Dental Materials Flashcards
What is fibre reinforcement?
Fibres embedded into a matrix (acrylic or resin) alongside an effective coupling agent (e.g. resin bonding agents, silanes) between the fibres
What are the 4 different fibre types used in fibre reinforced materials
- Carbon fibres
- Kevlar fibres
- Polyethylene fibres
- Glass fibres
What is a disadvantage of carbon fibres?
Black - not aesthetic
Which 2 fibre types are most commonly used in dentistry due to their translucent properties?
Polyethylene and Glass fibres
What are the properties of fibre-reinforced composite?
Biocompatible
Easily produced
Cost-effective
Improved flexural strength and fatigue resistance
Increased toughness and impact strength
Superior to metallic materials in terms of weight:strength ratio and modulus:weight ratio
Why are the benefits of fibre-reinforced composites only short-lived?
As resin absorbs water over time by diffusion.
This results in hygroscopic expansion of the resin which strains the fibre resin interface causing decreased flexural properties and failure of the material.
What is an ideal fibre weight (%) for a fibre-reinforced material?
5%
Must have sufficient fibres but not too many
Too many fibres can lead to abrasive and adhesive wear plucking
What are the clinical uses of fibre reinforced materials?
Acrylic dentures - to improve fatigue resistance and impact strength
Temporary bridges, periodontal splinting, and posts
Elimination of metal in PFM restorations - improves aesthetics and better biological tolerance
Why is pure gold no longer used as a restorative filling material?
Very time-consuming process
Insufficient rigidity and elasticity in high-stress situations
What are the 4 types of traditional casting cold alloys?
Type 1: LOW STRENGTH
- for castings subject to slight stress (e.g. inlays)
Type 2: MEDIUM STRENGTH
- for castings subject to moderate stress (e.g. inlays/onlays)
Type 3: HIGH STRENGTH
- for high-stress situations (e.g. onlays, thin cast backings, pontics, full crown)
Type 4: EXTRA HIGH STRENGTH
- for casting thin in cross section (e.g. saddles, bars, clasps, crowns, bridges, and partial dentures)
What is the composition of traditional casting gold alloys?
Au - decreases in stronger/more durable alloys
Cu - increases more in durable alloys
Ag - constant throughout different alloy types
Zn - constant throughout different alloy types
Pt/Pd - not used in type 1
From the % composition table of traditional gold allows, what can be evaluated as you move from type 1 to type 4?
As you move from type 1 to type 4 there is:
- Increase in hardness, proportional limit, and strength
- Decrease in ductility and corrosion resistance
Why does the harness increase as gold content decreases?
Due to solution hardening - the formation of solid solutions with gold
What effect does Ag have in traditional casting gold alloys?
Slight strengthening effect, and counteracts the red/copper tint.
What effect does Cu have in traditional casting gold alloys?
Increases strength and lowers MP
What will happen if the Cu content extends >16%?
The alloy will tarnish
What effect does Pt/Pd have in traditional casting gold alloys?
Increases both strength and MP of alloy
What effect does Zn have in traditional casting gold alloys?
Acts as a scavenger
What happens when all of the Zn runs out?
Once used up, its function is taken over by Cu
This is undesirable as it impairs the physical properties of the casting. Always have fresh alloy in the button to prevent this from occurring.
Which types of traditional gold casting alloy can be heat treated and which types cant?
Type 3/4 CAN be heat treated
Type 1/2 CANNOT be heat treated due to the lack of Ag and Cu that are necessary for atomic diffusions
What does heat treatment of type 3/4 traditional gold casting alloys result in?
Further hardening and a formation of a new crystalline structure
What must you do to avoid premature precipitation and order hardening of type 3/4 traditional casting gold alloys following heat treatment?
To avoid precipitation and order hardening, cool the casting rapidly from an excess of 600*C (red sprue indicates this)
Once the red colour is lost through cooling, plunge into cold water - this disintegrates the investment resulting in a fine grain structure
What is the risk associated with cold water cooling of a traditional casting gold alloy containing Pt/Pd?
