Dental materials Flashcards
Within the EU what body regulates the materials that come to market?
Medical devices directive
If the medical devices directive decides assessment of a material is favourable, what will indicate this?
A CE mark will be awarded
Give three examples of notified bodies and the number alongside the CE mark, that identifies them.
- TuV SuD Product Service (CE 0123)
- British Standards (CE 0086)
- Sgs prodcert (CE 0120)
Define each step of the three phases involve in a standardised transparent approach to material product selection.
Phase 1- regulatory and quality evidence
Phase 2- clinical trails
Phase 3- blinded assessment by restorative consultants
What specification does endodontic instruments fall under?
ISO 3630-1
In terms of physical properties of endodontic materials, define:
resistance force per unit of area of the material against deformation when an external force is applied, it is measured in newtons by square metres.
Stress
In terms of physical properties of endodontic materials, define:
the capacity of a material to withstand loads that are trying to elongate the material
Tensile strength
In terms of physical properties of endodontic materials, define:
stress the material can withstand without being deformed permanently.
Yield strength
In terms of physical properties of endodontic materials, define:
the change of the original dimension under stress of a material
Strain
In terms of physical properties of endodontic materials, define:
the greatest stress that an elastic solid can withstand without being deformed permanently
Elastic limit
In terms of physical properties of endodontic materials, define:
“The ability of a solid to undergo permanent deformation, a non-reversible change of shape in response to applied forces”
Plastic deformation
In terms of physical properties of endodontic materials, define:
breaking point of a material
Plastic limit
What are the two key properties of nickel titanium?
Super-elasticity and shape memory
What is the composition of stainless steel?
18% chromium
8-10% nickel
0.12% carbon
By what manufacturing process are hedstrom stainless steel files created?
Micro grinding of stainless steel block into shape of Endo-file
What are the advantages of manual stainless steel ?
- hard
- tortional effect
- cutting efficiency
- fracture resistant
- Low cost
- high corrosion resistance
What are the disadvantages of manual stainless steel used in Endo-files?
- rigid, so greater risk of procedural error
- less predictable canal shape after preparation
- incomplete debridement of canal
What is the composition of NiTi?
45% titanium
55% nickel
What are the three different crystal phases of NiTi? State which phase is an intermediate phase.
- Austenitic
- R-phase (intermediate)
- martensite
When using a NiTi rotary file for RCT, what crystal phase is the file mostly in before beginning treatment?
Austenitic phase
When using a NiTi rotary file for RCT, what crystal phase is the file mostly in when used in root canal treatment and subjected to stress which will produce a change in shape, volume and density of the file?
Martensitic phase
What are the two properties of NiTi alloys that allow for the change between Austenite to martensite and vice versa?
- thermoplasticity
- superelasticity
What crystal phase of NiTi is described:
Stable at high temperature and no stress
Austenitic phase
What crystal phase of NiTi is described:
Stable at low temperature and high stress
Martensitic phase
What will happen to a NiTi file if we increase the heat, or reduce stress on the file?
It will be in Austenitic phase where the file returns to its original shape (straight)
What will happen to a NiTi file if we lower the temperature, or increase stress on the file?
It will be in Martensitic phase, which will be more soft and flexible and this will be able to flex the file to fit into curved canals int he root.
Why are NiTi files unable to precurve?
Due to shape memory effect
What treatment would remove manufacturing errors in NiTi wires (e.g. grooves and chips)?
Electropolishing
Define, thermoplastic material that is formed around a crystalline structure.
Wax
What are the dental applications of wax?
- indirect modelling wax
- direct inlay wax
- joining components to facilitate union
- to make record blocks
What use does wax have as a modelling material in dentistry?
Used to form dental restoration or prosthesis
What are the three main advantages of using modelling wax in the construction of acrylic dentures?
- Withstands intra-oral temperature at try in stage
- Amendable to chairside/laboratory adjustment
- No residue in investment mould upon boiling out
What type of wax is used to make crowns/inlays/bridges?
Inlay wax
What are investment materials used for?
To encase the wax designed for the lost wax casting technique
What is another term for “the lost wax technique”?
Investment casting
What are the 5 main steps involved in the lost wax technqiue?
- Create wax pattern
- Sprue wax pattern
- Invest wax pattern
- Use furnace to burn wax pattern, creating a mould.
- Force molten alloy into mould
What is a refractory die?
“ any die material that is capable of enduring the high temperatures that occur during the firing or casting procedures.”
What is the purpose of refractory dies?
To support materials during processing (e.g. porcelain or ceramic veneers or cobalt chromium frameworks) that would deform/disintegrate otherwise.
Name 4 types of investment materials?
- Dental plaster or stone
- Gypsum bonded
- Silica bonded
- Phosphate bonded
Why would dental plaster or stone not be used for any alloy casting? And only used in the construction of acrylic dentures.
Because dental plaster and stone is not heat resistant
What investment material is most likely used in the construction of acrylic dentures?
