Synthetic polymers

· Polymers:

High molecular weight, chain-like molecules consist of distinct repeating groups of atoms (monomers).

· Natural: DNA, proteins, collagen, silk….

· Synthetic

· Polymerisation

monomers polymers

· There is two types of polymerization:

· Addition

· Condensation

· Addition polymerization(free radical addition):

· The addition of a reactive species (fr) with a monomer to form a larger reactive species which is capable of further addition with monomer.

R* + M → R − M*

R − M* + M → R − M − M*

R − M − M* + M → R − M − M − M* etc.

· Source of free radicals = initiators

· Benzoyl peroxide.

· Benzoin methyl ether.

· Camphoroquinone.

· Activation systems:

1. Heat

2. Chemical activator

3. Radiation

· 4 main stages of polymerization :-

· Activation

· Initiation

· Propagation

· Termination: combination of two growing chains to form one dead chain

· Condensation polymerization:

· Two molecules reacting together to form a third, larger molecule with the production of a by product.

X − M1 − X + Y − M2 − Y → X − M1 − M2 − Y + XY

X − M1 − M2 − Y + X − M1 − X → X − M1 − M2 − M1 − X + XY

X − M1 − M2 − M1 − X + Y − M2 − Y → X − M1 − M2 − M1 − M2 − Y + XY etc.

· Physical changes occur during polymerization:

· Phase changes: amorphous structures crystalline structures

· Temperature changes: exothermic in nature

· Dimensional changes: shrinkage

Resin based restorations

· The first material developed for use as a direct esthetic restorative was silicate cement (1800s).

· Properties of silicate cement:

1. Dissolution

2. Discoloration

3. Loss of translucency

4. Lack of adequate mechanical properties

5. Release fluoride

· Unfilled resins (Acrylic resins) 1930s-1940s

· Properties of unfilled resin :

1. Much less soluble and more color stable than silicate

2. Easy to use, polishable, and had good initial esthetics

3. High shrinkage upon polymerization

4. Large thermal dimentional change

5. Eventual discoloration

6. High wear rate

· Filled resin (Composite )(middle1950s)

· Composite Bis-GMA were developed and their use in dental restorative materials was proposed in 1960.

· On going development in filler technology.

· Compomer.

Dental composite

(Structure-Composition)

· Definition:

· Is a product which consists of at least two distinct phases normally formed by blending together components having different structures and properties.

· To produce a material having properties which could not be achieved from any of the individual components alone.

· Composition & Structure:

Resin matrix + filler + coupling agent = composite

· Resin Matrix:

Chemically active component of the composite.

· plastic monomer rigid polymer

· Radical addition reaction.

· Types:-

Most commonly used:

· An aromatic or urethane diacrylate oligomer (cross-linked polymer.)

· BisGMA (Bowen’s resin) MW polymerisation shrinkage

· UDMA

· TEGDMA, MMA (viscosity controllers)

· Oxirane & Silorane (ring opening mechanism)

· The resin matrix also contains:

· The activator\initiator system (to provide free radical)

Depends upon the type of reaction

· Inhibitor

- To prevent premature polymerization & hence increase the shelf life of the material

Such as hydroquinone 0.1 % or less

· Ultraviolet absorbers

· Photo stabilizers

· optical modifiers: pigments & opacifiers

· New products contains :

· Anti microbial

· Fluoride: fluoridated composite

· Fillers:

· Inorganic composition to improve properties (1950).

· Advantages of the fillers:

1- polymerization shrinkage (not eliminate)

2- coefficient of thermal expansion (80ppm\ºc for monomer , 8-10ppm\ºc for ceramic filler)

3- Improve mechanical properties such as compressive strength, hardness, rigidity.

4- Provide radiopacity, by use heavy metals such as barium & strontium

5- Provide means of controlling aesthetic features, as color, translucency, fluorescence

· The factors of interest in the selection of the filler are:

· Composition:

· Quartz (hard)

· Fused Silica (including colloidal silica, lithium aluminium silicate glass and silica glasses containing barium or strontium.) (colouring, refractive index, improves the surface finish of the composite).

