Dental materials Flashcards
what is a basic requirement of dental materials?
CE mark safety
what is an example of hydrocolloid
alginate
what is negative and positive replica?
negative = impression
positive = gypsum model
3 broad categories of material characteristics
- mechanical
- physical
- chemical
mechanical properties
hard/soft
strong/weak
flexible/rigid
ductile/brittle
stress
strain
shear
tensile
compressive
elasticity
physical properties
thermal conductivity
thermal expansion
viscosity
density
radiodensity
chemical properties
setting mechanism
setting time
corrosive potential
what is the formula of stress
stress = force/ area
strain formula
change in length/ original length
rigidity formula
stress/strain
what is young’s modulus
young’s elastic modulus = rigidity
hardness
Hardness is the ability of surface to withstand indentation
Compressive strength
the ability to withstand applied compressive forces without fracturing
compressive vs tensile strength
compressive - pushing force that reduces size of material
tensile - pulling force that increases size of material
failure mechanisms
- Fracture
- Hardness
- Abrasion
- Abrasion resistance
- Fatigue
- Creep
- Deformation
- De bond
- Impact
elasticity
- Elasticity is the ability to return to its original shape upon removal from mouth using force
- Elasticity – strain and recovery
abrasion
material surface removal due to external factors like toothbrushing
abrasion means erosion is also porbably involved (refer to oral functions)
fatigue vs creep vs deformation
fatigue is repetitive small stresses causing material FRACTURE/CRACKS
creep is the gradual dimensional change due to small forces (amalgam)
deformation is permanent change in materials dimensions WITHOUT FRACTURE
when does (permanent) deformation occu?
occurs when the stress is beyond the elastic limit
what tells you that enamel is more rigid than dentine
enamel has higher fracture stress
on a stress strain curve, what does the peak, the slope, the plateau and the end of the linear gradient tell you?
peak - fracture stress
end of the linear gradient = proportional limit
linear slope = stress and strain are directly proportional
slope gradient = rigidit
what is fracture stress
the force at which fracture occurs
what is proportional limit?
- The proportional limit in materials denotes the maximum stress they can withstand while still exhibiting a linear correlation between stress and strain. Functionally, it is the stress threshold beyond which plastic deformation takes place. The stress and strain are directly proportional to one another until they reach the proportional limit.
what are properties that dental adhesive must have?
high bond strength to tooth
impermeable bond
easy
durable bond
immediate high strength
enamel vs dentine bonding
enamel - mechanical
dentine - mechanical, chemical, van der waals
what does etching enamel with 30-50% phosphoric acid do?
roughens surface
removes contaminents
increases surface energy and wettability for the resin to flow into the etched enamel prisms
what is needed for enamel micro mechanical interlocking
enamel needs to be dry
amalgam is ana lloy made of?
mercury liquid
silver tin copper powder
what percentage of amalgam is silver?
70%
25% tin
5% copper zinc and mercury
what does the copper powder do?
increase strength and hardness
what is gamma phase?
silver and tin powder, gamma phase reacts with mercury liquid to form amalgam
why is Hg triple distilled?
to make it very pure
what are the two particle types of amalgam
lathe cut and shperical cut
Advantages of amalgam
strong
hard
durable
radiopaque
user friendly
Half the failure rate for all types of restoration in posterior teeth
disadvantages of amalgam
corrosion
leakage - does not bond to enamel or dentine
poor aesthetics
mercury:
- perceived toxicity
- environmental impact
is spherical cut or lathe cut better?
spherical
o Less mercury required
o Amalgam stronger under tension or compression
o Less sensitive to condensation (not compromised by poor application technique)
o Easier to carve
o Requires less force to pack compared to lathe cut
what is the setting reaction to make amalgam?
gamma phase power and liquid mercury =
gamma + gamma 1+ gamma 2
whats gamma , 1,2
gamma = silver tin
gamma 1= silver mercury
gamma 2 = tin mercury
out of gamma , 1, 2, which has poor corrosion resistance?
gamma2- poor corrosion resistance and weakest tensile strength
gamma - strongest tensile and good corrosion resistance
gamma1 - mid tensile and good corrosion resistance
setting dimensional changes between traditional and modern amalgam
traditional contract then expand alot (relative)
modern contract only but very small negligible amount
why are some amalgam zinc free? what is the disadvantages of zinc
zinc can react with water in saliva or blood to produced hydrogen gas
this gas causes pressure that can cause expansion or pulpal pain
what does corrosion do to amalgam?
