Applied Dental Materials Flashcards
what is an alloy ?
a mixture of 2 or more metallic elements
what advantages does an alloy have over pure metals ?
cost, biocompatibility, mechanical properties, ease of casting
what are the fabrication processes of alloys?
forming - plastic deformation e.g. rolling, extrusion
amalgamation
casting
powder metallurgy - sintering, injection moulding, 3D printing
what physical properties should an alloy have for dental application ?
low melting point (flow)
moderately high density (castability)
low coefficient of thermal expansion
what chemical properties should an alloy have for dental application ?
corrosion resistance
electrochemical corrosion resistance
what mechanical properties should an alloy have for dental application ?
high modulus (stiffness) moderately high yield strength (resistance to plastic deformation ) hardenable by heat treatment (retention of polish )
what biological properties should an alloy have for dental application ?
biocompatible - no toxic soluble phases
non-reactive
what are the 2 types of casting metal alloys ?
noble (precious metal)
base metal
which alloys are noble metal alloys ?
gold
silver - palladium
which alloys are base metal alloys ?
cobalt-chromium and nickel - chromium
titanium
what are the properties of noble metals ?
surfaces retain lustre unclean dry air
resist oxidation, tarnish and corrosion during heating casting and soldering
what are the properties of gold ?
malleable, ductile
tarnish resistant in air and water at any temp
insoluble in nitric, sulphuric and hydrochloric acids
low strength
small amounts of impurities e.g. lead, mercury have detrimental effect on its properties
what is the fusion temperature of gold ?
106.3 deg
what is the density of gold ?
19.3g/cm^3
what is the coefficient of thermal expansion of gold ?
14.2x10^-6/degC
what is the MOE of gold ?
80 GPa
what are the properties of platinum ?
tough, malleable, ductile
high corrosion resistance
high cost
higher melting step than porcelain
what is the fusion temperature of platinum ?
1755 deg
what is the density of platinum ?
21.37 g/cm3
what is the coefficient of thermal expansion of platinum ?
8.9x10^-6/degC
what is the MOE of platinum ?
147 GPa
what are the properties of palladium ?
decreased cost
prevents corrosion of silver in oral environment
absorbs hydrogen gas when heated improperly
what is the fusion temperature of palladium ?
1555 deg
what is the density of palladium ?
11.4g/cm3
what is the coefficient of thermal expansion for palladium ?
11.1x10^-6/degC
what is the MOE of palladium ?
112 GPa
what are the properties of silver ?
malleable and ductile,
harder than gold,
best known conductor of heat and electricity,
unaltered in clean dry air,
combines with sulphur, chlorine and phosphorous resulting in tarnish in oral environment,
oxygen will evolve during solidification resulting in pits and porosities
what is the fusion temperature of silver ?
960.5 deg
what is the density of silver ?
10.4g/cm3
what is the coefficient of thermal expansion of silver ?
19.7x10^-6/degC
what is the MOE of silver ?
120 GPa
give 2 examples of minor alloying elements
iridium
ruthenium
what are the effects of minor alloying elements on alloys ?
increases tensile strength and elongation by 30 %
increases tarnish resistance
slightly increases yield strength
what are the uses of type 1 gold casting alloy ?
inlays that are well supported with no large masticatory forces
high ductility allows burnishing to improve hardness and fit
what are the uses of type 2 gold casting alloy ?
inlays - better mechanical properties that type 1 but less ductile
what are the uses of type 3 gold casting alloy ?
less support and opposing stress
class II cavities in molars
inlays, onlays, crowns, bridges
what are the uses of type 4 gold casting alloy ?
partial denture components, long span bridges
what is the composition of silver-palladium alloys ?
25% minimum palladium with small quantities of copper, zinc, indium and (gold), bulk is silver
what are base metal casting alloys ?
alloy which contain no gold, silver, platinum or palladium
give examples of 3 base metal casting alloys and their uses
nickel - chromium : crown and bridge casting
cobalt - chromium : partial denture framework casting
titanium and titanium-aluminium-vanadium alloys : implants
what are the properties of titanium and titanium alloys ?
lightweight, strong, biocompatible, corrosion resistant, low cost
what is the density of titanium ?
