DMS - bonding/GI/Am/Comp/imp/alloys Flashcards
Constituents of enamel
- Hydroxyapatite (95% weight / 90% volume)
- Water (4% weight / 5-10% volume)
- Organic matrix (1% weight / 1-2% volume)
proteins, amelogenins, enamelins, peptides and amino acids
Constituentss of dentine
- Hydroxyapatite - (70% weight / 50% volume)
- Water (10% weight / 20% volume)
- organic matrix - (20% weight / 30% volume )
content of dentinal tubules
- odontoblast process
- Unmyelinated nerve terminals (sensory)
- dendritic cells (immune system)
- dentinal fluid/EC fluid
which is more minaeralised: surface or deep enamel
SURFACE ENAMEL MORE MINERALISED,
hardness < from cusp tip /incisal edge to cervical region
Bonding to enamel
acid etch technique - application of acid , this orughens the surface enamel causing characteristic etched pattern.
MOISTURE CONTAMINATION RUINS THIS
- rough surface allows micromechanical interlocking of resin filling materials.
- etching INCREASES surface energy of enamel surface (remove surface contaminants) and INCREASES the WETTABILITY of the enamel.
How can enamel be etched - acid?
30-50 organic/inorganic acid (aqueous solution of phosphroric acid-37%)
constitients of GI cement
-acid - tartaric/polyacrylic
-base -Silica, SiO2(Silicone dioxide) 30% -40%
Alumina, Al2O3(Aluminium dioxide) 15% -30%
Calcium Fluoride, CaF215% -35%
Aluminium Fluoride 2% -10%
Aluminium phosphate 4% -20%
Sodium fluoride 4% -10%
acid base reaction - glass and acid =salt and water
GI cement - setting reaction stages
- dissolution - glass surface is attacked by H ions causing the release of ions (Ca/F/Al/Na), leaves silica gel around unreacted glass
- Gelation - initial set. Bivalent Ca crosslinks with polyacid by chelation to form Calcium polyacrylate
Takes a few minutes, will appear hard but will need to be protected (vaseline) - Hardening - final set. Trivalent aluminium crosslinks with polyacid by chelation.
Takes a long time - 30min-weeks
why should GI be protected following gelation
-aluminium ions escape
-excessive drying - water lost
-saliva contamination - water absorption
=weak material that will break up easily
how does GI bond to tooth
chelation with Ca on tooth, reprecipitation of complex of calcium phosphate from apatite, H bonding/metallic ion bridging
what factors are needed to achieve a good bond
Clean surface
Conditioned surface
Conditioned, not etched.
Little or no tissue is removed.
Best conditioner appears to be polyacrylic acid
Purpose is to produce clean smooth surface
- advantages of GI
- Disadvantages
Adv - stable chemical bond to enamel and dentine
low microleakage and fluoride release, good thermal properties, no contraction on setting
DIS - Brittle
- Poor wear resistance
- Moisture susceptible when first placed
- poor aesthetics
- Poor handling characteristics
- Susceptible to acid attack and drying out over time
- Possible problems bonding to composite
- Etching damages surface
components of RMGIC
powder - fluoro alumino silcate glass, barium glass (radiopaque), vacuum dried polyacrylic acid,
potassioum persulphate (allows redox reaction in dark)
ascorbic acid, pigments
liquid -HEMA, polyacrylic acid and pendant methacylate groups, tartaric acid, water and photoinitiators
how does RMGIC set - Dual-curing
Dual curing
Initially on mixing the acid base reaction begins in the same way as conventional GIC.
On light activation a free radical methacrylate reaction occurs resulting in a resin matrix being formed
Quickly light activation is complete (20s)
Acid Base reaction continues within the resin matrix for several hours
how does RMGIC set - tricuring
Tri curing
Initially on mixing the acid base reaction begins in the same way as conventional GIC.
