Decrease the polymerization shrinkage of composite by:
a- Incremental placement with increase time of curing.
b- Incremental placement with high intensity light cure.***
Polymerization contraction:
The polymerization shrinkage of acrylic matrix-based composite resins is inherent to the polymerization reaction itself and depends on their chemical composition, the volume fraction of the feedstock and the degree of conversion (measurement of the degree of polymerization) during the polymerization which is never total and uniform.
The mechanical stresses occurring during and after the polymerization phase occur simultaneously in the mineralized fabrics, in the material and at the interface between the two. These constraints can have bad clinical consequences: flexion of the cusp up to fractures of the amelic limits, postoperative sensitivities and formation of peripheral hiatuses.
Interface constraints:
In principle, a high percentage of fillers decreases the polymerization shrinkage because of the reduction in the percentage of resinous matrix. However, hybrid composites do not necessarily have a lower setting shrinkage rate than microfilled composites (Table 3). This is partly explained by the presence of prepolymerized particles in the microcharged which do not participate in setting shrinkage. In addition, the addition of diluents to counterbalance the rapid increase in viscosity of the hybrid composites due to the increase in the number of particles increases the setting shrinkage.
Fluid composites shrink more than other composites (Table 3), due to, among other things, their lower percentage of fillers.
Finally, the essential element to take into account, more than the percentage of shrinkage is the stress at the tooth-material interface which depends not only on the percentage of shrinkage but also on the kinetics of polymerization.
To limit the clinical consequences, various procedures, adopted for a long time, remain of topicality: sealing technique (stratification, quantity of material / layer), polymerization (ex: "soft start" polymerization), factor of configuration (factor C) , etc ...
Clinical consequences:
- Tension in the dental tissues that can cause cusp flexions, embrittlement or ruptures of the enamel,
- Tears / - extended and deep at the joint with creation of a peripheral hiatus favoring marginal percolation, dyscolorations, inflammatory reactions pulp, recurrence of caries,
- Internal stresses in the material favoring partial or complete rupture of the resin-particle bond, appearance of cohesive fractures in the material.
- Decrease of the mechanical resistance.
Kinetics of the setting shrinkage of chemopolymerizable and photopolymerizable composites.
The slower setting of the chemopolymerizable composites facilitates the dissipation of the shrinkage stresses, although the final shrinkage percentage is of the same order).
This polymerization kinetics depends, whatever the type of polymerization, the mode of polymerization (Standard, Pulsed, Exponential), the type of lamp (power, duration, type of light source), the distance between the composite and the source and, the type of composites (monomers, hue).
Thermal properties:
They also intervene in the integrity of the peripheral joint:
- Coefficient of thermal expansion.
The coefficient of thermal expansion of composite resins is 2 to 4 times greater than that of dental tissues:
• 25.10-6 / ° C• 22.10-6 / ° C • 45.10-6 / ° C Reminder: Email: 11.4.10-6 / ° C & Dentine: 8.3.10-6 / ° C
Constraints may appear at the material / tooth interface during temperature changes.
Thermal conductivity:
In a simple way, it can be said that the thermal conductivity is the ability of a material to transmit the heat supplied to it.
The composite resins have a low thermal conductivity (1.09 Wm-1.K-1), close to that of enamel (0.93 Wm-1.K-1) and dentin (0.64 Wm). 1.K-1), in contrast to amalgam (83 Wm-1.K-1).
The difference in coefficient of thermal expansion between the composite resin and the dental tissues may cause stress at the material / tooth interface during temperature changes, but the thermal conductivity of the composite resins being similar to the dental tissues and weak, it is likely that prolonged temperature changes so that these constraints can be transmitted.
Water absorption and solubility:
A perfect seal would require a nanometer interface.
The water behavior is directly related to the quality of the polymerization.
• Water absorption between 0.2 and 2.2 mg / cm².
• Solubility in water after 2 weeks varies between 0.01 and 2.2 mg / cm²,
• The resulting volumetric expansion (0.3 to 4%) offsets the polymerization shrinkage.
Optical and radiographic properties:
The reduction in the number of hues available in a system and the emergence of a new terminology characterize the colorimetric evolution of the new composite resins. In recent years, some manufacturers have increased the number of shades to achieve restorations of a high aesthetic level. However, this approach runs up against the limits of visual perception and the precise discrimination of all hues by practitioners (ΔE must be> 3.3).
Dentin and enamel have different optical properties: dentin is characterized by a high opacity, a marked fluorescence and a large variability of its saturation related to the age of the patient; conversely, the enamel is translucent, opalescent and very weakly saturated.
The differences in opacity are obtained thanks to the differences in refractive index between the mineral fillers and the matrix; the different levels of saturation are obtained thanks to varying concentrations of metal oxides.
