Resin composite restorations
This section opens up the reader to Resin composite restorations with regard to their physical and mechanical properties and the types available.
1. Why is a silane coupling agent used in composite resin?
The organic resin matrix is bonded to inorganic filler by means of a silane coupling agent. It increases the strength of the composite and reduces the solubility and water absorption.
2. Why resin-modified glass ionomer and polyacid-modified composites are not the same?
The primary intention of both these materials is to preserve and utilize the favourable properties of glass ionomer. Resin modified glass ionomer is primarly glass ionomer, which sets chemically by acid-base reaction. Resins are added to this for improving the physical and esthetic properties. Incorporation of resin component makes this cement to set fast by light curing. Polyacid-modified composites are primarly resin composites which sets only by light cure. Some of the glassionomer components are added to this cement to incorporate the favourable properties of glass ionomer. They are also termed as compomers.
Ref: Theodore.M.Roberson,Harald.O.Heymann,Edward.J.SwiftJR.Sturdevant's Art and Science of Operative Dentistry.Mosby publications;2002;478
3. How do chemically activated resins polymerize?
The chemically activated resins have a benzoyl peroxide initiator and a tertiary amine activator. When the two pastes are mixed, the amine reacts with the benzoyl peroxide and free radicals are formed to start the polymerization.
Ref: Science of Dental Materials, Skinner, Prism books private limited,9th Edn,219
4. Why were Visible light activated systems developed?
Initially, ultra violet light was used to initiate free radicals. But the depth of penetration of UV light into the resin was limited and there was no penetration through the tooth structure. So visible light activated systems were developed which can polymerise thicker increments.
Ref: Science of Dental Materials, Skinner, Prism books private limited,9th Edn,219
5. How does polymerization of light activated systems occur?
The amine activator and the photoinitiator are present in the same paste of the light cure system. When it is not exposed to light, no reaction takes place. When light of the correct wavelength is exposed on this material, the photo initiator gets excited and free radicals are produced by reacting with the amine.
Ref: Science of Dental Materials, Skinner, Prism books private limited,9th Edn,220
6. How is the pattern of shrinkage in restorative resins?
The chemically activated resins shrink towards the centre of the bulk. The polymerization of light cured materials is first at the surface and then as the light penetrates the resin, the deeper portions are cured. So the shrinkage is towards the light source for light activated resins.
Ref: Science of Dental Materials, Skinner, Prism books private limited,9th Edn,221
7. What are Microfilled Composites?
Microfilled composites are those that use colloidal silica as the inorganic filler particles, to overcome the surface roughness produced by conventional composites. They provide a smooth finished surface necessary for esthetic restorations.
Ref: Science of Dental Materials, Skinner, Prism books private limited,9th Edn,224
8. What are the advantages of light activated systems over chemically cured resins?
Light cure materials are single component pastes. No mixing is required, sufficient working time is available and the resin gets cured on exposure to light only, the depth of cure is limited and incremental build up can be done in deep cavities.
Ref: Science of Dental Materials, Skinner, Prism books private limited,9th Edn,234
9. Why hybrid composites are widely used and considered as universal composites in the present scenario?
A highly polishable property with superior physical and mechanical properties is the ideal requirement for composite material. The conventional composite with the average particle size of 8 micrometer was not highly polishable. The microfil composite with the average particle size of 0.01-0.04 micrometer are highly polishable but lacks good physical and mechanical properties. The hybrid composite with the average particle size of 0.4-1 micrometer provides optimal esthetics and satisfactory physical and mechanical properties. Hence, they are used widely and considered as universal composites.
Ref: Theodore.M.Roberson,Harald.O.Heymann,Edward.J.SwiftJR.Sturdevant's Art and Science of Operative Dentistry.Mosby publications;2002;477