The REDOX reaction begins
On light activation a free radical methacrylate reaction occurs resulting in a resin matrix being formed
Quickly light activation is complete (20s)
The REDOX reaction continues for about 5 minutes after initial mixing
Acid Base reaction continues within the resin matrix for several hours
Final hardening of the acid/base phase with aluminium polyacrylateformation can take days
properties of RMGIC
Good bond to enamel and dentine Superior to conventional GIC ?? Difficult to know what is being measured Definitely better initially Better physical properties Lower solubility Fluoride release Better translucency and aesthetics Better handling DIS - Polymerisation Contraction Exothermic setting reaction both polymerisation and dark cure Swelling due to uptake of water HEMA is extremely hydrophilic Monomer leaching HEMA is toxic to the pulp it must be polymerisedcompletely Reduced strength if not light cured
properties for impression materials - elastomers
flow /viscosity • surface detail (reproduction) • contact angle / wettability • elastic recovery (%) • stiffness (flexibility) • tear strength • mixing time (min) • working time (min)
- surface detail properties of elastomers
- accuracy
- removal from undercuts
- dimentsional stability
- Quality of surface interaction between
material & tooth/soft tissue surfaces
viscocity/surface wetting/contact angle - replication of surface detail/viscoelastic behaviour
- Flow under pressure (”shark fin” test)
- Tear/tensile strength
- Rigidity - Setting shrinkage
- Thermal expansion/contraction
- Storage
explain the chemistry of elastomers
Elastomers - formed by polymerisation with cross-linking of polymer chains • Cross-linking:- – generates ELASTIC properties – causes FLUID SOLID transition • Polymerisation MAY produce BYPRODUCTS (H2O, H2, alcohol) which affect DIMENSIONAL STABILITY and cast compatibility
conventional silicone - constituents
base paste
catalyst paste
Base paste
– silicone prepolymer with terminal hydroxyl groups
– filler
• Catalyst paste (or liquid)
– crosslinking agents*
– activator - organo-tin compound
*alkoxy orthosilicate or organohydrogen siloxane
addition cured silicone - constituents
base paste
catalyst paste
Base paste - • polydimethylsiloxane - some methyl (CH3) groups replaced by hydrogen • filler - variations change viscosity • Catalyst paste - • polydimethylsiloxane - some methyl groups replaced by vinyl (CH2 =CH) • filler - variations change viscosity • platinum catalyst eg chloroplatinic acid
polyether constituents
base paste
catalyst paste
Base paste • imine terminated prepolymer - cross linking • inert filler - viscosity, strength • Catalyst paste • ester derivative of aromatic sulphonic acid - initiates polymerisation • inert oils - form paste • inert fillers - form paste
requirements of a DBA
Ability to flow Potential for intimate contact with dentine surface Low viscosity Adhesion to substrate Mechanical Chemical Van der Waals Combination of the above
how does van der waal forces cause bonding
Van der Waals Adhesion
Based on electrostatic or dipole interaction between
bonding agent and substrate
Strength of interaction depends on CONTACT ANGLE,
which is a good indication of WETTABILITY of a solid
by a specific liquid. A contact angle of <90o means the
solid surface is hydrophilic
Best adhesion/bonding is achieved when Van der
Waals forces are optimised
what is critical suface energy
A liquid must have a lower surface energy than the
surface it is being placed on for it to flow onto it and
stick.
A low surface energy liquid will spread on a higher
surface energy substrate because this leads to a
lower surface energy of the material as a whole.
dentine has a low surface energy so a DBA increases this
what is the smear layer
The smear layer is an adherent layer of organic debris that remains
on the dentine surface after the preparation of the dentine during
the restoration of a tooth.
It is 0.5 – 5 microns in thickness.
It is variably attached to the dentine surface.
It is generally contaminated with bacteria.
Originally it was thought of as a protective barrier reducing
permeability of the dentine and protecting the pulp.
Now it is considered to interfere with adhesion
components of total etch bonding agent
Dentine conditioner: An acid, usually 35% phosphoric.
Primer: Really the adhesive part of the agent with a
hydrophilic/hydrophobic molecule
Adhesive: A resin which penetrates into the surface of
the dentine attaching to the primers hydrophobic
surface
what is a dentine conditioner
Generally Phosphoric acid, can be EDTA or Nitric
acid in some older systems.
Removes smear layer
Opens dentinal tubules by removing smear plugs
Decalcifies the uppermost layer of the dentine
The etchant is washed off with water.
The collagen network in this top 10um of the dentine
is exposed and subsequently penetrated by the next
two components
what is a dentine primer
Primer
The primer is really the adhesive element in the
process. A coupling agent.
