Tooth Coloured Filling Materials 1 Flashcards
What properties are we looking for in a dental filling material?
- tooth coloured
- doesn’t wear away
define composite
A product with 2 distinct phases
- purpose is to combine 2 or more materials to produce one with superior properties
- dental composite contains inorganic filler and an organic binder
What does dental composite contain?
Inorganic filler
Organic binder
Advantages of dental composite
- aesthetic - tooth coloured
- conservation of tooth
- adhesion to tooth structure (through a bonding system)
- low thermal conductivity
- alternative to amalgam (mercury)
Disadvantages of dental composite?
- technique sensitivity
- decreased wear resistance
- polymerisation shrinkage (shrinks after curing) results in..
- Marginal leakage
- Secondary caries
- Postoperative sensitivity
Therefore dental composite not as good as amalgam
Classification of direct composite
Filler particle size (macro filled?)
How cured? (Chemically / light)
Clinical application - packable? Floatable? Bulk flow?
What is dental composite set by?
Dental composite set by free radical addition polymerisation
Free radical addition polymerisation
- Activation
- chemical
Organic amine + organic peroxide
- light
Camphorquinone + blue light - Initiation
- Propagation
- Termination
What does polymerisation result in?
Shrinkage
What can shrinkage lead to?
Poor bond to tooth so easily pulled away from dentine
Which leads to..
- poor retention
- staining
- sensitivity
- secondary caries
Monomers are used in dental composite
Why are these important?
Dental composite usually GMA + TEGMA or UDMA + TEGMA
Each material has C C bonds at both ends of monomer chain
- Dimethacrylate
The 2 C C bonds can be broken to initiate polymerisation
Bis-GMA
Extremely viscous - large benzene rings
Lowered by adding TEGDMA
- freely moveable
- increases polymer conversion
- increases cross linking
- increases shrinkage
Light cure why?
Convert from monomer to polymer
Stress set up at bonding interface
Stress relived up to gel point
After gel point composite = unyielding
Stress transferred to tooth
2mm depth of cure to minimise stress transfer
New materials don’t need this tho
Why are fillers used?
placed in dental composites to reduce shrinkage upon curing
- strontium glass
- barium glass
- quartz
- ceramic
- silica
- pre polymerised resin
how are fillers classified?
By material, shape, size
Filler classification
Fillers are irregular or spherical in shape depending on manufacturer
Spherical particles are easier to incorporate into a resin mix and to fillmore space leaving less resin
One size spherical particle occupies a certain space
Adding smaller particles fills the space between the larger particles to takeup more space.
There is less resin remaining and therefore, less shrinkage on curing
What does addition polymerisation result in?
Shrinkage
Acrylic restorations when first placed ->
Polymerisation is…
Bond to dentine is…
Polymerisation is highly exothermic
Bond to dentine is poor as dentine is wet and acrylic is hydrophobic
Light curing converts?
Monomer to polymer
Inorganic fillers reduce…?
- polymerisation shrinkage
- water sorption
- thermal expansion
Inorganic fillers increase…?
- compressive / tensile strength
- Modulus of elasticity
- Abrasion resistance
What is the 3rd phase?
The coupling phase
What does the coupling phase do?
It provides a chemical bond between the filler particle and the resin matrix
This facilitates stress transfer from the relatively weak matrix to the relatively stronger filler
This will typically be the organosilane (bifunctional molecule)
- Siloxane end bonds to hydroxyl groups on filler
- Methacrylate end polymerises with resin
In the middle is the silane coupling agent
What is the use of coupling agents
Coupling agents are used to improve adherence of resin to filler surfaces
What is the advantage of using coupling agents
Coupling agents chemically coat filler surfaces and increase strength
What are the disadvantages of silanes?
They age quickly in a bottle and become ineffective
Silanes are sensitive to water so the silane filler bond breaks down with moisture
Water absorbed into composite results in hydrolysis of the silane bond and eventual filler loss
Properties of dental composite
Compressible strength - 170-260MPa (Enamel 100-380, Dentine 250-350)
Tensile strength - 30-55MPa (Enamel 10, Dentine 20-50)
Coefficient of Thermal Expansion 20-77 (Enamel 11.4, Dentine 8.3)
Polymer shrinkage 3%
describe flowable composites
Their percentage filler content by weight (50% to 70%) is less than that of traditional hybrid composite resins which givesthem a lower viscosity.
Indications:
– Class V restorations
– Micropreparations
– Extended fissure sealings
– Adhesive cementation of ceramic restorations
– Blocking out cavity undercuts
– Initial (base) layer in any classification
Disadvantages of flowable composites
The flowable composite resins have lower filler volumes and thus they exhibit increased shrinkage and wear with decreased strength
Packable composites (macrofilled hybrids)
Bulk placed composites
Stiffer materials
Summarises dental composite
- BIS GMA resin + methacrylates
- Polymerise through photo initiated mechanisms
- Available in various shades and opacities
- Does not adhere to tooth tissue unless are specifically self etching
Bonding to enamel is predictable and relatively easy
What is the structure of dental enamel?
- 95% HAP
- 4% water
- 1% non collagenous protein
Etch and rinse
What is it and what does it do?
- An acid (mostly 30-40% phosphoric acid) is appliedand rinsed off.
- pH is 0.2 (very strong acid) and decalcification occursin 5-8 seconds
- Etching decalcifies portions of enamel rods
Acid etch technique
- Calcium salts are dissolved which exposes interprismatic and prism areas for interlocking tagformation with the bonding resin.
