DMS Flashcards
Define fracture
large force causes catastrophic destruction of materials structure
Define hardness
ability of surface to resist indentation
Define abrasion
material surface removal due to grinding. Loss of material surface layers leads to a roughened surface
Define abrasion resistance
The ability to withstand surface layers being removed.
Define fatigue
repetitive small forces cause the material to fracture. Most failures are not due to the application of a single load. When repeated loads are applied, small flaws in a material propagate allowing fracture when only a relatively small force is applied
Define creep
gradual dimensional change due to repetative small forces
Define deformation
an applied force may cause a permanent change in the materials dimension (but not fracture it)
Define de-bond
applied forces sufficient to break material to tooth bond
Define impact
Large, sudden forces causing fracture
List 8 mechanical properties
Hard, strong, rigid, ductile, soft, weak, flexible, brittle
List 3 chemical properties
setting mechanism, setting time, corrosive potential
List 5 physical properties
viscocity,thermal conductivity, thermal expansion, density, radiodensity
What kind of material is alginate?
an elatomeric irreversible hydrocollid
what is alginate composed of?
ester salts of alginic acid (either sodium, potassium or triethanolamine alginate)
Give a positive and negative point on using porcelain in fixed prosthodontics
positive - porcelain has good aesthetics
negative - microcracks tend to form at the fitting surface, making it prone to mechanical failure
Name some characteristics of porcelain
rigid, hard, strong, low tensile strength, tendency to form surface defects, brittle
Why are alloys used to support porcelain in crowns?
Metal limits the strain that porcelain experiences
describe cold working/work hardening
- producing a change in microstructure, with dislocations becoming concentrated at grain boundaries, but also a change in grain shape
- the properties of the material are altered, becoming harder and stronger with a higher value of yield stress
- ductility is decreased
Give three examples of hard working
- the formation of wires - an allow is forced through a series of circular dies of gradually decreasing diameter, resulting in a fibrous grain structure responsible for the springy properties possessed by most wires
- the bending of wires or clasps during the construction and alteration of appliances
- the swaging of stainless steel denture bases
what is swaging?
bending an alloy to the required shape
what are the components of steel?
- iron
- carbon (less than 2% otherwise steel will be very brittle)
- chromium (improves corrosion resistance)
- Nickle (improves corrosion resistance and strength)
what is autenite?
the solid solution of carbon above 723 degrees C
what is martensite?
it is formed when the austenitic solid solution is quenched.
It allows insufficient time for the alloy to undergo the transition from austenitic structure to pearlite structure.
This produces the very hard brittle steel; martensite
The brittleness can be reduced by tempering
Give two advantages of using stainless steel as a denture base?
- it has high values of modulus of elasticity and proportional limit achieving sufficient rigidity
- it conducts heat rapidly, ensuring the patient retains a normal reflex reaction to hot/cold
Give three requirements of stainless steel wires
- spring back ability
- value of stiffness to allow application of a suitable force for tooth movement
- easily joined by soldering or welding without impairing mechanical properties
define springiness of a wire
the ability to undergo large deflections without permanent deformation
spring back potential = yield stress/modulus of elasticity
What are acrylic resins composed of?
A combination of powder and liquid POWDER; - polymer; polymethylmethacrylate beads - initiator; a peroxide such as benzoyl peroxide - Pigments; salts of cadmium, iron or organic dyes LIQUID - monomer; methylmethacrylate - cross linking agent - inhibitor - activator (in self cure materials)
With acrylic resin, what are the stages in curing (acrylic polymerisation)?
- Activation; of inhibitor to provide free radicals
- Initiation; free radicals break C=C bond in monomer and transfer free radical
- Propagation; growing polymer chain
- Termination; of polymerisation
Name five ideal properties of acrylic
- dimensionally accurate and stable in use
- high softening temperature
- unaffected by oral fluids
- thermal expansion
- low density
- high thermal conductivity
- radiopaque
- non toxic
- colour/translucency
- easy and inexpensive to manufacture
- easy to repair
What are the ideal mechanical properties of acrylic?
- High Youngs modulus
- high proportional limit
- high transverse strength
- high fatigue strength
- high impact strength
- high hardness, abrasion resistance
Define free radical addition polymerisation
the chemical union of two molecules either the same or different to form a larger molecule without the elimination of a smaller molecule
Name five properties of acrylic
- non toxic
- non irritant (if no monomer released)
- unaffected by oral fluids
- mechanical properties are poor (increase in bulk to compensate)
- Fatigue/impact strength; fairly resistant
- High hardness/abrasion resistance
- thermal expansion OK
- Low thermal conductivity
- Low density
- high softening temperature
- dimensionally accurate and stable in use
Give the advantages and disadvantages of heat cured vs self cured acrylic
HEAT CURED - higher molecular weight; stronger - curing process may cause porosity SELF CURED - higher monomer levels (irritant) - fits cast better but water absorption in mouth makes it oversized - poorer colour stability
What materials are used for primary impressions?
