Dental Material Science Overview Flashcards
What are the desired properties of a dental adhesive?
- provide high bond strength to tooth tissues
- immediate high strength bond
- durable bond
- impermeable bond
- easy to use & safe
Describe how the acid-etch technique works when preparing to bond to enamel:
- long enamel prisms are filled with imperfectly packed hydroxyapetite crystals
- application of 35% phosphoric acid roughens surface
- roughened surface allows micromechanical interlocking of resin filling materials
- surface energy also increased which increases wettability
What does the stress-strain ratio indicate about a dental material?
How rigid or elastic a material is
What is the meaning of ‘creep’ in relation to dental materials?
gradual dimensional change due to repetitive small forces
What is the meaning of ‘fatigue’ in relation to dental materials?
repetitive small stresses causes material fracture
What is the meaning of ‘deformation’ in relation to dental materials?
applied stress causes permanent change in materials dimensions
- but NOT fractured
What is the meaning of ‘elasticity’ in relation to dental materials?
the ability of a material to recover its dimensions after experiencing a stress
What is composite resin made up of?
- Filler particles
- Resin
- Camphorquinone
- Low weight dimethacrylates
- Silane coupling agent
What filler particles can be found in composite resins?
Glass
- microfine silica
- quartz
- lithium aluminium silicate
- borosilicate glass
What filler particle size is found in conventional composite resin?
10-40um
What filler particle size is found in microfine composite resin?
0.04-0.2um
What filler particle size is found in fine composite resin?
0.5-3um
What resin monomers are used in composite resins?
- BIS-GMA
- urethane dimethacrylates
What are the key characteristics of monomer found in composite resin?
- difunctional molecule eg C=C bonds (facilitates crosslinking)
- undergoes free radical addition polymerisation
How is camphorquinone activated?
Blue light (430-480nm)
Once activated how does camphorquinone cause composite to harden?
- activated by blue light
- radical molecules produced
- initiation of free radical addition polymerisation of BIS-GMA
- changes in resin properties
What is the function of low-weight dimethacrylates in composite?
adjusts viscosity & reactivity
What is TEGDMA an example of?
Low weight dimethacrylate found in comp resin
What is the function of silane coupling agents in composite?
Allows good bond between filler particle & resin
What type of composite is better suited to anterior restorations?
- microfilled
- submicron
What does the addition of filler particles contribute to composite resin?
- improved mechanical properties
- lower thermal expansion
- lower polymerisation shrinkage
- less heat of polymerisation
- improved aesthetics
What is the estimated depth of cure of composite?
2mm increments
What is the average strength of composite resin?
350MPa
What clinical factors can affect the wear of composite resin?
- cavity size & design
- tooth position
- occlusion
- placement technique
- cure efficiency
- finishing methods
What is the typical bond strength between enamel/dentine & composite?
40MPa
How is stress transfer altered if composite is poorly bonded to tooth as opposed to well bonded?
Concentrates stress at the interface making failure more likely
Discuss the thermal properties of composite resin:
Thermal conductivity - low (good)
Thermal diffusivity - low (good)
Thermal expansion coefficient - high (bad… can cause microleakage)
What is the function of a lining material?
- prevent gaps between restoration & tooth
- acts as a pulpal protective barrier
How does a liner under a restoration protect the pulp?
Protection from:
- chemical stimuli (unreacted chemicals or pH of material)
- thermal stimuli (exothermic setting reaction)
- bacteria & endotoxins (microleakage)
What are the ideal properties of a lining material?
- easy to use
- good mechanical properties (eg strong)
- radiopaque
- marginal seal
- low solubility
- cariostatic
- biocompatible
- low thermal conductivity
Give examples of materials used as liners:
- CaOH
- RMGIC / GIC
What is the initial pH of setting CaOH liners? Why is this relevant?
initial pH of 12
- highly alkaline liner kills cariogenic bacteria that survive in acidic environment
What is the mode of action of a CaOH liner?
- bacteriocidal to cariogenic bacteria (high pH)
- causes irritation to odontoblast layer, necrosis followes which results in tertiary dentine production, calcified bridge walls the base of cavity off from pulp
What properties does CaOH have?