Dental plaster
What investment material is most likely used in the construction of gold casting alloys?
Gypsum bonded
What investment material is most likely used in the construction of base metal casting alloys?
Phosphate bonded
What investment material is most likely used in the construction of gold casting alloys, base metal alloys, cast ceramics and glasses?
Phosphate bonded
What are the ideal properties of investment materials?
- accurate shape, size and surface detail
- high compressive strength
- compensate for casting shrinkage
- alloy compatible
What are the principal components for alloy, porcelain and glass casting?
- binder
- refractory material (usually silica)
What property of investment mould means that it will expand in the presence of water?
Hygroscopic
Which type of investment material lacks porosity?
Silica bonded
what is the temperature required for wax burn out in the “lost wax technique” of base metal alloys (includes co/cr)?
815-900 degrees
What are the 4 casting faults that can occur in metal casting?
- Finning and bubbling
- Incomplete casting
- Porosity in casting
- Over/under sized casting
What is meant by finning/bubbling of cast?
Extra appendages on casting due to investment mould cracking under forces
Why might incomplete metal casting occur?
- poor space design or failure
- alloy not sufficiently heated
- lack of force
Why does porosity (fault) occur in metal casting?
due to alloy entrapment of oxygen taken up by Cu, Au, Ag,Pt and Pd in alloys when molten
What base metal casting alloy is most often used for partial denture frameworks and complete denture bases?
Co/Cr
What base metal casting alloy is most often used for applications in crown and bridge?
Ni/Cr
What is the main action of Co and Ni?
Hardness and strength
What is the action of Cr?
Confers both hardness and corrosion resistance through passivating effect
What is the passivating effect?
Chromium when exposed to atmosphere becomes oxidised, this forms chromium oxide which protects against corrosion
What is the fusion temperature for both co/cr and ni/cr?
1200-1500 degrees Celsius
What are the most widely used wrought alloys for making instruments, burs and wires?
Steel and stainless steel
What two phases of steel exist below 723 degrees Celsius?
- ferrite
- cementite
What is a mixture of ferrite and cementite termed as?
Pearlite
On a phase diagram, what is the term for any alloy to the left of the eutectic alloy (X)?
Hypo eutectiod
On a phase diagram, what is the term for any alloy to the right of the eutectic alloy (X)?
Hyper eutectoid
Give an example of a hyper-eutectoid alloy that could be used in clinical applications?
Cementite
Give an example of a hypo-eutectoid alloy that could be used in non-cutting instruments such as forceps?
Ferrite
How does stainless steel differ from just steel?
Steel with the addition of chromium
What is the benefit of stainless steel?
Improved corrosion resistance by passivating effect
Additions of what metals to steel reduce the critical temperature of alloys?
Chromium and nickel
What form of stainless steel is not amendable to heat treatment as the critical temperature is below where atomic movements are possible?
18/8 stainless steel
What are the three grades of stainless steel where there is less than 0.15% carbon content?
- soft
- half hard
- hard
What two processes can join stainless steel alloys?
- soldering
- welding
When welding, what is temperature change proportional to?
I^2Rt
What do the letters stand for in I^2Rt?
I= current
R= electrical resistance
T= time current passes
Martensitic stainless steel contains Ni and Cr, true or false?
False. It does not contain Ni, only 12% Cr
What is Martensitic stainless steel used for?
Construction of probes and cutting instruments
What is impression compound?
“A mucocompressive thermoplastic (so it softens when heated and hardens when cooled) and rigid impression material”
What are the two presentations of impression compound?
- Impressions compound in a 5mm thick sheet
- Green stick
When might an impression compound in a 5mm thick sheet be indicated for use?
In a stock tray for impressions of:
- edentulous ridges
- free end saddles for partial dentures prior to alginate wash impression
When is green stick indicated?
Used for border extensions of impression trays
What are the components of impression compounds? And their functions?
- Thermoplastic material: characterises softening temperature
- Filler: gives body & decreases thermal contraction
- Lubricant: improves flow
How can impression compound be conditioned?
Place block of composition into 55-65 degree Celsius napkin/cloth lined water bath
Why should you avoid placing impression compound in lined water bath for a prolonged time?
Because stearin acid will leach out causing damage
Why is green stick used?
To extend peripheral border of extension trays
What are the 3 steps to conditioning green stick?
- Heat in flame
- Apply to tray
- Temper in water before use
What form of prosthetics impression material can be used for complete dentures?
Zinc oxide euganol impression material
How many paste presentations of zinc oxide euganol impression materials are there?
2
Describe the two paste presentations of zinc oxide euganol impression materials.
Paste 1:
- ZnO
- olive oil
- zinc acetate + water
Paste 2:
- euganol
- kaolin/Talc
Describe the setting reaction of zinc oxide euganol impression materials?
-OH group of euganol acts as a weak acid which undergoes acid base reaction with ZnO, this forms a salt of zinc euganolate +water
What type of special tray must be used for zinc oxide euganol impressions? And what spacer is required?
