dms Flashcards
which alloy is more rigid?
alloy a
most rigid = steepest gradient
Which alloy has the highest Compressive Strength?
alloy b
The strongest alloy (with the highest compressive strength) by definition, is the one with the endpoint of its curve at the highest value (MPa)
Which alloy would be best for use when creating clasps for use around teeth with large undercuts > 0.4mm?
Alloy B
What property is this metal showing which makes it the best choice?
For an alloy appliance, being able to make minor adjustments to it at the chairside to ensure a comfortable fit is important, so B’s ductility makes it better.
What is cold working, how does it work and how does it improve the function of the metal once in use?
- Work done on the material at low temperatures
- Causes slip, moving all dislocations to collect at grain boundaries
- Changes the shape of the lattice structure
- Increases elastic limit, fracture stress and hardness
- Decreases ductility and impact resistance
Crucially this minimises the quantity of defects in each grain ensuring each grain (hence the alloy as a whole) is as resistant to fracture as it can be.
What is the elastic limit of an alloy?
The maximum stain/stress that can be applied to a material before causing permanent deformation
Which material, commonly used for making clasps for partial dentures, has the highest elastic limit?
stainless steel
What implications does this have for the depth of undercut this material can engage?
this material can be used for larger undercuts
In addition to the material from which a clasp arm is made, give TWO other factors which influence a clasp’s efficiency.
retention form
occlusal rest
design - proper use of undercuts
right amount of tension
a) Setting reaction of GIC. Describe what happens at each stage? (6)
i) Dissolution
ii) Gelation
iii) Hardening
(i) DISSOLUTION - Acid reacts into solution and release ions, leaves silica gel
- Poly acid attacks glass to release Ca2+, Al3+ and Fe- ions which react with polyions to produce a salt-gel matrix (around un reacted glass particles)
(ii) GELATION
- Initial setting stage: Poly acid cross-links with Ca2+ ion via chealation with carboxyl groups
(iii) HARDENING - Ca reaction with polyacrylic chains
- Trivalent Al3+ ensures effective cross-linking.
- Also stabilizes reaction to INC molecular strength and weight.
Regarding properties why is RMGIC better than GIC? (4)
resin adds extra strength and bonding
improved mechanical properties: higher fracture toughness, compressive strength, and wear resistance compared to GIC
better bonding strength: RMGIC has better bonding strength to tooth structure than GIC due to the resin component, which allows for stronger adhesion to enamel and dentin.
faster setting time: RMGIC sets faster than traditional GIC
increased esthetics: RMGIC is available in a wider range of shades than GIC, which allows for better color matching to natural teeth
c) Give and describe an advantage and disadvantage of GIC vs Composite. (4)
- better bond strength (BRITTLENESS of GI limits bond strength)
- better wear resistance (less brittle)
- easy handling (set on demand to extend working time)
- better aesthetics (wider range of shades)
Role of Tartaric acid? (1)
cross-linking agent -> promotes COVALENT BONDS BETWEEN RESIN AND FILLER particles
aid polymerisation to initiate and accelerate the reaction
acts as a chelating agent
a. Amalgam – define each and how they affect potential as restorative material (9 marks)
i. Creep
ii. Hardness
iii. Rigidity
is a mixture of metals, typically composed of silver, tin, copper, and mercury.
Creep: Creep is the tendency of a material to DEFORM SLOWLY over time under a constant load.
HIGH CREEP
material can shift and change shape in the tooth cavity, potentially leading to leakage and failure of the filling.
Hardness: a measure of a material’s RESISTANCE to deformation, scratching, or abrasion.
HIGH HARDNESS which makes it durable and able to withstand the forces of chewing and biting.
BUT can also make it difficult to manipulate during placement and shaping.
Rigidity: ability to RESIST DEFORMATION under load. LOW rigidity compared to other restorative materials such as ceramics or composites.
advantageous when need a DEGREE OF FLEXIBILITY is needed to avoid damage to the tooth structure.
