Structure of Dental Composites Flashcards
Define Composite
Two or more materials put together, with each contributing to the overall properties
Why were silicate cements used as direct filling materials in the early 20th Century? (2)
- Only tooth coloured material (aesthetics)
- Released fluoride
What are some disadvantages of silicate cements? (3)
- Erodes within a few years
- Soluble in oral fluids
- Discoloured with time
What were direct filling acrylics based off?
Polymethyl methacrylate (PMMA)
What are some advantages of PMMA as a direct filling acrylics? (4)
- Tooth like appearance
- Low cost
- Insoluble in oral fluids
- Easy to manipulate
What are some disadvantages of PMMA as a direct filling acrylics? (3)
- High setting shrinkage which leads to secondary caries
- Poor wear resistance
- Coefficient of thermal contraction different to tooth
How are some of the problems of PMMA as a direct filling acrylic resolved?
Resolved by adding quartz powder as a filler, which renders it to a composite
What do we use now as a composite filing material?
Bowen’s Resin
How do direct composite fillings work?
Placed in the unset state and then is cured (set) in the oral cavity
What are the 3 phases/components of a dental composite?
- Organic Matrix
- Inorganic filler
- Coupling agent
What is the role of the organic matrix in a dental composite?
It is the plastic resin that polymerises to form a continuos phase
What is the role of the coupling agent in a dental composite?
To promote adhesion between filler and matrix resin
What monomer does the organic matrix of a composite consist of?
Bis-GMA
What is Bis-GMA prepared from? (2)
BA
GM
- Bisphenol A
- Glycidyl methacrylate
What is the viscosity of Bis-GMA?
700,000 cps
Why is Bis-GMA used over MMA in composite fillings?
When MMA monomers polymerise there is a big volume shrinkage of the compound where it occur a lot less in Bis-GMA
What is the role of hydroquinone in the organic matrix?
Acts as inhibitor to prevent setting of composite in the lab
What minor substances could potentially be found in the organic matrix of a composite? (7)
- Hydroquinone
- Activator/Inhibitor for light curer
- Activator/Inhibitor for room temp curer
- Fillers
- Pigments
- UV stabilisers
- Optical brighteners
What are the advantages of incorporating fillers? (5)
- Reduced setting shrinkage
- Reduced coefficient of thermal expansion
- Enhanced strength and compressive hardness
- Provides radio-opacity
- Control of aesthetics
Why must the refractive index of both the filler and resin be the same?
To avoid the scattering of light otherwise full depth of light cure isn’t achieved
Why are many fillers used in composites?
Single filler insufficient to attain most of the above goals
What are the different types of fillers? (3)
- Conventional (macro and micro)
- Hybrid
- Microfine
What did the first composites use as macro fillers?
Used ground quartz
What material is used as a micro filler?
Colloidal silica
What does a micro filler do?
Increases viscosity of monomers so method of incorporation is further improved
What are the advantages of hybrid fillers? (4)
- Favourable physical, mechanical and optical properties
- Improved wear resistance
- Surface morphology superior to conventional but inferior to microfilled resins
- Radiopacity possible.
What is a disadvantage of hybrid fillers?
Increased surface roughness with time so resin wears away
Typically how much inorganic filler is loaded into a composite?
30-70%
Why are fillers hydrophilic?
Due to a surface layer of OH groups
What coupling agent is used to bond the filler to the resin?
Silane coupling agent
How does a silane coupling agent work?
The hydrophilic end of the coupling agent (OCH3) reacts with surface hydroxyl groups (OH) of the filler – via a condensation reaction
How are room temperature composites dispensed?
Two paste containing;
- Benzoyl peroxide
- DHPT
How are light cured composites dispensed?
One paste