It has a bifunctional molecue with a hydrophilic end to
bond to the hydrophilic dentine surface and a
hyrdophobic, methacrylate end to bond to the resin.
The molecule must also have a spacer group to make
it long enough to be flexible when bonding. Lack of
flexibility reduces bonding sites and bond strength.
This molecule or group of molecules is dissolved in a
suitable solvent. Ethanol, acetone or water
name an adhesive and how it works
It is predominantly hydrophobic.
It may contain some filler particles to make it stronger.
It will contain Camphorquinone to allow it to light
cure
Penetrates the primed dentine which now has a
hydrophobic surface.
Forms a micromechanical bond within the tubules and exposed dentinal collagen fibres.
Forms the HYBRID LAYER of collagen plus resin.
what are some isssues with total etch
Over etching à to collapse of the collagen fibres so no
resin can penetrate
Over etching à too deep an etch and the primer
cannot penetrate to the full depth of the etch.
Moisture dependent:
Too dry and the dentine surface collapses
Too wet and the primer is diluted à reduced strength
what are some issues with self etching primers
Self etching primers
These materials work in a different way from all the
previous materials.
They do not attempt to remove the smear layer. They
infiltrate it and incorporated themselves into it.
They are not washed off.
This removes the problem of how dry to make the
dentine.
Not as technique sensitive but bond doesn’t seem to
be as strong.
components of composite resin
- filler particles - quartz
- resin - Bis-GMA
- camphorquinone -produce free radicals to break c=c bonds
- low weight dimethacrylates - viscocity and reactivity
- silane coupling agent - ensure good bond
types of composite to be used in areas of mouth
- anterior
- posterior
- universal
-anterior - microfilled, or submicron
hybrid
– posterior - heavily filled
– universal - submicron hybrid
classification by handling characteristics
condensable - “amalgam feeling” - greater porosity – syringeable - good adaptation, less porosities, easy to apply – flowable - lower filler content, more shrinkage, difficult to apply, place for them -with fibre ribbons
effect of adding more filler particles
improved mechanical properties
– strength, hardness, rigidity etc
• improved aesthetics
• increased abrasion resistance
• lower thermal expansion (still not perfect)
• lower polymerisation shrinkage (still a problem)
• less heat of polymerisation (BUT not negligible)
• some radiopaque
how do self cure and light cure occur
self curing: benzoyl peroxide + aromatic tertiary amine free radicals (break resin C=C bonds) • light curing: camphorquinone + blue light (430-490 nm)
advantage of light cure acrylic
extended working time ie on-demand set • less finishing • immediate finishing • less waste • higher filler levels (not mixing two pastes) • less porosity (not mixing two paste
what is depth of cure
the depth at which the composite resin polymerises sufficiently, such that its hardness is about half that of the cured surface 2mm
comparison of:
conventional
microfine
hybrid
Conventional:
– strong but problems with finishing and staining due to soft resins and hard particles
• Microfine:
– smaller particles - smoother surface better aesthetics
for longer period
– but inferior mechanical properties (Elastic limit &
Young’s Modulus
hybrid - improved filler loading and coupling agents have
led to improvement in mechanical properties
factors affecting wear of composite
filler material • particle size distribution • filler loading • resin formulation • coupling agent
properties of composite
-abrasion resistance
-Thermal Conductivity
• low – which is good
-Thermal Expansion coefficient
- high - which is poor
(NB for composite: 28 ppm/ 0C
enamel: 11.4 ppm/ 0C
dentine: 8.3 ppm/ 0C)
Thermal Conductivity
• low – which is good
-bond to tooth
-aesthetic
-radiopaque
-handling/viscocity
-smooth finish
-NOT anticariogenic
-low setting shrinkage
-biocompatible
amalgam powder contents
liquid content
Silver, Tin- intermetallic compound Ag3Sn
- g phase, reacts with Hg liquid to form amalgam
• Copper- increases strength & hardness
• Zn - scavenger during production - preferentially
oxidises & slag formed / removed - some zinc free
• Hg in powder - (few materials)
– “pre-amalgamated” alloys - react faster