· Ceramics (strength, COTE)

- Barium oxide, strontium (radio-pacity)

· Average particles size and distribution:

· Determines the amount of filler that can be added to the resin, without the necessary handling characteristics being lost.

· Final surface finish of the composite restoration.

· Coupling agent:

· In-efficient bond leads to :

· Stress developed will not be resulting in : creep, fracture, wear of the restoration.

· Crack initiation sites.

· Coupling agent used are Silanes

· Most commonly used is

· γ – methacryloxypropyltriethoxy silane (γ - MPTS)

· coated on the filler particle surface by the manufacturer before mixing with the oligomer

· hydroxyl groups hydroxyl group of the glass particle (hydrophilic).

· methacrylate group resin via a carbon double bond (hydrophobic)

Dental composite

(Classification)

A. Filler Classification System

1- According to the filler particle size

megafill

Glass inserts

0.5-1 mm

macrofill

LPS, original (40)

10-100

midifill

MPS, traditional (4)

1-10

minifill

SPS

0.1-1

microfill

VSPS, fine particle

0.01-0.1

nanofill

Atomic clusters (0.005)

0.001-0.01

· Conventional composites:

· Traditional composite

· Glass filler particles of 10-20µm up to 40 µm

· Filler loading of 60 – 80 % by wt

· Poor surface finish dull appearance

· low wear resistance

· Microfilled composites:

· Colloidal silica , 0.01-0.05 µm ( average 0.02 µm)

· Filler loading only 20 % ( 50% if use pre-polymerized fillers)

· Smooth surface finish

· Hybrid composites:

· Hybrid or blended composites involve mixture of two particle sizes

· large filler particles of 15-20 µm + small size colloidal silica of 0.01-0.05 µm

· filler loading is 75% large p. + 8% small p.

· can be loaded up to 90% by wt

· Nano composite:

· Nanotechnology provide composite resin with filler particles that are dramatically smaller (0.001-0.01).

· The material was created using two fillers:

· Nanomers (individual filler particles, roughly spherical in shape).

· Nanoclusters (loosely agglomerated collections of nanoparticles)

· The adhesion between conventional macroscopic material (40 nm to 0.7 nm) and the tooth structure (1nm to 10 nm) is very dissimilar .

· So nano composite optimizing the adhesion of restorative biomaterials to the mineralized hard tissues of the tooth enhancing the mechanical strength, marginal adaptation, and seal improving the longevity of the adhesive restoration.

· Nanofilled composites use nanosized particles throughout the resin matrix.

· Nanohybrids take the approach of combining nanomeric and conventional fillers.

· It provides unique characteristics to the material (physical, mechanical, and optical).

2- According to amount of filler:

Composites

Volume %

Weight %

Unfilled resin, bonding agents, sealants

0

0

Homogenous microfills

30

50

Flowable (1st generation)

40

Macrofills, midifills

50

75

Hybrids, heterogenous microfills, flowable(2nd generation)

60

packable

70

85

3- According to particle size distribution (or mixture):

Hybrid: mix of two particle size fraction of fillers

4- According to method of filler loading:

i- homogeneous filled composites:

mixture of resin & filler

ii- heterogeneous filled composites:

mixture of pre-cured composite pieces with resin & filler

5- According to handling characteristics

I: Flowable composite

a. 1st generation:

· filler content, so decreased viscosity.

· particle size.

· mechanical properties inferior to traditional hybrid composite.

· used as pits & fissure sealants or small anterior restorations

b. 2nd generation:

Filler content but higher than 1st generation.

The most suited applications for flowable composite:

· As the 1st increment layer in the restoration

· As repair resin for margins or non occluding surfaces

ii- traditional \ conventional composite

iii- packable composite :

· Condensable composite.

· filler loading by 1-2% volume.

· Less stickiness, higher viscosity and higher stiffness than traditional hybrid composite.

· more opaque, limited shades range.

· Inferior surface finish.

· No improvement in mechanical properties.

· Limited applications, small to moderate posterior cavities.

