22. Composite Resins Flashcards
L22: What are the ideal mechanical properties of a direct filling material?
- Strength;
- Rigidity;
- Hardness;
- Bonding to tooth/ compatible with bonding systems.
L22: Other than mechanical properties, what other ideal properties are there for direct filling materials?
- Thermal properties;
- Aesthetics;
- Radiopaque (opaque to X-rays);
- Handing/ viscosity;
- Anticariogenic;
- Smooth surface finish/ polishable;
- Low setting shrinkage;
- Biocompatible.
L22: What are the two main constituents of a composite resin?
- Glass filler particles (hard);
- Resin (soft).
L22: Alongside glass filler particles and resin, what else do composite resins contain?
- Camphorquinone (photo initiator);
- Low weight dimethacrylates;
- Silane coupling agent.
L22: Give an example of a type of filler particle:
- Microfine silica;
- Quartz;
- Borosilicate glass;
- Lithium aluminium silicate;
- Barium aluminium silicate.
L22: What is the conventional particle size for filler particles in a composite resin?
10-40 um
L22: What size of filler particles do hybrid composites contain?
A range, 0.01-0.1um and 1-10um
L22: What resin monomers are used in composite resins?
- Bis-GMA;
- Urethane dimethacrylates.
L22: What are the key characteristics of resin monomers in a composite resin?
- Difunctional molecule (C=C bonds, to facilitate cross linking);
- Undergoes free radical addition polymerisation.
L22: How does camphorquinone initiate bonding of composite resin?
- Activated by blue light;
- Produces radical molecules;
- These initiate free radical addition polymerisation of Bis-GMA;
- Changes in resin properties;
- i.e. increased MW, increased viscosity and strength.
L22: What is the role of low weight dimethacrylates?
Improve material - adjust viscosity and reactivity
L22: What is the role of saline coupling agent?
Improve bonding - favours glass-resin bonding
L22: What type of restorations are composites used for?
- Where aesthetics are important;
- Class III, IV and V permanent restorations;
- Class II - limited occlusal wear;
- Labial veneers;
- Inlays, inlays, indirect technique;
- Cores;
- Modified forms as luting cements (some dual cured).
L22: What are the three main classification classes for composite resins?
- Filler type;
- Curing method (light/ self);
- Area of use;
- Handling (condensable, syringeable, flowable).
L22: Sometimes a range of filler particle sizes are used to accommodate composite volume, how does this effect the mechanical properties?
Stronger, harder, more rigid
L22: What other effects does adding filler particles (to fill volumes) have?
- Improved aesthetics;
- Increased abrasion resistance;
- Lower thermal expansion;
- Lower polymerisation shrinkage;
- Less heat of polymerisation;
- Some radiopaque.
L22: What wavelength is blue light (used to cure light-cured composites)?
430-490 nm
L22: Why is a certain wavelength of light used to cure composites?
Absorption peak at ~ 450nm for camphorquinone
L22: What are the advantages of using light-curing system,s?
- Extended working time, i.e. on-demand set;
- Less finishing;
- Immediate finishing;
- Less waste;
- Higher filler levels;
- Less porosity.
L22: How does hardness (due to light curing) and depth vary?
- Most blue light absorbed at the surface;
- Hardness decreases with depth.
L22: What is the ‘depth of cure’?
- The depth to which the composite resin polymerises sufficiently;
- (where hardness will be half that of the surface);
- Indicates increment thickness to use.
L22: What is the typical depth of cure for hybrid and bulk composites?
- Hybrid: 2mm;
- Bulk: claim up to 6mm.
L22: What are the potential problems with curing bulk fill composites?
- Can curing light be positioned the same as in a lab test?;
- Can optical rod be placed addicted to tooth/ composite?;
- Does blue light illuminate material?;
- Will it polymerise (cure) fully?;
- What impact does ‘stray’ blue light have on soft tissues?.
L22: What are the general potential problems with light curing?
- Light/ material mismatch (overexpose);
- Premature polymerisation from clinical lights;
- Optimistic depth of cure;
- Recommended curing times too short;
- Polymerisation shrinkage.
L22: What are the potential problems with light curing, regarding patients?
- Exothermic reaction (heat release in composite material, conducts to adjacent dentine/ enamel), risk of irreversible damage to pulp;
- Divergent light beam, may cause thermal damage to tissues.