And how might you eliminate this risk?
Coring (undesirable)
Eliminate this and improve corrosion resistance by carrying out homogenisation heat treatment - heat to 700*C for 10 minutes, quench
What colour are low gold content alloys?
White
What is the difference in properties between silver palladium alloys and gold alloys?
Silver palladium alloys have a lower density and ductility compared to gold alloys affecting castability
Silver palladium alloys may dissolve oxygen causing porosity unlike gold alloys (undesirable)
What are ceramics?
Composite materials
What are the properties of ceramics?
Potentially aesthetic
Prone to crack propagation
Brittle
How do ceramics lose strength over time?
Si-O bonds undergo alkaline hydrolysis from solubilisation of Na2O and K2O into hydroxide form leading to static fatigue.
This also occurs to filler particles in resin composites contributing to wear.
What are the 3 classes of ceramics?
- Predominantly glassy materials - high aesthetics
- Particle filled glasses
- Polycrystalline ceramics - high strength
What is the structure of the filler particles in particle filled glass ceramics?
Usually crystalline structure
What filler particles are present in particle filled glass ceramics to improve mechanical properties, opalescence, colour and opacity?
Leucite - dispersion strengthening
Al2O3 - etches at greater rate facilitating micro-mechanical attachment
What are the properties of polycrystalline ceramics?
Have no glassy component
Atoms packed in dense arrays - tougher/stronger than glass ceramics, inhibits crack propagation
Relatively opaque - unlike metals they transmit some light
Multilayered - different levels of flexural strength/translucency
How are polycrystalline ceramics manufactured?
By pressing them into oversized die or machined from block and sintered
What are ceramic trial pastes composed of?
Glycerine
Water
Titanium dioxide
Amorphous silica
Polyethylene glycol pigments
What is the most advantageous property of trial pastes?
They exhibit shear thinning - can be removed easily as viscosity changes
What must occur prior to cementation of the ceramic?
Silanisation of the fit surface of the ceramic
What happens to silane in the presence of oxygen?
It will degrade
Why must you NOT use conventional acid base cements with ceramics ?
As its etchant effect can exacerbate flaws
There is only one type of polycrystalline ceramic that can be used with conventional acid base cements.
Name this type of ceramic
Zirconium Oxide based ceramics
What type of acid is used to etch Feldspathic based ceramics?
Hydrofluoric acid
What can be used as an alternative to etching feldspathic based ceramic with hydrofluoric acis?
Can be etched with air abrasion
What do alumina based glass infiltrated ceramics and alumina oxide crown copings require prior to cementation?
Tribochemical silica coating (3M)
What is porcelain made up of?
Clay (Kaolin), quartz (sand), and felspar
What are the properties of traditional dental porcelain?
Hard - risks wear of opposing teeth
Relatively resistant to chemical attack
Good thermal insulator
How is traditional dental porcelain classified?
High fusing - 1300-1400C
Low fusing - 850-1100C
How do high-fusing dental porcelain and low-fusing dental porcelain differ in composition?
High-fusing dental porcelain contains 0% glasses and 4% clay
Whereas low-fusing dental porcelain contains 15% glasses and 0% clay
What forms on repeated heating of the glass component within the traditional dental porcelain?
Leucite
How does leucite containing dental porcelain compare to normal dental porcelain?
Leucite containing porcelain has a higher co-efficient of thermal expansion
Why would the manufacturer add leucite to dental porcelain?
To have a slightly higher expansion/contraction than the underlying alloy so that the porcelain is in tangential compression - this prevents crack propagation within the ceramic
What is Feldspar?
A mixture of aluminosilicates
- Potassium aluminosilicate
- Sodium aluminosilicate
Why might particles of metal oxides be added to dental porcelain?
To promote light scattering and colouration (opalescence)
How is traditional dental porcelain built up?