B. Matrix Classification System

1- According to matrix Composition:

i- BISGMA or BISGMA-like

ii- UDMA

2-According to activation method:

i- Auto-Cured (Self-Cured)

ii- Light-Cured.

iii-Dual-Cured (VL and Self-Cured)

iv-Staged-Cure (2 Stages of Visible Light Curing)

· Chemical Activation :

1- two paste system :

Each paste contains a blend of resin & filler

· One paste : 1% peroxide initiator (benzoyl peroxide).

- The other paste : 0.5% tertiary amine activator as (N, N’ dimethyl-p-toluidine) or (P-toly dimethanol amine).

2- powder liquid system :

Powder : filler + peroxide initiator

Liquid : monomer + co monomer + chemical activator

3- paste liquid system :

Paste : monomer + co monomer + filler +

peroxide initiator

Liquid : monomer + chemical activator

4- encapsulated system :

Part 1 : filler + peroxide initiator

Part 2 : monomer + co monomer + chemical activator

· Light Activation :

· Supplied as a single paste, contains :

· Monomer + co monomer + filler + initiator

UV light :

· The initiator : benzoin methyl ether

· Certain UV wave length benzoin methyl ether absorption of the light free radicals initiate polymerization

· Mercury discharge lamp:

· Expensive

· Intensity of light output reduces as the lamp gets older

· Has a limited depth of cure

Use of UVL radiation diminished over time due to possibilities of :

· Soft tissue burns

· Damage to the eye

· Visible light :

· The initiator : mixture of diketone (camphorquinone) + amine

· Correct wave length & intensity of visible light initiator free radicals initiate polymerization

· Light activated material can be supplied in the form of syringes or compules ( mini syringes )

Advantages:

· Mixing isn’t required

· Command polymerization : controlled working time

Disadvantages

· limited curing depth

· Poor accessibility to certain post. & IP locations

Light activation units :

Purpose :

· To deliver high intensity radiation of correct wave length to the surface of the material to activate polymerization.

· The critical wave length used by most units & materials is 470 nm, which corresponds to the blue region of the visible spectrum.

Types:

1- quartz tungsten halogen (QTH)

2- plasma arc

3- argon lasers

4- light emitting diodes (LEDs)

Compatibility & Testing:

· For composite material & light curing unit

· Should be done regularly

· All light sources fail, either catastrophic or gradually with aging of the bulb/ LED

· By device consist of light sensitive diodes, that can be used as light intensity meters, when the reading falls below a critical value it suggests that the unit needs attention, perhaps a bulb needs replacing.

· Depth of cure can achieved by VL (3-4 mm) is more than that achieved by UVL (max<2 mm)

· Depth of cure, affected by:

1- Shade of composite

2- Quality of light source

3- Method of cure

4- Time of cure

· Oxygen inhibition:

· When there is air interface with the resin, the resin will not cure leading to an oxygen inhibited surface layer

· Few micrometers.

· Adv: incremental placement

· Needs to use a matrix strip with the last increment

Properties of composite

A. Physical properties:

1. Thermal properties

Linear Coefficient of Thermal Expansion (LCTE) :

Dental tissues : 9 - 11 ppm

Unfilled resins : 72 ppm

Composites : 28 - 45 ppm

Amalgam : 25 ppm

· As filler content the LCTE

If interfacial bond fails micro leakage

results in :

· Staining.

· Pulpal sensitivity.

· Pulpal irritation.

· Diffusion of bacterial endotoxins (recurrent caries)

2. Water sorption

Water sorption occurs through:

i- direct absorption by the resin

ii- adsorption onto the filler surface

Depends on:

i- The resin content

ii- the quality of resin\filler bond

Leads to

· Loss of bond

· Filler particles lose their surface cohesion

· High rate of wear

3. Color match

· Should be available in various shades & different degrees of translucencies.

· Shading is achieved by adding pigments, usually small amount of metal oxide.

· Trancelucency and opacity are adjusted by adding an opacifier, as titanium dioxide & aluminum oxide.

· Discoloration:

Chemically cured composites have less colour stability, due to presence of tertiary amine accelerator (oxidation).

Types of discoloration:

· Marginal.

· Surface.

· Bulk.