- quick setting time
- radiopaque
- easy to use
- low compressive strength (bad)
- soluble (if cavity leaks lining will disappear)
What type of reaction results in Zinc Oxide Eugenol?
acid-base reaction
Why are ZOE materials unsuitable to be used with composite resin?
ZOE releases eugenol which inhibits the set of resin based filling materials
- softens them
- causes discolouration
What type of material is Kalzinol?
Resin modified ZOE
Why are glass ionomer lining materials good?
- similar thermal expansion to dentine
- high compressive strength
- good marginal seal
- fluoride release
- cytotoxic to residual carious bacteria
- GIC bonds directly to composite resin
What problem can be associated with RMGICs as liners?
Incomplete cure can cause unreacted HEMA to damage pulp
Why is RMGIC usually placed over CaOH when lining a cavity?
Prevents dissolution of CaOH beneath restoration (as CaOH soluble)
Explain the acid etch technique of bonding to enamel:
- 35% phosphoric acid applied to enamel
- roughens the surface of enamel & increases surface energy
- roughened surface allows micromechanical interlocking of resin filling materials
- increased surface energy leads to better wettability, allowing resin to flow and adapt better to rough surface
Why is bonding to dentine difficult?
- permeable tubules so dentine is wet
- smear layer present
- inconsistent material (eg aged dentine more mineralised)
- dentine is hydrophillic & bonding agents usually hydrophobic
- low surface energy
What is the smear layer found on dentine?
layer of organic debris that remains on dentine surface after preparation of dentine
- generally contaminated with bacteria
How do dentine bonding agents alter the surface energy of dentine?
DBAs increase the surface energy of dentine surface to allow composite to flow & stick
What are the components of a Total Etch DBA?
Dentine conditioner: acid (usually 35% phosphoric)
Primer: the adhesive part of the agent with a hydrophilic/hydrophobic molecule
Adhesive: resin which penetrates into the surface of the dentine attaching to the primers hydrophobic surface
What is the purpose of a dentine conditioner when bonding to dentine?
- removes smear layer
- opens dentinal tubules by removing smear plugs
- decalcifies upper layer of dentine
What is the function of primer in denting bonding agents?
- contains bifunctional molecule with hydrophilic end & hydrophobic end
- hydrophilic end bonds to the hydrophilic dentine surface
- hydrophobic end (methacrylate) bonds to the resin (C=C bond opens and forms covalent bond)
What is an example of a coupling agent found in many dentine bonding agent primers?
HEMA
What is the function of the adhesive in dentine bonding agents?
- mixture of resins usually BIS-GMA and HEMA
- predominantly hydrophobic
- penetrates the primed dentine (which now has a hydrophobic surface)
- forms a micromechanical bond within tubules known as the HYBRID LAYER
What problems are associated with the total etch technique when bonding to dentine?
- over etching of dentine causes collapse of collagen fibres & resin cannot penetrate
- over etching causes too deep of an etch and primer cannot penetrate to full depth
When might amalgam be contraindicated?
- if aesthetics are paramount to pt
- pt has history of sensitivity to mercury or other amalgam components
- loss of tooth substance is such that a retentive cavity cannot be produced
- where excessive removal of sound tooth tissue would be required to create retentive cavity
What are some advantages of using amalgam?
- durable
- long lasting if placed properly
- short placement time
- radiopaque
- cheap
What are some disadvantages of using amalgam?
- poor aesthetic qualities
- no bond to tooth surface
- high thermal diffusivity
- cavity prep may require destruction of sound tooth tissue
- marginal breakdown common
- local sensitivity reactions eg lichenoid lesions (type IV)
- tooth discolouration
- amalgam tattoo
- mercury toxicity ?
Why does amalgam cause tooth discolouration?
corrosion products migrate into porous tooth surfaces
- darkened tooth
Why does an amalgam tattoo occur?
fine amalgam particles migrate into the soft tissue causing discolouration
What is this likely to be?
Amalgam tattoo
Describe the cavity design requirements of amalgam?
Retention form:
- cavity floor should be parallel to occlusal surface
- approx. 1.5-2mm deep
- gingival floor of interprox. box should be approx. 90 degrees to axial wall
- undercuts to prevent losing restoration
What cavity designs are available to treat interproximal caries when restoring with amalgam?