Powder mixed with water
Built up to desired shape on metal foil/core
Compaction
Firing
Re-firing
Apply glaze (low MP glass) to completed build-up
How might you limit/stop crack propagation of the dental porcelain?
Limiting crack propagation:
- aluminium core
- add alumina powder to porcelain - opaque
- sintered alumina core +/- zirconium oxide
- metal core - PFM
Stopping crack propagation:
- compressive forces stop crack propagation
- low MP glass glaze helps by tensioning surface
Why does the porcelain in a PFM contain leucite?
As leucite increases the coefficient of thermal expansion so that the metal core and porcelain are better matches
What are the main benefits of PFM over traditional dental porcelain?
Reduces stress concentrations, lowers risk of crack propagation
What are the 4 available PFM alloys?
High gold alloys
Low gold alloys
Silver palladium
Nickel chromium
What are the 2 alternatives to PFM alloys?
Captek system
CAD-CAM
What does the Captek system utilise?
Capillary technology
What 5 systems are in place to avoid ceramic shrinkage during firing?
- Pressed ceramic powder/polymeric binder that expands and crystallises to fill lost wax mould
- Casting of special glass (Glass Ceramic) into a lost wax mould followed by heat treating
- Pressing solid ingots of filled glass (leucite/lithium disilicate) into a lost wax mould
- Lightly sintered Aluminium Oxide (&later magnesium aluminate spinal & Zirconia/alumina) to form touching necks between touching particles and infiltrate with glass
- CAM
How do you maximise the clinical durability of porcelain?
Maximum occlusal thickness
Highest elastic modulus substrate possible for the core
Bond the restoration
- ceramic-cement
- cement-tooth/substructure
Develop broad NOT pinpoint occlusal contacts
- spread the load and avoid stress concentrations
What are the 3 allotropes of Zirconia?
Cubic
Tetragonal
Monoclinic
Name 6 ceramic products
- Porcelain jacket crown
- Veneer
- Dentine bonded crown
- Hot-pressed / injection moulded ceramic crown
- Crown with improved coping strength
- CAD-CAM crown
What is a a porcelain jacket crown?
Feldspathic porcelain with added alumina
Why are porcelain jacket crowns no longer made?
As they are historic and prone to fracture
What technique is used to form a hot pressed / injection-moulded ceramic crown?
Lost-wax technique
Which ceramic materials can be used with the lost wax technique?
Leucite-reinforced glass ceramic (IPS Empress)
Lithium di-silicate glass ceramic (IPS Empress 2 and E-max)
Why are DICOR crowns no longer used?
No longer used as the paint chips off
Which company developed ceramic crowns with improved coping strength?
And what is the coping made out of?
VITA
Coping made of a slurry of:
- Alumina - In-Ceram Alumina
- Alumina and magnesium - In Ceram Spinell
- Alumina and zirconia - In Ceram Zirconia
What are the VITA ceramic crowns with improved coping strength veneered with?
Veneered with feldspathic porcelain to achieve aesthetics
What are the features of an In-Ceram crown?
- Feldspathic porcelain
- Slurry coping
- Rounded shoulder or chamfer margin
- Shrinking refractory die
Are CAD-CAM crowns veneered with feldspathic porcelain?
Yes
How can you increase the strength of CAD-CAM crowns?
With the addition of a Yttrium stabilised zirconia milled core
What state must the blocks be in, in order to be milled?
Must be in their ‘green state’ - not hard
Once the copings have been milled, where do they go?
Into the furnace to be sintered/hardened up - this takes a few hours
What technique allows staining after sintering?
Lava technique
How much shrinkage does sintering cause and how does the CAD-CAM machine respond to this?
20%
The machine counteracts shrinkage by scanning the barcode on the block and increasing the coping size by 20% to counteract shrinkage.