- This is purely mechanical bonding, not trueadhesion.
- Bond strength ~20 MPa
Bonding to dentine - the problem area?
Dentine contains:
– 33% by volume of organic component (mainly Type I collagen).
– 45% by volume of inorganic component(mainly hydroxyapatite).
– 23% by volume of water.
It is a moist, living tissue (hydrophobic) = issue
Dentine bonding agents
- Ultimate aim is to bond a hydrophobic, highly viscous composite to a hydrophilic adherent.
- We therefore need something to change the properties of the dentine surface or smear layer from hydrophilic to hydrophobic
What is a smear layer?
- Created by any mechanical cutting of dentine.
- Essentially dentine debris of variable thickness.
- In addition smear plugs are formed which block dentinal tubules
Advantages of smear layer
Can protect dentine and ultimately the pulp from penetrationof bacteria
Eliminates diffusion of dentinal fluid
Disadvantages of smear layer?
Impairs bond of composite to dentine.
It’s self is only relatively loosely bound.
Can harbour bacteria.
4 strategies have been used to achieve optimum bonding
The smear layer is left alone
The smear layer is removed
The smear layer is modified
The smear layer is practically removed and partially modified
Mechanism
The basic mechanism of bonding to enamel and dentin is essentially an exchange process involving replacement of minerals (removed from either of the hard dental tissues) by resin monomers, which, uponsetting, become micro-mechanically interlocked inthe created porosities
Superficial surface demineralisation of HAP enamel
Or
Removing HAP filler from the dentine and the resin then locking into the dentine
What is the dentine hybrid layer
It is a process that creates a molecular-level mixture of adhesive polymers and dental hardtissues – the hybrid layer
Define hybridisation
Hybridisation ~ the infiltration of resin monomers into the collagen fibrillar matrix of demineralised dentin, followed by polymerisation.
Why is etching important?
Etched dentine has removed some HAP and so end up with exposed collagen fibres
What and why do we apply on top of the etch?
Then infiltrate with primer which acts as a deterent to wet collagen and provide a hydrophobic surface for the resin - dual functionality
Resin is applied on top and then bond
- Steps in bonding to dentine
Describe the benefits and process of Acid etching
- Improves the retention of the restoration
- Increases the surface area of the dentine
- Removes “smear layer” from prep
- Allows for penetration of bonding agent into dentine
- Protect pulp exposures before using!
- Phosphoric acid (35-37%) gel or liquid
- Isolate teeth, apply etchant, wait (5-15 seconds)
- Rinse – don’t desiccate! – blot prep to remove wate
- Steps in bonding to dentine
Describe the benefits and process of Primer
- Resin - monomer
- Improves wettability of prep
- Penetrates etched dentin tubules
- Applied in a thin layer; thinned with air; blot
- May require light-curing
- Steps in bonding to dentine
Describe the benefits and process of Bonding adhesive
- Un-filled or lightly filled resin
- Adhesive bonds to collagen fibers in dentin - mechanically “locks-in” therefore a “Hybrid Layer”
- Applied in a thin, uniform layer
- Light-cured 10-20 seconds
- New “generations” being developed
Dental composite then applied
Is acid etching good for enamel and dentine?
Yes
Gives good bonding area
It removes / modifies smear layer and exposes dentinal tubules
Describe what we do after etching and why
This etching step is followed by a priming step.
- Primer is ampiphillic and low viscosity
- Solvents are added to adjust primer viscosityand improve its wetting
- Usually optimised for dentine surface characteristics rather than enamel
Etch and rinse primers are usb classified by the primer solvent
What are they?
- Acetone based e.g. All-bond 2, One Step
- Ethanol based e.g. Optibond solo
- Water based eg scotch bond MP
What is the final step?
Final stage, application of the hydrophobic resin and cure, resulting in a three step process
Simplified two-step etch-and-rinse adhesives combine the primer and adhesive resin into one application
Self-etch primer adhesive
Make use of acidic monomers that simultaneously etch and prime dentin.
The application of the self-etch primer is followed by the application of a hydrophobic bonding resin.
Without the need for rinsing, application time of self-etch adhesives is shorter and the technique-sensitivity lower
Disadvantages of self-etch
As one moves away from phosphoric acid towardsacidic organic monomers, the challenges increase!
No option has yet been tested that is as effective asphosphoric acid for etching tooth structure - No typical microetching patterns on enamel
pH initially only in the range 1-3 and this increases asthe etching proceeds
Requirements of self etch primer
Upon application the SE primer must:
– Dissolve the smear layer
– Decalcify intertubular dentine
whilst–
- Penetrating to embed superficial collagen andproduce an effective hybrid layer
Many complicated competitive events!
Consequently, systems relatively unstable
Self etch adhesive
Just as was the case with etch-and-rinse adhesives,simplified adhesives that combine the (self-etch) primer with the adhesive resin were developed tocreate what is known as the one-step self-etch adhesives or so-called ‘all-in-one’ adhesives.
Self etching enamel dentine adhesives
Self etch adhesive requirements
- Some challenges as discussed above and inaddition it must create a surface compatible to hydrophobic restorative materials
- May need to acid etch in advance in complexclinical situations.
- The acidity of the bonding agent can interferewith the setting of self–cure composite andare CI
A pathway for successful bonding to dentine?
A pathway for successful bonding to dentine:
– wetting
– infiltration to produce a ‘hybrid zone’
– mechanical interlocking
– stress resistance
Ultimate goal is marginal integrity and sealing tubules to prevent ingress of bacteria