L22: What are the potential problems with light curing, regarding clinicians?
Occular damage, safety shields/ glass should be used
L22: What are the mechanical property requirements for composite resins?
Hardness
(a) strong, 350MPa (fracture stress)
(b) rigid (Young’s modulus), 15GPa
Bond to tooth/ compatible with bonding systems
L22: What are the clinical property requirements for composite resins?
Large posterior cavity (high strength, high YM, high abrasion resistance
L22: How do conventional, microfine and hybrid composites compare?
- Conventional: strong but problems with finishing and staining due to soft resins and hard particles;
- Microfine: smoother surface (better aesthetics) but inferior mechanical properties (elastic limit and Young’s Modulus);
- Hybrids: compromise between above two.
L22: What is hardness in relation to a composite surface?
Resistance to scratching and indentation
L22: What is abrasion (wear) in relation to a composite surface?
Removal of surface layers when two surfaces make frictional contact
L22: Abrasion, of tooth or composite surfaces, can lead to surface roughness, what can this effect?
- Appearance;
- Plaque retention;
- Sensation when in contact with tongue.
L22: How does surface roughness compare between conventional and microfine composites?
- Conventional: 80um;
- Microfine: 10um.
L22: What material properties can affect wear?
- Filler material;
- Particle size distribution;
- Filler loading;
- Resin formulation;
- Coupling agent.
L22: What clinical properties can affect wear?
- Cavity size and design;
- Tooth position;
- Occlusion;
- Placement technique;
- Cure efficiency;
- Finishing methods.
L22: How does composite bonding to enamel and dentine differ?
Enamel: acid etch technique, dentine: universal bonding system
L22: What is the typical bond strength of composite to both enamel and dentine?
40MPa (dependent on surface preparation of tissue, composite brand and test method)
L22: Why is good bonding important?
- Reduce microleakage;
- Counteract polymerisation shrinkage;
- To minimise cavity design (small, no need for undercuts);
- Stress transfer (restoration does not have to withstand full stress, can be transferred to tooth/ bone; if poor bonding to tooth, stress concentrated at interface and more likely to fail).
L22: How do mechanical properties compare between microfine and hybrid composites?
- Hybrids better for all properties;
- Higher compressive strength, elastic stress, tensile strength, flexural strength, elastic modulus, hardness;
- See L22.
L22: How do mechanical properties of composites compare to enamel and dentine?
- Compressive strength similar to (microfine) or greater than (hybrid) enamel and dentine;
- Tensile strength, slightly greater than dentine and enamel;
- Elastic modulus much less than enamel, comparable to (hybrid) dentine;
- Hardness is much less than enamel but similar to dentine.
L22: How do the mechanical properties of composites compare to amalgam?
- Amalgam greater compressive strength (just);
- Comparable tensile strength (amalgam slightly greater);
- Amalgam twice elastic modulus;
- Comparable hardness (amalgam slightly greater).
L22: Describe the thermal properties of composites.
- Low thermal conductivity, good for protection of the pulp;
- High thermal expansion coefficient, poor so risk of microleakage.
L22: How do the thermal expansion coefficients compare of amalgam, composite and GIC; and how do these compare to enamel and dentine?
- Composite highest (worst), then amalgam;
- GIC comparable to enamel but higher than dentine;
- Enamel higher (worse) than dentine;
- Ceramin and gold alloy lower and more comparable to enamel and dentine.
[enamel: 11, dentine: 8, composite: 25-68, amalgam: 22-28, GIC: 10-11, ceramic: 8-14, gold alloy: 12-15]
L22: What other advantageous properties do composites offer?
- Aesthetics, good and range of shades;
- Radiopaque, some are so able to distinguish secondary caries;
- Smooth surface finish/ polishable (but product/ technique sensitive);
- Handling/ viscosities, range throughout products (good for different cavities/ uses).
L22: What other disadvantages are there to using composites?
- (Most) do not offer anticariogenic properties;
- (Although low) polymerisation shrinkage still a problem, stresses can develop and make de-bonding likely;
- Biocompatibility generally ok but some concerns about resins in general (not all monomer polymerised).
L22: Which would you use of the following: RMGI, compomer and composite resin, for high, medium and low caries risk patients?
- High risk: RMGI (if frequent attenders);
- Medium risk: compomer (caries under control, regular attenders);
- Low risk: composite resin.