4.5g/cm3
what are the problems with casting titanium alloys ?
high melting point (1700deg)
chemical reactivity
need well controlled vacuum in processing
technology required makes casting expensive
what are the different components of dental porcelain ?
kaolinite, quartz, feldspar
what is kaolinite ?
china clay, hydrated aluminosilicate, Al2O3.2SiO2.2H2o
what is quartz?
silica Sio2
what is feldspar ?
mixture of potassium and sodium aluminosilicates
what is the composition of dental porcelain ?
little/no clay, mainly feldspathic glass with crystalline inclusion of feldspar
what are the additional components of dental porcelain and what are their functions ?
borax (sodium borate) - added to feldspar, reduces melting temperature
metal oxides e.g. iron - pigments
metal oxides eg zinc oxide - opalescence
what determines the opacity of dental porcelains ?
particle size
index of refraction
fabrication process
what are the physical properties of dental porcelains ?
low thermal conductivity - does not conduct heat to pulp but thermal stresses can cause cracks
what are the chemical and biological properties of dental porcelain ?
stable, biocompatible
what are the mechanical properties of dental porcelain ?
brittle, hard, strong - tensile
what are the common failure modes of dental porcelains ?
catastrophic failure
static failure
what factors reduce the strength of dental porcelain ?
surface roughness
internal voids
porosity
what is static fatigue ?
decrease in strength over time even in absence of applied load
why does static fatigue happen ?
Si-O bonds slowly hydrolysed,
generates OH which elevates pH,
dissloves feldspathic component Na2O and K2O,
accelerated by dynamic mechanical loading
how can the mechanical properties of dental porcelain be improved ?
alter processing - fabrication condition optimisation
produce stronger porcelains - composition and microstructure optimisation
reinforce - provide solid supportive core
what are the strengths of porcelain ?
aesthetic, high compressive strength, inert, hardness, high elastic modulus
what are the weaknesses of porcelain ?
brittle, fatigue, tooth wear
what os the general preparation of porcelain ?
powder mixed with water and binder, moulded and carved, compacted and dried, fired and cooled, glazed or polished
how is the porcelain powder/frit prepared ?
kaolin, silica, feldspar and other components are ground, mixed, fused, rapidly cooled and ground again
what is the function of feldspar ?
melts at the lowest temperature fusing the other components together
how is porcelain prepared for traditional crowns ?
porcelain frit mixed with water,
binder of sugar starch improves working properties,
aqueous plastic porcelain compacted onto platinum foil mould,
porcelain shrinks into foil,
assures good fit of crown on tooth
how is porcelain compacted ?
light vibration/patting,
modern manufacturers use cold isostatic pressing to make dense green porcelain ceramics
why is porcelain compacted ?
settles particles, reduces shrinkage,
gives uniform contraction over whole surface,
brings excess water to surface
describe the process of drying and firing porcelain
place into warm atmosphere to dry (1000deg sometimes under vacuum
what can happen if wet porcelain is fired at high temperatures straight away ?
it can bloat
how is porcelain glazed ?
coated with low-fusing transparent glass
what are the advantages of glazing porcelain ?
improves appearance by giving better translucency,
gives an impervious smooth coating that protects against chemical attack,
lower hardness to reduce wear
what alloys are typically used for porcelain fused to metal crowns ?
high gold, low gold, silver-palladium, nickel-chromium
what new development could allow crowns to be constructed at the chair-side ?
CAD/CAM
What is the filler in a GIC?
Alumina and silica in a calcium fluoride flux
What minor components are in gics ?
Sodium and aluminium fluoride and calcium and aluminium phosphate for flux
What is the function of tartaric acid in gics ?
Chelating agent
What poly acid is in gics ?
Poly acrylic or mixture of acrylic and itaconic or acrylic and Maleic acid
What are the stages of setting of gics?
Dissolution, gelation, hardening
Describe the adhesion of gics to tooth structure
Hydrogen bonding to collagen in dentine
Ionic bonding to hydroxyapatite in enamel
What is the bond strength of a GIC ?
2-7 mpa
Where does bond failure occur in a GIC ?
Cohesive failure within the material, not at bind interface
What are diamond GIC products ?
Contain poly phosphoric and poly acrylic acid
Faster reaction with immediate saliva resistance
What resin is in a compomer ?
UDMA
What is the resin in resin modified gics ?
Water soluble HEMA
What else is required in RMGICs ?
CO for free radicals
Poly acid molecules have methacryloxy groups for further polymerisation linkage
What are the three types of retention ?
Macro mechanical eg amalgam
Micro mechanical eg composite
Chemical eg GIC
What are resin tags ?
Resins with good surface wetting properties penetrate pores in enamel and form resin tags
What is used for pre etch prophylaxis ?
Proprietary pastes containing pumice, oil, glycerine, fluoride
What is used to etch enamel ?
36 % ortho phosphoric acid
Why are etchant gels better than etchant liquids ?
Easier to control due to viscosity and colour
What types of etch patterns are produced by enamel etching ?
Intra prismatic,
Inter prismatic,
Mixed appearance
How long should the tooth be washed for after etching ?
For at least as long as the etching time
What should happen if the tooth is contaminated with saliva after etching ?
Re etch for 5 seconds as glyco proteins will precipitate onto surface
What is the strength of enamel bonding ?
