary cure via redox allow?
Shorter setting tine
Longer working time
Increased compressive and tensile strength
Higher bond to tooth
Decreased solubility
What are the problems with RMGIC as a luting cement?
HEMA monomer is cytotoxic and swells
What can RMGIC not be used as a luting cement with?
Porcelain crowns
Posts
What are the benefits of composite luting agents?
Better physical properties
Lower solubility
Better aesthetics
What are the downsides of composite luting agents?
Need dentine bonding agent
Technique sensitive
Discuss the bonding between an indirect composite restoration and a composite luting agent?
Composite bonds composite
Bond strength is lower to inlay fitting surface than new composite
Bond is micromechanical and chemical (C=C) on the fitting inlay surface
Dual cure required as light penetration through inlay will be poor
Discuss the bonding to porcelain crowns?
Porcelain is brittle and requires to be bonded to tooth to prevent fracture
Porcelain must be etched with HF to etch the surface and create a rough retentive surface
Surface is hydrophobic, surface wetting agent required
What is an example of a silane coupling agent?
Gamma methacryoxypropyltrimothoxysilane
How does a silane coupling agent work?
Applied to etched porcelain: strong bond between oxide group on porcelain surface and silane
Other end of the silane has a C=C which bonds the composite luting agent
What must be done with bonding to a metal crown?
Etching; electrolytic etching
Sand blasting: needs a bonding agent
Why must dual cure be used for metal crowns?
Light cannot penetrate metal
What must be done when bonding precious metals?
Change composition to alloy alloy formation
Increase copper content and heat 400C
How does a self adhesive composite resin work?
Metal coupling agent incorporated into composite resin
Simplifies bonding process
What is an example of a self adhesive composite resin?
Panavia
What are examples of self etching composite resin cements?
RelyX
Unicem
What are self etching composite resin cements?
Combination of composite resin cement and self etching DBAs
What are the benefits of a self etching composite resin cement?
Very good moisture control
Good bond to dentine
Good compressive strength
Good tensile strength
Good hardness
Good wear resistance
What are the downsides of a self etching composite resin cement?
Limited smear layer removal
How do self etching composite resins work?
Acidic groups bind with calcium in hydroxyapatite forming stabilising attachment between tooth and resin
Ions from dissolution of filler neutralise the remaining acidic groups forming chelate reinforced methacrylate network
What is the 2 paste system of temporary cements?
Base: zinc oxide, starch and oil
Accelerator: resins, Eugenol, ortho-EBA, carnauba wax
What is the role of carnauba wax in temporary cements?
Prevents full set
When should a Eugenol containing temporary cement not be used?
If the permanent luting cement is composite based
What is the composition of stainless steel?
Iron 72%
Chromium 18%
Nickel 8%
Titanium 1.7%
Carbon 0.3%
What makes hard stainless steel?
Work hardening
What are the reasons for stainless steel fracture?
Overworked
Mechanical abrasion
Fatigue
Weld decay
What are the features of austenitc steel?
Crystal lattice structure
Soft unless cold hardened
What does chromium provide to stainless steel?
Lowers temperature for martensite formation
Corrosion resistance
What are the features of martensite?
Formed quenching from high temperatures
Inter grown plate-like crystals with a distorted cubic structure arising from the presence of carbon atoms in the iron structure
What does nickel provide to stainless steel?
Help achieve austenitic structure at room temperature by lowering critical temperature for austenic structure breakdown on cooling improves strength and corrosion resistance
What does titanium provide to stainless steel?
Prevents precipitation of chromium at grain boundaries when the alloy is heated during welding or soldering
Carbon combines with titanium in preference to chromium
What are the differences between austenite and martensite?
The average unit of austenite is a perfect cube
Martensite is distorted by interstitial carbon atoms that do not have time to diffuse during quenching
Material is harder as particle layers cannot slide past one another
What are the ideal features of an investment material?
Porous
Expands
Easily removed from cast
What are the two components of an investment material?
Binder and refractory
What does a binder do?
Forms coherent mass
What does a refractory do?
Allows expansion
Withstands high temperatures
What is hydroscopic expansion?
Water molecules attracted between crystals forcing crystals apart
What factors lead to hydroscopic expansion?
High water concentration
High water temperature
Long immersion time
Higher silica content
What are the types of dental stone/plaster?