— Impression compound:
i. Non elastic material which records poorer
surface detail and this is why it isn’t used for
definitive impressions. It isn’t cheap to use and
can be messy.
ii. It is good for primary impressions due to the
high Mucocompressive viscosity
— Alginate:
i. Elastic material which means it will flow into
undercuts and can record surface detail for
primary impressions.
What materials are used for definitive impressions?
— Silicone:
i. It is dimensionally stable and can be made into
a variety of consistencies which means it
accurately record surface detail
— Polyether/impregum:
i. It is dimensionally stable and has very accurate
recording of surface detail
— Alginate:
i. Can also be used for master impressions as if is
elastic so can flow into undercuts if they are
present.
What is gypsum?
- Gypsum plaster is calcium sulphate dihydrate which is used to
cast a positive replica of an oral cavity. - Type 1 plaster/impression plaster is used in making primary
impressions of edentulous oral cavity in complete denture
fabrication. - Type III dental stone is an alpha form of calcium sulphate
hemihydrate which is used to make master casts
Describe three properties of cobalt chrome that make it a suitable alloy for use in a partial denture
- rigid
- component parts integral to base
- high abraision resistance
Give two drawbacks to cobalt chrome as a denture base
- Difficult to repair
- difficult to add more teeth
List four types of elastomeric impression materials
- addition silicones
- polyethers
- polysulphides
- condensation silicones
What are some dental investment materials?
- Dental stone or plaster
- Gypsum bonded materials
- Phosphate bonded materials
- Silica bonded materials
What are the requirements of an investment material?
- Expand; compensate for cooling shrinkage of alloy
- Porous; allow escape of trapped gases on casting
- Strong at room temp; ease of handling
- Strong at casting temp; withstand casting forces
- Smooth surface; easy finishing
- Chemically stable; porosity, surface detail
- Easy removal from cast; technician time
- Handling; not complicated
- Relatively inexpensive; destroyed
How are elastomeric impression materials formed?
- Formed by the polymerisation with cross linking of polymer chains
- Cross linking generates elastic properties. Causes fluid > solid transition
- Polymerisation may produce by-products (H2O, H2, alcohol) which affect dimensional stability and cast compatibility
Name two elastomeric impression materials
- Conventional silicone
- Addition cured silicone
- Hydrophilic silicone (incorporate non-ionic surfactant; wets tooth surface, more easily wetted by water containing die materials)
- Polyethers (impregum)
What are the ideal properties of an elastomeric impression material?
- Low viscosity; interaction between material and tooth/soft tissue
- Low contact angle/good wettability; must make intimate contact with teeth/mucosa
- High elastic recovery
- Good dimensional stability
- High tear resistance
Give some positive and negative points of PMMA as a temporary material
- Powder and liquid formulation
- Self curing
- Good marginal fit
- Good transverse strength
- Polishable
- Poor abrasion resistance
- High shrinkage
- High thermal release
- Free monomer may be toxic
What is the significance of temperature change during the setting of temporary materials?
There are some studies that suggest a link between thermal change and pulpal damage.
- 2o increase; no effect on pulpal histology
- 5.5o increase; significant tissue changes over first few days. Most pulps overcome trauma
- 11o increase; two thirds suffer irreparable necrosis.
- Pulpal damage is also affected by the duration of the temperature change.
Why is low viscocity important for a luting agent?
To allow seating of the restoration without interference.
- Viscocity increases as the material sets, so restoration must be seated quickly and pressure maintained.
- ideal film thickness is 25 microns or less
Name some ideal physical properties of a luting agent.
- Good marginal seal (chemically bonded to tooth and restoration)
- Good aesthetics
- Low solubility
- Cariostatic (fluoride releasing, antibacterial)
- Biocompatible; non toxic, not damaging to pulp
What are some ideal mechanical properties of a luting agent?
- High compressive strength
- High tensile strength
- High hardness value
- Youngs modulus similar to tooth
What are some different types of luting agent?
- Dental cement
- Glass ionomer cement
- Composite resin luting ageng
Describe a silane coupling agent
- Applied to the etched porcelain surface, ideally as a mono-layer
- Very strong bond between oxide groups on the porcelain surface and the silane
- The other end of the silane molecule has C=C which reacts with the composite resin luting agent
- This works in the same manner as a dentine bonding agent does with tooth
How does a composite luting agent bond to an indirect composite restoration?
- composite bonds to composite
- Bond strength is lower to inlay fitting surface than to new composite
- Bond is micromechanical to rough internal surface of inlay.
- Bond is also chemical to remaining C=C on the fitting surface of the inlay
- Use dual cure cement as light penetration through the inlay will be poor
How does a composite luting agent bond to porcelain?
-The porcelain is etched with hydrofluoric acid to produce a rough retentive surface but is still not hydrophobic and compatible with composite resin luting agents. A surface wetting agent is required
How does a composite luting agent bond to metal?
- Metal surface must be roughened by etching or sandblasting.
- Sandblasting does not give the same undercut surface as etching and so chemical bonding is required
Describe a self adhesive composite resin luting agent
- The metal coupling agent is incorporated into the composite resin
- This simplifies the bonding process
- MDP is used in panavia
- This is an anaerobic self cured material
- Good film thicknes
- Opaque
- Moisture sensitive
- Expensive
