COMPOSITE RESINS II & III

what are dental composite resins composed of 

• methacrylate resin (bulk)

• silica powder (filler)

• photoinitiator

• stabiliser 

• typically with pigments

what is considered a composite material

• where two or more materials are combined in such a way to produce improved properties they are commonly termed composites

e.g. fibreglass

more detailed composition of dental composite resins

• matrix: mixture of methacrylate resin monomers 

  • most commonly bis-GMA (formed by the reaction of glycidyl methacrylate with bis-phenol A)

• filler: typically a silica powder (SiO₂)

  • other materials like zirconia are also added

• photoinitiator

  • e.g. camphorquinone

• inhibitors: for shelf life

what is the main resin monomer in all composites 

bisphenol A-glycidyl methacrylate (bis-GMA)

what other monomers are added to composites and what properties do they contribute 

• to reduce viscosity and improve mixing 

• e.g. triethylene glycol dimethacrylate (TEGDMA), urethane dimethacrylate (UDMA)

how are dental composites set

by either chemical cure or light activation

how does chemical curing work

• two pastes are mixed together

  • an activator + a free-radical initiator

• this approach is effectively obsolete in restorative materials - previously done in the 60s and 70s

» material polymerises on its own accord

how do light activated systems work

• contain a photoinitiator

• setting is initiated by exposure to an appropriate lamp

» it is controlled when the material polymerises

what wavelength is camphorquinone activated 

approx. 470nm - hence visible blue light is used

what does the term ‘vinyl’ refer to chemically

‘vinyl’ refers to the presence of an electron-rich C=C double bond

name this molecule

what is another term to denote how light activate systems cure/ set

set via free-radical/ vinyl polymerisation

why are photoinitiators needed

• the methacrylate vinyl group is a site awaiting release of its internal energy which will be used subsequently to polymerise other methacrylate groups

• the key to starting the process of unlocking this internal energy is the creation of a reactive chemical species that seek out the high-density electrons in the C=C

• photoinitiators are used to start this process and open up the C=C

what does free radical/ vinyl polymerisation of composite resins lead to 

inevitably leads to polymerisation shrinkage

name 3 other photoinitiators apart from camphorquinone

• Lucirin

• PPD

• Ivocerin

what do photoinitiators intend to do

• aid the efficiency of photoinitiation of polymerisation and should match the output of the light curing unit 

outline the purpose of coupling agents

• coupling agents improve adhesion of resin to filler surfaces to increase strength and toughness

• they chemically coat the filler particle surfaces 

give an example of a coupling agent and outline it

silanes

• have been used to coat fillers for over fifty years in industrial composites and dental composite resins

• common silane agents:

  • vinyl triethoxysilane

  • gamma or 3-methacryloxypropyltrimethoxysilane

what are disadvantages of silanes

• silanes age during storage and lose potency

• sensitive to water

• water absorbed into composite restorations may result in hydrolysis of the silane bond and gradual loss of properties 

state properties of dental composite resins

• tough and relatively wear resistant

• durable

• insoluble

• aesthetic

• can achieve good bond strength with tooth tissue

• relatively safe and biocompatible if handled correctly 

what are 2 disadvantages of dental composite resins

• hydrophobic so need bonding system to adhere to hydrophilic tooth tissue

• contract during setting (polymerisation shrinkage)

what did the Norwegian Institute of Public Health conclude in their 2016 review

• there was no difference in adverse health effects of composite resins compared to amalgam

• risks were very low

• although quality of evidence was moderate to low

how many phases are in a composite resin

3

what are the three phases in a composite resin and outline them

• organic phase (resin matrix)

  • monomer, initiator, inhibitors, pigments

• dispersed phase (inorganic filler)

  • glass, quartz, colloidal silica

• interfacial phase (coupling agent)

what is the purpose of each phase

• organic phase forms the polymer backbone to provide tensile strength

• dispersed phase improves mechanical properties (wear and compressive strength) and decreases shrinkage

• interfacial phase provides an adhesive bond between organic and dispersed inorganic phases

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all components of the dental composite contribute to the properties of the final restoration

if a tooth that needs to be restored has a very exposed pulp, what is used to protect the pulp before the restoration is put in 

liner 

what does composite polymerisation result in 

a set matrix containing dispersed filler particles (typically silica)

what has been a major evolution in composites

• filler particles have decreased in size overtime (the more filler you can get into a composite the less it will shrink)

• filler particles have now gotten down to the nano scale

why are big filler particles not ideal

• cannot achieve translucency

• material would be much more opaque - unaesthetic

• smaller filler particles = less polymerisation shrinkage

table showing relationship between filler size and properties

what are claimed advantages of nanofilled over conventional composites

• potentially superior fracture toughness, high strength and excellent wear resistance

• high polishability

• very low polymerisation shrinkage

• stable and natural interface

• high surface to volume ratio which allows higher filler loading to give workable consistencies

• nanosized fillers are unable to scatter or absorb visible light which increases translucency » realistic appearance of natural tooth tissue

studies showing properties of nanocomposites

• several studies reported reduced wear of nanofilled Filtrek Supreme® compared to hybrid composites and other tooth-coloured restoratives

• these studies provide little info. on the mechanisms responsible for reported properties

• subject is controversial - at least one paper reporting increased wear for nanocomposites (Turssi et al. 2006)

—

» shows that improvements in material properties do not always manifest as improvements in clinical performance

nano VS micro composites

• lab tests show that nanomaterials are more resistant to wear than micro

• but composites do not generally fail through wear

how do composites usually fail

• fracture

• microleakage

• secondary caries

what are the advantages associated with decreased filler size

• increased resistance to wear and potentially greater strength

• increased ability to polish

• improved translucency

• combined with increased filler volume potential to decrease polymerisation shrinkage

what aspects of composite resin determines clinical outcome

arguably not the selection or quality of the composite that (mainly) determines clinical outcome but:

• the correct use of a modern bonding system

• good clinical technique e.g. to eliminate moisture contamination or effects of shrinkage

polymerisation shrinkage is inevitable in _____________

methacrylates

what are the risks of bulk composite placement

• inadequate cure at depth

• shrinkage may cause weak or incomplete interface with tooth tissue » risk of

  • microleakage, discolouration of margins, secondary caries

how do we clinically minimise polymerisation shrinkage

• composite is placed and cured in layers

generally, what is the depth considered to be a reasonable thickness for an individual composite layer

2mm

what is limiting the ‘thinness’ of layers

• oxygen inhibition of polymerisation 

• i.e. if the composite layer is too thin the oxygen from the atmosphere will inhibit polymerisation

timeline showing the evolution of composites