30 mpa
What reduces bond efficiency to enamel ?
Technique eg lack of bevel,
Contamination after etching,
Micro cracks developing from cavity prep,
Unsupported or fractured enamel margins
Why is bonding to dentine not as reliable as bonding to enamel ?
Difference in composition
Difference in structure
Describe the difference in composition between enamel and dentine
Dentine contains 12 times as much water and twice amount of organic material
Enamel is homogeneous in structure,
Dentine is heterogeneous in structure with different amounts of HA and collagen in peritubular and inter tubular dentine
What is a smear layer ?
Smeared HA crystals in a matrix of partially degraded collagen, may contain bacteria
2 layers :
Thick outer layer on peri and inter tubular dentine
Thin inner layer of plugs in dentinal tubules
What type of smear layer is created by water spray and tungsten carbide burs?
Thin smear layer 7um
What type of smear layer is produced by diamond burs ?
Thick loose smear layer 15 um
What are the adverse effects of smear layer removal ?
Tissue fluid outflow,
Bacterial invasion,
Dentine sensitivity,
Adverse pulpal response
What are the ideal properties of adhesive resin ?
Non toxic to pulp,
Hydrophilic,
Good surface wetting,
Long term stability
What is in a dentine bonding resin ?
Methacrylate to bond with overlying composite,
R - organic linking molecule,
X - hydrophilic molecule that interacts with the etched dentine surface
How is bonding to dentine surface achieved?
Mainly entanglement or micro mechanical retention,
Bonding to appetite salts in etched dentine
Bonding to organic collagen matrix of etched dentine
What is the bond strength of composite to etched enamel ?
20-25 mpa
What does bonding by entanglement produce ?
Hybrid zone responsible for 80 % of bond strength
How does bonding to appetite salts occur ?
Appetite salts bond ironically to ions in enamel, dentine and GIC ,
Consist mainly of phosphated esters of bis GMA eg PENTA and MDP
Describe additional covalent bonding with the organic matrix
Materials contain HEMA, PMDM, aldehyde hydrophilic resins
Describe the three step etch prime and bond technique
Simultaneous etching of enamel and dentine with phosphoric acid,
Smear layer removal and exposure of collagen in inter tubular dentine,
Application of hydrophilic primer to exposed collagen and tubules,
Adhesive application to copolymerise with primer
What is primer resin dissolved in ?
Acetone or alcohol which displaces water from collagen due to its volatility
Give examples of self etching primers
NRC,
Clear fill SE bond
Describe the 2 step self etch primer and adhesive method
Simultaneous etching and priming of tooth structure using weaker acids eg maleic and itaconic acids,
Adhesive application to copolymerise with polymer,
Eliminates separate etch and wash step
Give examples of one bottle primer and adhesive
Prime and bond,
Optibond solo
Describe the 2 step etch and combined primer and adhesive method
Simultaneous etching or enamel and dentine using phosphoric acid,
Smear layer removal, exposure of collagen,
Application of one bottle combined primer and adhesive
Give examples of self etching primer and adhesives
Prompt L pop,
Xeno
Describe the one step etch, prime and adhesive method
Simultaneous etching, priming and adhesive application to enamel and dentine,
Smear layer removal, neutralisation of etch component and forms hybrid layer in one step
Describe the mechanism of action of Xeno
Hydrolysis of pyro EMA,
Produces phosphoric acid groups,
Phosphoric acid groups bond to calcium ions in HA in dentine and enamel,
Phosphoric acid groups neutralised resulting in etched dentine and enamel
How do polyakenoate cements bond to tooth structure ?
Don’t rely on micro mechanical retention,
Utilise ion exchange at the enamel or dentine surface to produce dynamic ionic bond
How is the smear layer removed for polyalkenotae cement ?
Apply 10% poly acrylic acid for 10 seconds,
Washed for 15 seconds,
Plugs remain in tubule ends
How are bonds formed between polyalkenotae cement and tooth structure ?
Free polyalkenoic acid penetrates enamel and dentine surface and the carboxyl group displaces phosphate ions,
Phosphate takes calcium with it to maintain electrical neutrality,
These combine with cement matrix and form an ion rich soup at the interface of the tooth and cement,
Firmly binds to tooth and cement
What is the soup ?
Calcium/strontium/phosphate/alkenoate
How do polyalkenoate cements bond to tooth structure ?
Hydrogen bonding,
Metallic ion bridges between carboxyl groups an poly acid and amino acids of collagen
How do gic bonds fail ?
Weakest material is matrix holding glass particles together,
When gic is stressed it cracks cohesively between glass particles and not at ionic interface
What is the bond strength of polyalkenoate cements to tooth structure ?
5-9 MPa