can cause weakened marginal integrity, might lead to fracturing or leakage
what decreases the strength of amalgam?
undermixing
slow rate of packing
corrosion
why does creep occur?
amalgam is viscoelastic
low level stress constantly can cause amalgam to FLOW and result in permanent deformation
what affects marginal intergrity (3 pts)
creep
corrosion
cavity design
o Marginal integrity is in relation to the structure of the outline of the cavity, eg. Ditched margins from fracture or high margins from creep
whydo we need to use liner when using amalgam
it has poor thermal properties , can damage pulp
thermal expansion is 3 times
thermal conductivity is high
thermal diffusivity is high
ideal thermal properties of amalgam
same expansion as dentine and enamel
low conductivity and diffusivity (ie poor conductor)
does amalgam bond to tooth
o Does not bond
o Purely mechanical
o Requires undercuts so more tooth tissue removal than desired
is amalgam anticariogenic
no
what determines the force needed to pack amalgam?
shape of particles, with spherical particles requiring less force
2 other names of copper enriched amalgam
- Aka Non gamma 2
- Aka high copper
what is the criteria to be classified as copper enriched amalgam
more than 6% copper
adv of copper enriched
- Higher early strength and longer term
- Less creep
- Less corrosion cause no gamma 2 *****
- Increased durability at margins , less likely to fracture at margins
which type of amalgam has the fastest early strength?
copper enriched single compostion
which type of amalgam has the least creep
copper enriched single composition
what is in copper enriched dispersion amalgam
conventional lathe cut alloy
extra silver-copper spherical cut
added
what creates the protective halo around spheres in copper enriched amalgam?
copper tin
whats the copper % in single comp copper enriched amalgam?
12-30%
therefore least creep and highest compressive strength
3 components of bonding system
etch
prime
adhesive
what does etch do
o remove contaminants and increases surface energy of enamel
o Opens dentinal tubules
o Removes smear plugs
o Decalcifies uppermost layer of dentine
o Exposes up to 10 microns of collagen network
do we want high or low surface energy/ wettability for dentine and enamel
want enamel and dentine to have higher surface energy and wettability so that the adhesive can flow over easily.
purpose of high wettability
adhesive can flow over easily.
what increases surface energy of dentine.
DBA increases surface energy of dentine.
enamel adhesion is through micromechanical interlocking while dentine is ______?
molecular entanglement
why do we need to prime the surface of dentine?
PRIMER GIVES THE SURFACE HIGH ENERGY SO THAT THE RESIN CAN FLOW
dentine is wet, it is hydrophilic, but the resin adhesive is hydrophobic
therefore we need primer to help then interact
what is smear layer
organic debris that remains after the preparation of dentine
about 0.5-5um thick
why do we need to removed smear layer?
- weak bond as the smear layer not firmly attached to the underlying dentine
- Generally contaminated with bacteria
- Interferes with adhesion
what removes the smear layer
phosphoric acid
why cant dentine be too dry or too wet when putting primer and adhesive
- If too dry, collagen fibres will collapse and the primer cannot infiltrate because of decreased porosity
- If too wet, primer is diluted and reduced strength
what happens when you over etch dentine?
collagen fibres collpase
too deep an etch and primer cannot penetrate full length
chemical structure of primers
Hydrophobic end - bind adhesive, has C=C bonds for polymerisation
Hydrophilic end – bind dentine, binds to the polar groups on HA or amine groups on collagen
characteristics of DBA
- SURFACE WETTING AGENTS
- Ability to flow
- Low viscosity
- Adhesion to substrate
o Mechanical
o Chemical
o Van dear waals
is it ionic or covalent bond to organic dentine?