- self retentive box preparation (minimal prep box)
- proximo-occlusal prep
What is the consequence of moisture contamination during placement of amalgam?
- reduces strength
- increases creep
- increases corrosion
- increases porosity
What are the functions of a matrix band?
- recreates walls of cavity
- allows adequate condensation of materials
- creates contact point with adjacent tooth
- confines filling material to cavity (reduces overhangs)
Why are wedges essential when placing matrix bands?
- produces adaptation of matrix at cervical margin
- prevents overhangs
- aids contour of restoration
- holds matrix in place
How is amalgam placed into cavities?
Placed via amalgam carrier and then condensed down with large plugger
What occurs if amalgam is not properly condensed into cavity?
- lack of adaptation to cavity/more likely to fall out
- poor bonding between layers
- inferior mechanical properties
What phase of amalgam is corrosion associated with?
Gamma 2 phase
(most amalgam is now non-gamma 2, high copper, so less of a problem)
What is creep in reference to amalgam?
slow internal stressing and deformation of amalgam under stress
What helps to prevent creep formation in amalgam?
copper incorporated to decrease creep (Ag-Sn-Cu phase stronger)
When is mercury release from amalgam at its greatest?
During insertion and removal
What is the purpose of the Minimata Convention on Mercury treaty?
Aim is to protect human health and the environment from anthropogenic emissions and releases of mercury and mercury compounds
Who should amalgam not be used in as per Minimata Treaty?
- children under 15
- pregnant women
- breastfeeding women
What are some uses of glass ionomer cement?
- restorative
- core build ups
- linings
- luting cements
What is GI composed of?
Acid (liquid)
- polyacrylic acid
- tartaric acid
Base (powder)
- silica SiO2
- alumina
- calcium fluoride
- aluminium fluoride
- aluminium phosphate
- sodium fluoride
What alters the translucency of Glass Ionomer?
Ratio of alumina/silica
- more silica = more translucent
What particle size of power in GI is required to use it for luting purposes?
<20um
What are the 3 phases of the glass ionomer setting reaction?
- dissolution
- gelation
- hardening
Describe dissolution in relation to the glass ionomer setting reaction:
- acid into solution
- H+ ions attack the glass surface
- Ca, Al, Na & F ions are released
- Leaves silica gel around unreacted glass
Describe gelation in relation to the glass ionomer setting reaction:
- calcium ion crosslinking with the polyacid by chelation with the carboxyl groups
- formation of calcium polyacrylate
(initial hard set of the material)
Describe hardening in relation to the glass ionomer setting reaction:
Formation of aluminium polyacrylate due to crosslinking of trivalent aluminium
(does not start for at least 30 mins)
What happens if there is contamination of GIC during setting?
- aluminium ions diffuse out of material
- water absorption
ALL LEAD TO A WEAK MATERIAL
What has been added to GICs to improve ease of use?
Tartaric acid
- shortened setting time
What is the bonding mechanism between GIC and tooth?
Chelation between carboxyl groups in the cement & Ca on the tooth surface
What are some negatives of GIC when compared with composite resins?
- poor tensile strength
- lower compressive strength
- poor wear resistance
- lower hardness
- higher solubility
Give some uses of GICs:
- dressing
- fissure sealants
- temporary filling
- luting cement
- restorations
- base / lining
Give the constituents of the powder of RMGIC:
- fluoro-alumino-silicate glass
- barium glass
- polyacrylic acid (vacuum dried)
- potassium persulphate (redox catalyst to provide resin cure in dark)
- ascorbic acid
- pigments
Give the constituents of the liquid component of RMGIC:
- HEMA (resin)
- Polyacrylic acid with pendant methacrylate groups
- Tartaric acid
- Water
- Photo-intitiators
What are the cons of using RMGIC as opposed to regular GIC?
- polymerisation contraction
- swelling due to water uptake of HEMA
- HEMA is toxic to pulp if left unpolymerised
What is the composition of traditional amalgam?
Powder:
Ag (70%)
Sn (25%)
Cu (3%)
Zn (1%)
Hg (1%)
Liquid:
Hg
What is the γ phase of amalgam?