-liquid - mercury
name 2 types of amalgam particles
lathe cut – coarse, medium, fine – formed by filing ingots • spherical, spheroidal – range of particle sizes – formed by spraying molten metal into inert atmosphere
amalgam reaction
Ag3Sn + Hg –> Ag3Sn + Ag2Hg3 + Sn7Hg9
gamma gamma gamma 1 gamma 2
properties of gamma/gamma 1/gamma 2
g good strength & corrosion resistance • g1 good corrosion resistance • g2 weak and poor corrosion resistance
describe the issues with zinc
interaction of zinc with saliva / blood -
Zn + H2O ZnO + H2
• bubbles of H2 formed within amalgam
– pressure build up causes expansion
– downward pressure cause pulpal pain
– upward - restoration sitting proud of surface
• Hence zinc-free materials
name some properties of amalgam
Strength – generally consider compressive but others important – early (1hr) - Traditional materials, poor(ish) – late (> 24hrs) - OK – see also comparison later • Abrasion Resistance - high, suitable for posterior teeth/too high for deciduous -3x thermal expansion of tooth -high thermal conductivity -no bond to tooth Aesthetics - poor • Radiopaque - yes • Anticariogenic - no • Smooth surface - yes, if polished well, may deteriorate over time • Setting shrinkage - modern materials tend to have net overall shrinkage
factors decreasing strength of amalgam
undermixing • too high Hg content after condensation • too low condensation pressure • slow rate of packing – increments do not bond • corrosion
what is creep
When a material is repeatedly stressed
for long periods at low stress levels ie
stress below elastic limit, it may flow,
resulting in permanent deformation
ADV of spherical particles
less Hg required • higher tensile strength • higher early compressive strength • less sensitive to condensation • easier to carve
name 2 types of copper enriched amalgam
dispersion modified
single composition
dispersion modified copper enriched amalgam
benefits
originally Ag-Cu spheres + conventional lathe cut alloy
(now some single composition dispersed alloys - spheres & lathe
cut particle same composition)
• originally thought spherical particles would act as
strengthening agent, but
• increased copper content gave beneficial modifications
to setting reaction
setting reaction of dispersion modified
- g + Hg g + g1 + g2
- g2 + Ag-Cu Cu6Sn5 + g1
benefits of copper enriched
Higher early strength
• Less creep
• Higher corrosion resistance
• Increased durability of margins
setting reaction of single composition
Ag-Sn-Cu + Hg Ag-Sn-Cu + g1 + Cu6Sn5
what is a stress concentration point in an endo file
Abrupt changes in the geometric shape of a file that leads to a higher stress at that point
what is strain
the change in dimension over the original (deformation)
what is the elastic limit
A set value representing the maximal strain that when applied to a file, allows the file to return to original dimensions
what is plastic deformation
Permanent bond displacement occurring when elastic limit exceeded
what is plastic limit
the point at which a plastic deformed fine breaks
what is cyclic fatigue
Freely rotating in a curvature
•Generation of tension/compression cycles
•Cyclic fatigue
•Failure
Tension
what is work hardening
Strengthening of a metal by plastic deformation
•Crystal structure dislocation
•Dislocations interact and create obstructions in crystal lattice
•Resistance to dislocation formation develops
•Observed work hardening
sodium hypochlorite - which ions predominate at different pH - neutral/acid and alkaline
NaOCl ionises in water into NA+ and the hypochlorite ion, OCl-
•Establishes equilibrium with hypochlorous acid (HOCl)
•Acid/Neutral HOCl predominates
•pH 9 and above OCl- predominates
•HOCl is responsible for antibacterial activity
factors important for NaOCl function
Concentration •Volume •Contact •Mechanical agitation •Exchange
how to remove smear layer
17% EDTA
•10% Citric Acid
•MTAD (Mixture of a Tetracycline isomer, an Acid, and a Detergent
•Sonic and Ultrasonic irrigation
what is in GP
20% Gutta-percha
•65% Zinc Oxide
•10% Radiopacifiers
•5% Plasticizers
constituents of green stick
carnauba/stearic acid/wax/talc
2 types of imp material
MUCOSTATIC:
(eg zinc oxide eugenol, low viscosity alginates)
- fluid materials that displace the soft tissues slightly
- ie give an impression of the undisplaced mucosa.