Gypsum bonded
Silica bonded
What are the ideal properties of dental cement?
Easy to use
Radiopaque
Aesthetic
Cariostatic
What are the types of dental cements?
Zinc phosphate
Zinc poly carboxylate
GIC
RMGIC
What is weld decay?
Carbon chromides forming at grade boundaries
Increases brittleness and corrosion
What are the key features of an impression material?
Low viscosity
Low viscoelasticity
High tear strength
High elastic recovery
Good wettability
Good surface detail
What is static fatigue?
Time dependent reduction in function regardless of applied strength
What are the pros of alumina cores?
Good aesthetics
High flexural strength
Cheap
What are the pros of zirconia cores?
Hard
Strong
Excellent fit
What colour is copper oxide?
Green
What colour is chromium oxide?
Green
What colour is cobalt oxide?
Blue
What colour is manganese oxide?
Lavender
What colour is iron oxide?
Brown
What colour is iron oxide?
Brown
What colour is nickel oxide?
Brown
What are the stages of cast and pressed ceramics?
Wax up
Investment
Cast from heated ingot
Ceraming
Staining
What are the ceramic stages?
Crystal formation
Crystal growth
What is a study cast?
Positive replica of the dentition produced from an impression
What are the uses of study casts?
To record positions, shape and dimensions of teeth
Aids visualisation/assessment of dentition
Enables manufacture of dental prostheses
What is the reaction for the manufacture of gypsum?
Calcium sulfate dihydrate + heat –> Calcium sulphate hemihydrate
(CaSO4.2H2O) + heat —> (CaSO4)2. H2O + H2O
What are the types of gypsum?
Plaster (beta hemihydrate)
Stone (alpha- hemihydrate
Improved stone (densite)
What are the features of plaster (b hemihydrate)?
Heated in open vessel
Large, porous
Irregular crystals
What are the features of stone (a hemihydrate)?
Heated in autoclave
Non- porous
Irregular crystals
Requires less water
What are the features of improved stone (densite)
Heated in presence of Ca and Mg chloride
Compact, smooth particles
What is the reaction for setting gypsum?
Calcium sulfate hemihydrate + water –> calcium sulfate dihydrate
(CaSO4)2.H2O + 3H2O –> 2CaSO4.2H2O
What happens during the initial set of gypsum?
Dihydrate crystals come in contact
Expansion begins
What happens during the final set of gypsum?
Hardens
Excess water evaporates
What is the mixing ratio for dental plaster?
50/60ml : 100g
What is the mixing ratio for dental stone?
20-35ml : 100g
What is ditching?
Ditching is a failure of amalgam which is characterised as marginal breakdown where the restoration breaks away from the cavity margins
What are the causes of ditching?
Inappropriate cavity preparation
Creep
What is wettability?
A measure of how well a material spreads/adheres to the surface
When does wettability increase?
Wettability increases when the critical surface energy of the surface is higher than the liquid
How does light curing work?
Blue light activates camphorquinone which releases free radicals
The free radicals released polymerase the monomers
What is the depth of cure?
2mm
What are the properties of gypsum?
Reproduction of surface detail
Strength
Surface hardness (low)
Setting time
Exansion
What is the compressive strength of gypsum?
28-38MPa
What is the expansion of gypsum?
0.05-0.3
What is the effect of increasing the powder in gypsum?
Decreases setting time
Increases expansion
What is the effect of increasing spatulation in gypsum?
Decreases setting time
Increases expansion
What is the effect of increasing impurities in gypsum?
Decreases setting time
Increases expansion
What is there effect of increasing temperature on gypsum?
Increases the rate of diffusion of ions but decreases hemihydrate solubility
What are examples of additives to gypsum?
Potassium sulfate
Borax
What is the role of potassium sulfate as an additive in gypsum?
Produces synergite (K2(CaSO4)2.H2O)
Rapid crystallisation (increased growth)
Decreased setting time
What is the role of borax as an additive in gypsum?
Forms calcium borate (deposits on dihydrate)
Delays setting process
What are the disadvantages of gypsum?
Low tensile strength
Poor abrasion resistance
Very brittle
Surface detail (less than elastomer)
Poor wetting of some impression materials
What are the advantages of gypsum?
Dimensionally accurate
Stable
Low expansion (<0.1)
Good colour contrast