Ag3Sn (reacts with Hg liquid to form amalgam)
What is the function of copper and zinc in amalgam?
Copper
- increased strength & hardness
- corrosion resistance
Zinc
- scavenger during production
- oxidises
What is the setting reaction of amalgam?
What amalgam phase is weak and has poor corrosion resistance?
γ2 phase (Sn7Hg9)
What effect could occur in a freshly placed amalgam restoration as a result of the presence of zinc in the amalgam alloy?
Explain the mechanism.
Expansion of material
- interaction of zinc with saliva/blood
- bubbles of H2 formed within amalgam
- pressure buildup causes expansion
- downward pressure cause pulpal pain
What handling factors are the properties of amalgam dependent on?
- proportioning & trituration (composition and how it is mixed)
- condensation
- carving & polishing
Give examples of some factors that can decrease strength of amalgam:
- undermixed
- low condensation pressure
- slow packing rate (increments do not bond)
- corrosion
Explain how ditching can occur around an amalgam restoration. What can this lead to?
- an amalgam placed in cavity experiences repeated & prolonged low level stresses
- amalgam “flows” and changes shape
- amalgam now sitting proud of tooth surface
- ditched margins present
- amalgam now vulnerable to microleakage or fracture at margins
What are the thermal properties of amalgam like? How is this relevant to patients?
- High thermal expansion (may lead to marginal gaps)
- High thermal conductivity (may need to use a liner in deep cavities to prevent pulp damage)
- High thermal diffusivity (amalgams temp rises quickly also a risk to pulp)
How can corrosion of amalgam be reduced?
- use copper enriched amalgams
- ensure well polished margins
How do copper enriched amalgams differ from conventional amalgams?
- Non γ2 phase (Sn7Hg9)
- high copper content (MUST BE > 6%)
What are the benefits of NON- γ2 phase Amalgams?
- higher early strength
- less creep
- higher corrosion resistance
- increased durability of margins
Explain the difference between mucostatic and mucocompressive materials:
Mucostatic = fluid materials that displace the soft tissues slighty (eg tissues dont move)
Mucocompressive = viscous materials that record an impression of the mucosa under load
How can the load time of an impression material be reduced in clinical practise?
Impression tray removed with a sharp pull
Why are non-elastic impression materials not suitable for impressions of bulbous crowns?
Impression will:
- be deformed
- fracture/tear
What are the ideal properties of an impression material?
- low viscosity so able to flow
- no dimensional change on setting
- low thermal expansion (to cope with mouth to room temp change)
- elastic
- high tear strength
- non-toxic
- acceptable taste & smell
Give an example of a non-elastic impression material:
impression compound
What type of material is alginate?
irreversible hydrocolloid impression material
What is the composition of alginate?
- sodium alginate
- calcium sulphate
- trisodium phosphate
- filler
Give some pros and cons of alginate as an impression material?
Pros:
- easy to use
- good setting time
- nearly elastic
- cheap
Cons:
- poor tear strength
- will uptake water if too dry/will release water if too moist LEADS TO DIMENSION CHANGES
Why do we sometimes bond porcelain to metal for restorations?
- helps eliminate defects/cracks on porcelain surface
- aloy acts as a support & limits the strain that porcelain experiences
Describe the meaning of the statement of “elastic limit” in relation to endodontic files:
The set value representing the maximal strain that can be applied to a file before permanent deformation occurs
What is the meaning of the “plastic limit” of endodontic files?
The point at which a plastic deformed file breaks
- permanent bond displacement when elastic limit exceeded
How can you prevent endodontic instrument fracture?
- creation of a manual glide path to prevent torsional fatigue
- ensure files are in constant motion
- avoid use of files in abruptly curved or dilacerated canals
What is work hardening?
Strengthening of a metal by plastic deformation
- crystal structure dislocation
- dislocations interact and create obstructions in crystal lattice
- resistance to dislocation formation develops
- stronger metal
What is the role of irrigants in endodontic treatment?
- facilitation of debris removal
- lubrication
- dissolution of organic & inorganic matter
- biofilm disruption
- kills microbes
What is typically used to irrigate canals in endodontic treatment?