MUCOCOMPRESSIVE
eg impression compound, high viscosity alginates/elastomers)
- viscous materials that record an impression of the mucosa under load
ie give impression of displaced soft tissue.
components of alginate
Component % Function Salt of alginic acid - 12% reacts with Ca ions (eg Na alginate) Calcium sulphate - 12% provides Ca ions Trisodium phosphate - 2% delays gel formation Filler - 70% cohesion, strength modifiers, - small improve surface, flavourings - taste, chemical indicators - pH colour change
ideal partial denture alloy properties
rigid (YM) strong (UTS, EL) hard ductile precise casting (shrinkage) melting point (investment material) density
4 examples of partial denture alloys
ADA Type IV Gold
- White Gold (Ag-Pd)
- Co-Cr
- Titanium
type IV gold composition
Au (60-70%) 65% Zn (1-2%) 2% Cu (11-16%) 14% Ag (4-20%) 14% Pd (0-5%) 3% Pt (0-4%) 2%
effect of copper as alloying element with gold
1) solid solution in all proportions
2) solution hardening
3) order hardening - if 40-80% Gold
and correct heat treatment
4) reduced melting point
5) no coring - solidus close to liquidus
6) imparts red colour (if sufficient quantity)
7) reduces density
effect of silver as alloying element with gold
1) solid solution in all proportions
2) solution hardening
3) precipitation hardening with COPPER
& heat treatment
4) can allow tarnishing
5) molten silver absorbs gas (e.g. CO2)
6) whitens alloy - compensation for
copper
effect of platinum as alloying element with gold
1) solid solution with Gold
2) solution hardening
3) fine grain structure
4) coring can occur
(wide Liquidus - Solidus gap
composition of CoCr
Co (35-65%) 54% Cr (25-30%) 25% Ni (0-30%) 15% Mo (5-6%) 5% C (0.2–0.4%) 0.4%
CoCr properties as partial denture alloy
much harder than Gold * wear in mouth better * finishing/polishing time consuming Elongation : 4% * low ductility * work hardens rapidly * adjustment difficult * precision casting
Definition:
Elastic limit
ductility
EL - maximum stress without plastic
deformation
Ductility - amount of plastic deformation
prior to fracture (ie measure of
the extent that a material can be
shaped/manipulated = (y-x)%
In alloys, how do crystals grows
-Atoms at these sites act
as nuclei of crystallisation
-Crystals grow to form dendrites (3D branched
lattice network)
-Crystals (or GRAINS) grow until they impinge
on other crystals
-Region where grains make contact is called
GRAIN BOUNDARY
what are equi-axed grains
if crystal growth of equal dimension in
each direction – EQUI-AXED grains
- what is quenching and what effect does it have on grains
- what effect does slow cooling have on grains
fast cooling (QUENCHING):- -more nuclei -small fine grains slow cooling :- -few nuclei -large coarse grains
what is a grain
Grains:
- each grain is a single crystal (lattice)
with atoms orientated in given directions
(Dendrites)
what is grain boundary
Grain boundary:
- change in orientation of the crystal
planes (impurites concentrate here)
what are dislocations
-what is slip
Dislocations are imperfections/defects in
the crystal lattice
-SLIP is due to Propagation of Dislocations
and involves rupture of only a few bonds at
a time
what factors can impede the movement of dislocations
INCREASES elastic limit UTS hardness DECREASES ductility impact resistance
what is cold working
-effect
working done at LOW TEMPERATURE
(ie below recrystallisation temperature)
-causes SLIP – so dislocations collect at
grain boundaries
modifies grain structure: higher -Elastic Limit -UTS -hardness - lower -ductility -impact strength -lower corrosion resistance
what is annealing
heating metal (or alloy) so that greater thermal vibrations allows migration of atoms (ie re-arrangement of atoms) -residual stress eliminated
definition
- phase
- solution
PHASE - physically distinct homogeneous
structure (can have more than one component)
SOLUTION - homogeneous mixture at an
atomic scale
on crystallisation, what 3 forms can alloys be in
ON CRYSTALLISATION, metals may :-
– a) be insoluble, no common lattice - 2 phases
– b) form intermetallic compound with specific
chemical formulation (eg Ag3Sn)
OR
– c) be SOLUBLE and form a SOLID SOLUTION,
ie form common lattice… 3 types of solid solution.