How is RMGI superior to amalgam (2 marks)
in terms of aesthetics and adhesion
RMGi can match adjacent teeth
RMGI is able to form a chemical bond with a tooth structure, providing BETTER ADHESION
Why would lining materials be used with amalgam? (3 marks)
protect the pulp = creates a barrier to prevent from conduction of heat and cold
improve the seal = reduce micro leakage
additional support = if remaining tooth structure is weak
What advantage of GI cement?
fluoride release
glass ionomer fact sheet
POWDER- silica, alumina, Ca, Al phosphate
LIQUID - POLYACRYLIC ACID WITH TARTARIC ACID
Dissolution - Polyacrylic acid react with glass, release of Ca,Al,Mg,Na leave silica gel
Gelation - Ca chelation with Carboxyl group cause crosslinks
Hardening - Al react with Carboxyl cross crosslink increase strength. (20Mpa)
Properties - poor handling, poor strength, poor aesthetics, good fluoride release
Describe 2 advantages of Cobalt Chromium
- better finish = easy to polish
- harder (more resistant to scratching and indentation) = better wear-resistance
- very strong in thin section
- cheap
- low corrosion
Disadvantages of Cobalt Chromium compared to type 4 gold
- more difficult to produce defect-free casting
- difficult to adjust at chair-side
- polishing/ finishing = more time consuming
Advantage of Type 4 gold over cobalt chromium
- easier to produce defect-free casting
- zinc incorporation prevents oxidation (tarnish)
- better ductility (elongation) = easier to adjust at chair-side
- less shrinkage
- finishing and polishing less time consuming
Advantage of Type 4 gold over cobalt chromium
- easier to produce defect-free casting
- zinc incorporation prevents oxidation (tarnish)
- better ductility (elongation) = easier to adjust at chair-side
- less shrinkage
- finishing and polishing less time consuming
Components of stainless steel advantages & disadvantages
- Iron, carbon, chromium, nickel
- Chromium: corrosion resistance
- Nickel: causes the critical temperature to be lowered
Advantages:
* High proportional limit
* High tensile strength
* Ability to be hardened by heat treatments
Disadvantages:
* Susceptibility to corrosion
Work hardening & know what it means
- Cold working
- Strengthening of a metal by plastic deformation which occurs because of dislocation movement
Benefits of increased ductility & increased springiness
- Stainless steel is used in long clasps to engage 0.75mm undercuts, & used as wire in a splint
- It must therefore be flexible and have the ability to undergo prolonged periods of stress without subjecting to permanent deformation
- Increased ductility would allow wires and clasps to bend without fracturing
- Increased springiness allows SS to undergo large deflections without permanent deformation when it is placed under stress as it will be regularly.
State why dentine bond strengths to composite may be less than ideal in the abrasion cavities on root face dentine
(2 Marks)
- Tertiary dentine may be present and this has less or no tubules so there is no penetration to form a hybrid layer
- There is no enamel on root surface which forms a better bond with composite
- Usually a smaller cavity which provides less surface area to bond with
- Damage via abrasion can cause peritubular deposits which prevents penetration
Composite resins are subject to polymerisation contraction shrinkage. State two methods of reducing the stresses associated with this effect.
- Ensure the composite is placed in increments with each one only bonding to one cavity wall which reduces the configuration factor (ratio of bonded to unbonded surfaces)
- Ensuring a good bond between the tooth surface and the composite by sufficient etching and bond material helps prevent polymerisation contraction shrinkage
- Use a composite with higher filler as this provides less shrinkage
A lining has now been placed. What is the most suitable material to use as a lining in this situation and give two advantages why?
(3 Marks)
Calcium Hydroxide
biocompatible = no adverse reactions
stimulates tertiary dentine formation
A lining has now been placed. What is the most suitable material to use as a lining in this situation and give two advantages why?
(3 Marks)
Calcium Hydroxide
biocompatible = no adverse reactions
stimulates tertiary dentine formation
Why might dentine be unsuitable for bonding resin cement too? (2 Marks)
- moisture content
- collagen matrix
When placing direct restorations in teeth, it is often necessary to place a lining prior to the definitive restorative material.
- What is the purpose of a lining material?
(3 marks)
protective layer between the tooth structure and restorative material
pulp protection
dentinal sealing = for exposed dentinal tubules
improved adhesion
Glass ionomer materials are routinely used as linings below silver amalgam restorations. These materials can be either conventional or resin modified glass ionomer.
- What are the advantages of Resin Modified Glass ionomer when used in this context?
(4 marks)
improved bond strength = improved adhesion
reduced post-op sensitivity
improved handling = increased flowability
aesthetics = can match a natural colour
In what types of cavities could the use of glass ionomer, as a permanent restoration, be appropriate in adult patients? (1 mark)
non-load bearing cavities = buccal on posterior teeth??