2.5 or 3% Sodium Hypochlorite (NaOCl)
What is used to remove the smear layer?
17% EDTA
What is the function of sealers in endodontic treatment?
- seals space between dentinal wall & core
- fills voids & irregularities in canal
- lubricates during obturation
What are the properties of an ideal endodontic sealer material?
- exhibits tackiness to provide good adhesion
- establishes a hermetic seal
- radiopacity
- easily mixed
- no shrinkage on setting
- non-staining
Why might a GI sealer be a better option rather than a ZOE sealer?
- Eugenol in ZOE reacts with resin and prevents setting of composites
- glass ionomer bonds directly to dentine
What type of sealers can be used in endodontic treatment?
- ZOE sealers
- GI sealers
- resin sealers (best option)
Why are ZOEs good options as endo sealers?
- Zinc Oxide antimicrobial
Why are Glass Ionomer endodontic sealers good?
dentine bonding properties
What are the downsides of using glass ionomer endodontic sealers?
- removal for re-treatment difficult
- minimal antimicrobial activity
How long do resin endo sealers take to set?
8 hours
What are the ideal properties of a luting agent?
- easy to use
- radiopaque
- good marginal seal
- not soluble
- biocompatible
- cariostatic
Explain the ideal viscosity and thickness of luting agents:
- low viscosity to allow seating of restoration without interference
- film thickness should be as thin as possible 25um or less
Explain how a glass ionomer cement bonds to the tooth surface:
- ion exchange with calcium in enamel and dentine
- hydrogen bonding with collagen in dentine
Give an example of a Glass Ionomer luting cement material:
AquaCem
Give an example of a composite luting agent:
NX3 (nexus)
If you don’t light cure a dual cure composite, what happens?
25% reduction in physical properties
What would you use to bond a composite resin inlay to a tooth? How does it bond?
Dual cure composite cement with a dentine bonding agent. Bonds via:
- micromechanical bond to rough internal surface of inlay
- chemical bond to remaining C=C bonds on fitting surface of inlay
Why does porcelain need to be bonded to teeth?
Because it is brittle so needs to be bonded to tooth to prevent fracture
How does porcelain bond to teeth?
dentine bonding agent + composite luting resin + silane coupling agent [applied to porcelain]
Explain how porcelain is prepared to be bonded to a tooth as a veneer:
- Porcelain etched with Hydrofluoric acid [produces rough retentive surface]
- Silane coupling agent applied to etched porcelain surface [strong bonds between oxide groups on porcelain & the silane]
- Other end of silane molecule has C=C bond which reacts with composite rein luting agent
Although required, what is the disadvantage of using a dual cure composite (eg RelyX) to bond a porcelain veneer?
can cause discolouration
How are metals prepared to be bonded to teeth?
- sandblasted
- electrolytic etching
How are non-precious metals (eg MCC) bonded to tooth surfaces?
- dentine bonding agent
- composite luting resin
- metal bond agent is applied to the metal [eg 4-META, which has an acidic end and a C=C end, the acidic end reacts with the metal oxide and renders surface hydrophobic to allow for bonding]
What kind of composite luting resin should be used when bonding metal restorations to teeth?
Dual cure material
[can be used to cement most crowns, bridges & posts]
What luting agents can be used to cement MCCs?
- GIC [AquaCem]
- RMGIC
[Dual cure composite + DBA]
[anaerobic cure composite]
[self adhesive composite]
What luting agent is used to bond a metal post?
GIC [AquaCem]
What luting agent is used to bond a fibre post?
NX3 [dual cure composite + DBA]
or Self Adhesive Composite
What luting agent is used to bond a veneer?
- NX3
- RelyX [although can discolour]
What luting agent is used to bond a RRB?
Panavia 21 [anaerobic cure composite]
What luting agent is used to bond a zirconia crown?
- GIC [aquacem]***
- RMGIC
- Dual Cure Composite + DBA [RelyX]***
- Anaerobic Cure Composite
- Self Adhesive Composite
What luting agent is used to bond a composite inlay?
- NX3
- RelyX
What luting agent is used to bond a gold restoration?
- GIC [AquaCem]
- RMGIC