Mineralised dentine -> ionic bond
Organic dentine -> covalent bond
2 types of DBA
total etch - removes smear layer
self etch - incorporates smear layer
what are some common primers
Bifunctional molecules eg. HEMA (hydroxy ethyl methacrylate) or NTG-GMA
what is the function of the adhesive
form hybrid layer of collagen plus resin
what is the name of the thing in adhesives that can light cure
Camphorquinone
benefits of self etch
- Do not need to control how much to dry the dentine
- Reduced sensitivity since no chance of collapse of collagen architecture
- HA crystals remain around the collagen , protecting against hydrolytic breakdown
- remaining calcium ions can allow ionic bonding
key component in self etch
- Acidic methacrylate phosphatase
bifunctional monomers ^ with a hydrophilic end to infiltrate dentine and a polymerizing hydrophobic end like a bis GMA
Molecular entanglement definition
absorbed component of adhesive can form long chain polymers, this polymer meshes with the substrate, leading to high bond strenght
Adhesion Decalcification Concept
phase 1 all acids strong or weak adhere to the calcium ions in HA
phase 2 option 1
mild self etch will remain bonded and stabilize the HA crystals, protected the vulnerable collagen fibrils
phase 2 option 2
strong acids like phosphoric acid will debond with the loaded calcium ions, resulting in decalcification
properties of bonding adhesive
high bond strength
immediate bond
durable bond
impermeable bond
easy
safe
definition of critical surface energy
the surface tension of a liquid that will JUST spread on the surface of a solid
a low surface liquid will spread better on a higher surface energy substrate
what material is the adhesives
mixture of resins, usually BisGMA and HEMA
filler particles
is adhesive hydrophilic or hydrophobic
hydrophobic
how deep does the self etch incorporate the smear layer into the hybrid layer?
2um
potential problems with self etch
if smear layer is thick, the mild self etch may not be able to penetrate through it
Function of Cavity lining materials
- Protective barrier from
- Heat
- Chemical
- Bacteria
- Make intimate contact with tooth surface especially dentine
- Protect from microleakage
- Prevents gaps
whAt is a pulpal irritant if not cured properly
HEMA
2 types of cavity lining materials
base
liner
differences between base and liner
- Base
o Thicker
o Placed in bulk
o Replace dentine
o Block out undercuts - Liner
o Thin coating over EXPOSED dentine
o Seals dentine
o Prevents penetration of bacteria into dentine
o Thermal insulation
o Therapeutic – calm inflammation
o Palliative – reduce symptoms
what thermal properties should cavity lining materials have
low thermal conductivity - pulp sensitivity
similar thermal expansions - wont stress
- low thermal diffusitivity
- Compressive strength of cavity lining materials?
- Compressive strength
o High
o Around same as dentine is ideal
- Elastic modulus/ rigidity of cavity lining materials?
o Ideally same as dentine
o Teeth bend
o When tooth bends, liner should be able to bend with the tooth so that it will not debond
Marginal seal of cavity lining materials?
o Chemically bond to dentine ideally like GIC
o Impenetrable
o Permanent
should solubility be Low or high for cavity lining materials?
LOW
o Inside cavity is wet, dentinal fluid
o if Soluble, it will wash away when there is microleakage or dentinal fluid
what cavity liner is cariostatic
o Fluoride releasing GIC
o Antibacterial
o CaOH also produces alkaline pH which kills bugs
what material is most commonly used for pulp caps?
CaOH aka dycal
direct or indirect pulp cap
what does Setting calcium hydroxide do? (liner)
alkaline pH kills cariogenic bacteria
Irritates pulp, tertiary dentine laid down, calcification walls off cavity from pulp
does zinc phosphate or Zinc polycarboxylate bond to tooth
Polycarboxylate binds to the calcium in the tooth so there is chemical bonding, long chain bonding surface of the tooth to the zinc oxide
whats ZOE
- Zinc oxide eugenol
what is ZOE used for
- Base in deep cavities
acid base rxn formula for ZOE
Zinc oxide + eugenol = zinc oxide eugenolate matrix
what can ZOE materials not be used with
- ZOE materials cannot be used under composite resin materials, comp doesn’t set above ZOE, eugenol also softens comp and causes discolouration
what the advantage and disadvantage of eugenol being highly soluble
Eugenol disintegrates in contact with water bad
eugenol released can reduce pulpal pain good
what makes RMZOE different from zoe
resin gives stronger backbone to the set material
- Increase in compressive strength allows it to be a liner
- Low solubility compared to ZOE high solubility
what does eba stand for EBA ZOE cement
- Ethoxybenzoic acid
- EBA ZOE vs ZOE and RMZOE
- Stronger than ZOE and RMZOE
- Less soluble than ZOE and RMZOE
which material is the ONLY MATERIAL to seal dentinal tubules, preventing microleakage and sensitivity
GIC AND RMGIC
- Bond and Seals dentine!
- Bonds to composite resin over it
what should be placed over calcium hydroxide?