B. Chemical properties:

Setting contraction

· unfilled resin (6%) composite (1.5-3%).

· Should be as low as possible.

· Problems of polymerization shrinkage:

· Stress creation cause post operative pain + cuspal fracture.

· Rupture of adhesive bond compromise the marginal sealpain+ secondary caries+ marginal staining

· Factors that affect the Polymerization shrinkage:

Manufacturer’s factors:-

· Filler loading.

· Type of resin monomer.

microfilled composites (2-4%)

fine-particle composites or hybrids (1-1.7%).

Manipulative factors:-

· Cavity design.

· Clinical application technique.

Ways to overcome these problems:

· improved dentine bonding agents to improve marginal integrity

· The recommendations is to use composite when all margins are in enamel.

· Use low modulus lining materials to act as stress absorber.

· Use chemically cured composites in the base of boxes.

· Use of (soft start) light curing unit.

· Incremental placement technique.

· Composite inlay.

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C. Mechanical properties

depends on :

1- filler content

2- type of filler

3- efficiency of filler-resin coupling process

4- degree of porosity in the set material

· Chemically activated composites contain 2-5% porosity that introduced during mixing

· Light activated composites contain very little porosity

· Accordingly:

Light activated

Chemically activated

Amalgam

Compressive strength

260 MPa

210 MPa

350 MPa

Fatigue limit

higher

lower

highest

· Wear & surface hardness

Wear mechanisms:-

· Abrasive Wear (2or 3 body)

· Fatigue Wear (repeated loading cyclic stress

crack)

· Corrosive Wear (hydrolytic, erosive acid attack)

Factors:-

· Filler content, particle size, and hardness.

· Tooth position in the arch.

· Porosity.

· Degree of polymerization (heat-processed composite inlays).

· Coupling agent.

· Method of finishing (Wear resistance decreases with the use of carbide or diamond finishing burs).

· Biocompatibility

· Release of components from polymerized composite.

· uncured resins (LMW (MMA, TGDMA)), diluents, initiators, and additives such as UV stabilisers, plasticisers.

Depends on:-

· Type of composite (contents, solubility).

· Degree of curing.

· Some studies showed that some of these products may be associated with:

· Cytotoxic (local, systemic),

· Delayed hypersensitivity reactions,

· Bacterial reactions (micro, nanoleakage).

May not be interpreted as an unacceptable indication of these materials….. Why?

· The released amount is very scanty.

· Hyper Sensitivity Reaction very rare.

Indications:

· Mild-moderate class 1,2 of post teeth (direct/ indirect).

· Class 3,4,5.

· Veneers to treat discoloration.

· Closure of diastema and correction of morphology.

· To restore erosion or abrasion defects (aesthetic).

· Core build up materials.

· Cementation of indirect restorations.

· Pit & fissure sealant.

· Periodontal splinting of periodontaly weakened teeth.

· Repair of fractured tooth/ ceramic restoration.

· Bonding orthodontic appliances.

· Fiber-reinforced bridge.

Contraindications

· Isolation is difficult.

· Very high occlusal forces.

· Class 5 (aesthetic isn’t important).

· Lack of technical skills.

· Lesion extends up to root surface/subgingivally.

· High caries susceptibility/ poor oral hygiene .

Clinical procedure

· Local anaesthesia.

· Preparation of operative field (cleaning).

· Composite selection.

· Position of the tooth preparation.

· Large cl 1,2,4 (higher filler loading).

· Ant (aesthetic concerns (micro/nanofillers)).

· Cervical (highly polishable).

· Aesthetic requirements.

· Discoloration, defective shape, diastema, caries in ant teeth.

· Opacity & translucency.

· Shade selection

· More important for ant teeth.

· Multiple shades are required to attain optimal aesthetics.

dentin shade

· Dentin

dentin opacity

enamel shade

· Enamel

enamel transluency

· Isolation.

· Tooth preparation (to bevel or not bevel).

· Pulp protection ??? (RMGIC/ CaOH)

· Bonding.

· Acid etching (37% phoshoric acid)/primer/ bonding agent.

· Composite placement.

· Polymerization.

· Final contouring, finishing and polishing.

Thank you