Close-fitted special tray with 1mm spacer
What does “Tg” stand for?
Glass transition temperature
What are the ideal physical properties of a denture base?
- appearance
- high Tg
- dimensional stability
- low specific gravity so that denture does not drop out of mouth
- high thermal conductivity
- radiopaque
What are the ideal mechanical properties of denture bases?
- rigid
- high flexural strength
- high fatigue limit
- high impact strength
- high abrasion resistance
What are the ideal chemical properties of denture bases?
- insoluble in oral fluids
- no uptake of saliva/water
What are the ideal biological properties of denture bases?
- safe to use
- non toxic
- non irritant
- none sustenance of bacterial or fungi growth
What are the ideal miscellaneous properties of denture bases?
- cheap
- long shelf life
- easily manipulated and processed
What materials constitute the powder component of denture base materials?
Polymer- PMMA beads
Initiator - benzoyl peroxide
Pigments- salts of Cd/Fe or organic dyes
What materials constitute the liquid component of denture base materials?
Monomer- MMA
Cross linking agent- ethyleneglycoldimetharcylate
Inhibitor- hydroquinone
Activator
Why is it very important to get the correct powder to liquid ratio when mixing and processing for acrylic denture base materials?
This is because the monomer shrinks, by achieving the correct ratio this reduces shrinkage
Describe the elemental composition of cobalt chromium alloy denture bases and the proportions each of these elements take up.
Cobalt: 35-65%
Chromium: 25-35%
Molybdenum: 4% or more
Nickel: 0-30%
What is the major alloy component of Co/Cr alloy denture bases?
Cobalt
What component of Co/Cr alloy denture bases hardens by solution hardening and undergoes the passivating effect?
Chromium
What components of Co/Cr alloy denture bases improves casting, handling and mechanical properties?
- molybdenum
- nickel
What is the Co/Cr alloy fusion temperature?
1200-1500 degrees Celsius
What is the purpose of a tissue conditioner?
It is temporarily applied to fit surface of denture to permit a more equal distribution of load and assumption of a more normal mucosal position.
Give an example of when a tissue conditioner would be used?
In an immediate denture a few weeks after extraction, following infection of denture bearing area to adapt fit to healing tissues.
What is the purpose of a functional impression material?
Applied to fit surface of denture to allow a more definitive impression to be taken under functional conditions
What are the three purposes of a permanent resilient lining (soft lining)?
- It is applied as a “shock absorber” to avoid pain from movement of the denture upon nerves (e.g. mental nerve)
- avoids pain from movement of obturator/facial prosthesis against bearing structures
- useful to utilise undercuts by flexing in and out
Describe the properties of a tissue conditioner once set?
Soft but not capable of undergoing substantial deformation (elastic)
Describe the properties of functional impression material once set?
Soft but deforms plastic ally to record impression of denture bearing area or neutral zone
What property do most tissue conditioners and functional impression materials share that means they flow under pressure but are resilient under dynamic forces such as chewing?
They are visco-elastic
What are the two different presentations of tissue conditioners and functional impressions materials?
- powder/liquid
- preformed sheets of acrylic gel
What are the 4 types of resilient lining materials?
- Heat cured acrylics
- Self cured acrylics
- Heat cured silicones
- Self cured silicones
What are the disadvantages of using silicone as lining material?
- Silicones are susceptible to Candida albicans growth
- Difficult to achieve bond between silicone and PMMA denture base (peels away)
What is the correct thickness required for a resilient lining material?
2-3mm
What two materials can be used to make artificial teeth?
- acrylic
- porcelain
Why is porcelain produced in moulds that are 30% larger than required?
Because Porcelain shrinks when its fired
How are acrylic teeth retained to denture base?
By chemical interaction
How are porcelain teeth retained to denture base?
Via holes or pins
Which material used to make artificial teeth has a more realistic appearance?
Porcelain
What are the disadvantages of porcelain artificial teeth?
- click on chewing
- transmit greater load to denture bearing area
- more likely to chip and fracture
What compound control the setting rate of alginate?
Trisodium phosphate
What is the melting point of paraffin wax?
48-70 degrees Celsius
If microcrystalline is added to paraffin wax, how does this change the melting and softening point?
It increases the melting point (65-90 degrees Celsius) and lowers the softening point
What are the two major causes of porosity?
- Polymerisation shrinkage associated contraction porosity
- Gaseous porosity (volatilisation of monomer)
Why does contraction porosity occur in PMMA dentures?
Because the monomer contracts by some 20% of its volume during processing
How does gaseous porosity occur?
If the temperature of the resin exceeds 100 degrees Celsius before the polymerisation process is completed then a gaseous monomer will be formed - which is the cause of gaseous porosity.
Why must polymerisation of acrylic dentures be carried out slowly and under pressure?
To prevent gaseous porosity and to avoid contraction porosity