what is a sustitutional solid solution
- random
- ordered
Substitutional atoms of one metal replace the other metal in the crystal lattice/grain. - A. RANDOM:- metal atoms similar in:- SIZE, VALENCY, CRYSTAL STRUCTURE (eg fcc) eg AuAg, AuCu – B. ORDERED:- metal atoms in regular lattice arrangement, conditions as above
what is interstitial solid solution
- Interstitial
– atoms markedly different in size
– smaller atoms located in spaces in lattice/grain
structure of larger atom (eg Fe-C)
on a phase diagram, what does the liquidus and solidus represent
LIQUIDUS
line representing the temperatures which different
alloy compositions begin to crystallise
SOLIDUS
line representing the temperatures which different
alloy compositions have completely crystallised
what does rapid cooling of a molten alloy cause
RAPID COOLING of MOLTEN ALLOY
prevents atoms diffusing through lattice
-causes CORING
as composition varies throughout grain.
what does coring leave an alloy susceptible to
CORING:
may reduce corrosion resistance of the
solid form of alloy
what is homogenous annealing
-why is it completed below recrystallisation temperature
HOMOGENISING ANNEAL
-once solid cored alloy formed
REHEAT to allow atoms to diffuse and so cause
grain composition to become homogeneous
NOTE: keep below recrystallisation temperature,
otherwise grains altered
why are alloy inherintly more # resistant that metal
More energy/force is needed for the defect to
overcome the different-sized atoms, and move along
lattice to the grain boundary.
-Hence, it requires greater stress to move dislocations
in a solid solution – making alloys inherently more
fracture resistant (ie stronger) than metals
why do alloys forming an ordered solid solution have better properties, what are they
Alloys forming an ORDERED SOLID SOLUTION (atoms distributed at specific lattice sites) have a distorted grain structure (eg Au-Cu) which IMPEDES dislocation movement and so improves mechanical properties (EL, UTS, hardness)
Iron is allotropic, what does this mean
-what temperature do these occur
Allotropic - undergoes TWO solid state
phase changes with temperature.
(1) Temp. > 1400 C
BCC lattice structure; low Carbon solubility (0.05%)
(2) 900 < Temp. < 1400 C
FCC lattice; higher Carbon solubility (2%)
(3) Temp < 900 C: as (1)
definition of
- austentite
- ferrite
- cementite
- pearlite
AUSTENITE: interstitial solid solution, FCC; exists at high temp (ie >720 0C) FERRITE: very dilute solid solution; exists at low temp CEMENTITE: Fe3C ; exists at low temp PEARLITE: Eutectoid mixture of Ferrite and Cementite
how is martensite produced
quenching of austenite (fast cooling)
what does slow cooling of austenite produce
pearlite
composition of stainless steel
-how much chromium (%) must be in the alloy to be classed as stainless
iron/nickel/chromium/carbon
13% chromium
purpose of chromium in stainless steel
lowers Austenite to Martensite temperature \: lowers Austenite to Martensite rate \: decreases % carbon at which Eutectoid formed \: corrosion resistance
purpose of nickel in SS
lowers Austenite to Martensite transition
temperature
- improves UTS
- improves corrosion resistance
uses of austenitic SS
- dental instrument (not cutting edge)
- wires - ortho
- denture bases
composition of SS for ortho wire
18% Chromium
8% Nickel
0.1% Carbon
74% Iron
requirement for ortho wires (properties)
high springiness ( EL / YM) (IE undergo large deflections without permanent
deformation)
• stiffness (YM) - depends on required force for tooth movement
• high ductility - bending without fracture
• easily joined without impairing properties - soldered, welded
• corrosion resistant
Springiness ( EL / YM)-Ability of a material to undergo large deflections (to form arc) without
permanent deformation (ie it returns to its
original shape)
how does weld decay occur and at what temperature
-effect
Occurs between 500 - 900 0C Chromium carbides precipitate at grain boundaries -alloy becomes brittle • less chromium in central region of solid solution • more susceptible to corrosion
how to minimise weld decay
Low carbon content steels - expensive 2. Stabilised stainless steel - contain small quantities of titanium or niobium - forms carbides preferentially - not at grain boundaries
how are enamel crystalites deposited in relation to ameloblast membrane
Crystallite orientation is determined during enamel formation
•
Crystallites are deposited at right angles to ameloblast membrane