Give the advantages of glass ionomer materials over composite resin as a permanent restorative.
(5 marks)
gi chemically bonds to the tooth structure
gi releases fluoride
gi is more biocompable
gi has low polymerisation shrinkage
gi is easier to place, higher compressive strength, low sensitivity to moisture
One of the disadvantages of glass ionomer materials is their relatively poor aesthetics. How are the aesthetic properties altered by:
a) The silica content of the glass ionomer powder?
(1 mark)
increased silica = more translucent = improved aesthetics
b) The size of the glass particles in the glass ionomer powder?
(1 mark)
smaller particles = more polished surfaces = more translucent
larger = more rough and irregular = more opaque
Outline briefly two drawbacks of CoCr
- Low ductility. This means it does not have a great ability to deform under stress and has a very high YM; clasps are not flexible and can’t fit an undercut >0.25mm
- CoCr is difficult to adjust once made, as it work hardens rapidly. It therefore requires precision casting when first made.
- Aesthetics: The colour means the denture cannot be easily hidden is the base covers the palate
List 5 key differences between CoCr and type IV gold alloy
- CoCr is less dense than type 4 gold – will be more defects, however it will be lighter.
- CoCr has a higher hardness, this will make it harder to polish but effects will last.
- CoCr has a lower ductility than type 4 gold. Cannot deform under stress
- CoCr has a higher modulus of elasticity (YM)
- CoCr has a higher shrinkage
Definition of flowable composite
- Lower filler content, therefore less viscous than conventional composite
- Used for filling pit/fissure system, small fractures & luting agent
- Higher polymerisation shrinkage
- Lower fracture strength
- Place with fibre ribbons
Wettability of flowable composite
- Adhesion requires an intimate contact between the adhesive material and the substrate.
- To obtain a maximum contact area between a liquid adhesive and a solid surface, the substrate must show evidence of high wettability with regard to that liquid (hydrophilicity of dentine & material)
- Using acid etch technique
- Opportunity for excellent bond between composite & enamel
Light curing benefits
- Extended working time
- Less finishing
- Less waste
- Less porosity
Criticism for using flowable composite
- Reduced mechanical properties
- Lower availability of shades
- More difficult to sculpt due to decreased viscosity
- Discolouration over time
If a dentist uses one 6mm increment will this suffice?
- No, 20 seconds to cure 2mm.
- Larger than 2mm increments will result in an under polymerised base
Which direct restorative material relies on chelation between carboxyl groups and the calcium in the tooth as a major component of the bonding mechanism?
glass ionomer
what material would be most appropriate for:
i) 0.25 mm undercut
ii) 0.5 mm undercut
iii) 0.75 mm undercut
0.25mm - CoCr
0.5mm - Gold (Au)
0.75mm- Stainless Steel
what is polymerisation shrinkage?
occurs following the conversion of monomer molecules to a polymer structure through the replacement of van der Waals spaces with covalent bonds, leading to a decreased free volume
coefficient of thermal expansion?
the extent to which a material expands upon heating
creep?
the slow plastic deformation that occurs over time
due to application of static or dynamic force over time, after the material set
elastic modulus?
a measure of the rigidity of a material, defined by the ratio of stress to strain (below elastic limit).
fatigue?
when cyclic forces are applied, crack might happen by small increments
in certain time the force will result in # through the remaining material.
hardness
resistance to penetration.
resilience?
the energy absorbed by a material undergoing elastic deformation
stiffness ?
an indication of how easy it is to bend a piece of material without causing permanent deformation or fracture.
depends upon elastic modulus, size, and shape of the specimen.
strain?
change in size of a material that occurs in response to a force
the change in length divided by the original length.
stress
internal force per unit cross-sectional area acting on the material
can be classified according to the direction of the force: tensile (stretching), compressive or shear
thermal conductivity
ability of a material to transmit heat.
thermal diffusivity
rate at which temperature changes spread through a material.
toughness
the amount of energy absorbed up to the point of #.
a function of the resilience of the material and its ability to undergo plastic deformation rather than #.
wear
the abrasion (mechanical or chemical) resistance of a substance.
wettability
ability of one material to flow across the surface of another, determined by the contact angle between the two materials and influenced by surface roughness and contamination
the contact angle is the angle between solid/liquid and liquid/air interfaces measured through the liquid.
yield strength
(or elastic limit.)
the stress beyond which a material is permanently deformed when a force is applied.