RMGIC
what is the ideal thermal diffusivity of all liners
all commercially available liners have similar or lower thermal diffusivity than tooth enamel
disadvantages of CaOH
low compressive strength
unstable
soluble
lining will disappear when there is microleakage and contact with moist dentine
what sort of reaction does zinc phosphate undergo?
acid base reaction
zinc oxide powder with phosphoric acid liquid
problems with zinc phosphate
low initial pH causes pulpal irritation
exothermic reaction
does not bond to tooth
final set takes too long 24 h
brittle
what is the solubility of GIC and RMGIC
- Less soluble than any other liners except zinc phosphate
is GIC or RMGIC more soluble?
GIC More soluble than RMGIC
functions of GIC
- Restorative fillings
- Lining
- Luting
2 types of GIC
original glass and polyacrylic acid
anhydrous freeze dried polyacrylic acid added to powder
what is added to gIc to speed up setting time without changing the working time?
tartaric acid
what makes GIC more radioopqaue
Lithium and strontium salt
aluminium
what makes gic more radiolucent
silica
what is the benefits of anhydrous gic compared to traditional gic?
anhydrous is easier to mix
for gic:
smaller glass particles = faster or slower setting time?
smaller = more rxn = faster settig time = more opqaue cement
higher molecular weight of acid in gic adv vs disadv
adv = better mechanical properties
disadv = harder to miix
3 phases of acid base rxn in GIC
dissolution
gelation
hardening
what happens during the dissolution phase in gics
a. Acid added to powder
b. Acid attacks glass surface
c. Calcium aluminium sodium and fluoride released from glass surface
d. Silica gel left around unreacted glass
what happens during the gelation phase in gics
a. Calcium ion that were liberated from the silicate glass crosslink with the polyacrylic chains
b. Bivalent calcium can link 2 chains
c. Or sometimes it links different sections of the same chain (ie not as much crosslinking as you would like)
d. Hard material but not fully set yet
when does gelation start?
e. Gelation starts as soon as you start mixing
what is the bonding in gelation vs hardening
gelation = earlier bonding of bivalent calcium
hardening = later bonding of trivalent aluminium
what occurs during hardening that makes gic much stronger?
a. Trivalent aluminium ions ensures good crosslinking of polyacrylic chains
b. Increases strength
c. But takes a few days to week
d. Improves mechanical properties of material
why do we need to keep GIC dry
- Keep dry
- Not set yet
- Aluminium ions could diffuse out
- Don’t get trivalent loading
- Saliva might cause absorption of water
- Weaken the material
how do we keep GICs dry?
i. Varnish
ii. Resins
iii. Vaseline BAD
iv. Best is using DBA and light cure
why should we Never condition surface with phosphoric acid before GICs?
Phosphoric acid removes the calcium on tooth surface that is needed for GIC to bond to
compare gics to composite
GIC have poorer
-tensile strength
-wear resistance
-hardness
-compressive strength
-higher solubility making it more susceptible to erosion by acids
how does GICs act as a fluoride reservoir
recharge fluoride, fluoride reservoir
Refresh fluoride content with toothpaste and mouthwash
what rxn occurs in rmgic
dual curing = acid base + polymerisation
tri curing = acid base + polymerisation + redox
what is the purpose of redox in tricuring of rmgc
light cure cannot penetrate super deeply
in case the light cure doesnt fully polymerize the acids, redox can also produce the free radicals needed for polymerisation to occurs
what are the materiasl in rmgic
glass similar:glass from silcate cement
acid
- HEMA resin
- polyacrylic acid with pendant methacrylate groups (C=C)
how is rmgic better than gic
- Improved physical properties, stronger
- Better aesthetics
- Better handling since theres light curing so no need to keep dry for entire duration
- Lower solubility
disadvantages of rmgic
- Polymerisation contraction
- Exothermic
- Swelling due to HEMA uptake of water
- HEMA toxic if not polymerised completely so might consider use of pulp cap
- Redox catalysts benzoyl iodides and bromides are cytotoxic
how long does gic take to fully set
about days to a week or longer
what happens if GIC is contaminated
aluminium ions diffuse out, no trivalent crosslinking
weaker bonding which will break or have poorer aesthetics
prevent contamination after gelation phase, use resin or dba to create a protective layer
how does gic and rmgics have a good marginal seal
they bond to teeth
the polyacrylic acid binds to the calcium in the tooth structure
what do you condition tooth with before gic
polyacrylic acid NOT phosphoric acid