List 6 tooth coloured crown materials
- Feldspathic porcelain (oxide ceramic)
- Lithium disilicate (oxide ceramic)
- All polycrystalline ceramic (silica ceramic)
- Lava crowns, Procera crowns
- Glass (leucite reinforced)
- Ceromeric - porcelain-filled composite
- Composite
Name 2 trade names for zirconia
3M ESPE Lava - regular zirconia
Vita In-Ceram - Yttrium stabilised zirconia
What are the advantages of a lithium disilicate ceramic?
Increased strength, excellent aesthetics, can etch and silanate, ceramic of choice
What is used to help accommodate luting cement thickness?
Die relief - works by occupying the space where luting cement would go
What are the 2 different types of luting cement?
Active luting cement - bonds to tooth and restoration
Passive luting cement - fills the gap between tooth and restoration, no bond between tooth and restoration
What does a low film thickness of a luting cement allow?
Allows full seating of the restoration and good marginal adaptation.
List 4 different types of passive luting cements
- Zinc phosphate (DeTrey Zinc)
- Zinc Polycarboxylate (Poly F)
- Glass ionomer luting materials (Aquacem)
- Resin modified glass ionomer luting cements
(Rely X luting, Rely X luting plus)
What is the setting reaction of zinc phosphate luting cement?
Sets with an acid-base reaction
Dissolution of surface of ZnO powder - insoluble hydrated zinc phosphate matrix crystals, effectively binding unreacted ZnO particles
What is the setting reaction of zinc polycarboxylate luting cement?
Acid dissolves zinc oxide
Zinc ions form cross-links between carboxyl groups on polyacrylic acid polymer chains
Unreacted powder bound in matrix of zinc polyacrylate
What is the setting reaction of glass ionomer luting cement?
Chemical set
Dissolution - Ca2+ ions released first then Al3+
Gelation (hardening) phase - Al3+ ions bring about hardening
Al3+ - trivalent, slower to be released from glass, ensures strong cross-linking of polymer chains
What are the 2 different types of resin-based luting cements and what might they be used for?
Conventional resin luting cement - for use with all-ceramic restorations, indirect composite, or quartz fibre posts
Chemically adhesive resin luting cement - for adhesively bonding metals (e.g. Modified Bis-GMA)
Which cement materials can you not use under all-ceramic crowns and why?
Zinc phosphate cement
Zinc polycarboxylate cement
Glass Ionomer cement
Resin-modified glass ionomer cement
As they are water soluble - water sorption can lead to hygroscopic expansion putting stress on the fit surface of the crown
Which type of luting cement can you used under all ceramic crowns?
Conventional resin luting cement
What are the 7 principles of Caldicott Guardian consent?
- Justify the purpose for using confidential information
- Don’t use personal confidential data unless absolutely necessary
- Use the minimum necessary personal confidential data
- Access to personal confidential data should be on a strict need-to-know basis
- Everyone with access to personal confidential data should be aware of their responsibilities
- Understand and comply with the law
- The duty to share information can be as important as the duty to protect patient confidentiality
What popular method can be used to analyse data?
Survival Analysis
What are the lab predictors of durability?
Fatigue testing
What is the cyclical force that would be applied to test the upper load limit during fatigue testing?
2 Newtons
What method is used to assess dependability?
Weibull Statistical Distribution
What does a low Weibull distribution function indicate?
Wide distribution of values - Possible unreliability
What does a high Weibull distribution function indicate
Close grouping of values - Better dependability
What is static fatigue?
When prolonged and constant cyclic stress weakens a material until it breaks apart
In CAD-CAM, what are the 6 degrees of freedom of the non-contacting scanner?
Surge - forward/backward
Heave - up/down
Sway - left/right
Yaw - normal axis
Pitch - transverse axis
Roll - longitudinal axis
Which CAD-CAM scanner technique yields information about elevation?
Parallel Confocal Imaging Technique
Which 2 different CAD-CAD scanner techniques yield information about depth?
The Moire Effect
Active wavefront sampling
What are the 3 different rapid prototyping techniques in CAM?
Stereolithography
Selective Laser Melt
Selective Laser Sintering
What are the 3 types of ceramic blanks?
Green
Partially sintered
Fully dense