The heavy elements contained in the charges (high atomic number) allow the radiographic visualization.
adhesion:
A composite resin does not adhere spontaneously to the dental tissues. For adhesion to dental tissues, an adhesive system must be used:
- Enamel etching (micro-retentions), dentin (opening of the tubules), conditioning or elimination of the smear layer,
- Amelo-dentine coupling agent: the best results are obtained with loaded adhesives and having a solvent that does not contain acetone (volatile, highly dependent operator).
Polymerization contraction:
The polymerization shrinkage of acrylic matrix-based composite resins is inherent to the polymerization reaction itself and depends on their chemical composition, the volume fraction of the feedstock and the degree of conversion (measurement of the degree of polymerization) during the polymerization which is never total and uniform.
The mechanical stresses occurring during and after the polymerization phase occur simultaneously in the mineralized fabrics, in the material and at the interface between the two. These constraints can have bad clinical consequences: flexion of the cusp up to fractures of the amelic limits, postoperative sensitivities and formation of peripheral hiatuses.
Interface constraints:
In principle, a high percentage of fillers decreases the polymerization shrinkage because of the reduction in the percentage of resinous matrix. However, hybrid composites do not necessarily have a lower setting shrinkage rate than microfilled composites (Table 3). This is partly explained by the presence of prepolymerized particles in the microcharged which do not participate in setting shrinkage. In addition, the addition of diluents to counterbalance the rapid increase in viscosity of the hybrid composites due to the increase in the number of particles increases the setting shrinkage.
Fluid composites shrink more than other composites (Table 3), due to, among other things, their lower percentage of fillers.
Finally, the essential element to take into account, more than the percentage of shrinkage is the stress at the tooth-material interface which depends not only on the percentage of shrinkage but also on the kinetics of polymerization.
To limit the clinical consequences, various procedures, adopted for a long time, remain of topicality: sealing technique (stratification, quantity of material / layer), polymerization (ex: "soft start" polymerization), factor of configuration (factor C) , etc ...
Clinical consequences:
- Tension in the dental tissues that can cause cusp flexions, embrittlement or ruptures of the enamel,
- Tears / - extended and deep at the joint with creation of a peripheral hiatus favoring marginal percolation, dyscolorations, inflammatory reactions pulp, recurrence of caries,
- Internal stresses in the material favoring partial or complete rupture of the resin-particle bond, appearance of cohesive fractures in the material.
- Decrease of the mechanical resistance.
Kinetics of the setting shrinkage of chemopolymerizable and photopolymerizable composites.
The slower setting of the chemopolymerizable composites facilitates the dissipation of the shrinkage stresses, although the final shrinkage percentage is of the same order).
This polymerization kinetics depends, whatever the type of polymerization, the mode of polymerization (Standard, Pulsed, Exponential), the type of lamp (power, duration, type of light source), the distance between the composite and the source and, the type of composites (monomers, hue).
Thermal properties:
They also intervene in the integrity of the peripheral joint:
- Coefficient of thermal expansion.
The coefficient of thermal expansion of composite resins is 2 to 4 times greater than that of dental tissues:
• 25.10-6 / ° C
Constraints may appear at the material / tooth interface during temperature changes.
Thermal conductivity:
In a simple way, it can be said that the thermal conductivity is the ability of a material to transmit the heat supplied to it.
The composite resins have a low thermal conductivity (1.09 Wm-1.K-1), close to that of enamel (0.93 Wm-1.K-1) and dentin (0.64 Wm). 1.K-1), in contrast to amalgam (83 Wm-1.K-1).
The difference in coefficient of thermal expansion between the composite resin and the dental tissues may cause stress at the material / tooth interface during temperature changes, but the thermal conductivity of the composite resins being similar to the dental tissues and weak, it is likely that prolonged temperature changes so that these constraints can be transmitted.
Water absorption and solubility:
A perfect seal would require a nanometer interface.
The water behavior is directly related to the quality of the polymerization.
• Water absorption between 0.2 and 2.2 mg / cm².
• Solubility in water after 2 weeks varies between 0.01 and 2.2 mg / cm²,
• The resulting volumetric expansion (0.3 to 4%) offsets the polymerization shrinkage.
Optical and radiographic properties:
The reduction in the number of hues available in a system and the emergence of a new terminology characterize the colorimetric evolution of the new composite resins. In recent years, some manufacturers have increased the number of shades to achieve restorations of a high aesthetic level. However, this approach runs up against the limits of visual perception and the precise discrimination of all hues by practitioners (ΔE must be> 3.3).
Dentin and enamel have different optical properties: dentin is characterized by a high opacity, a marked fluorescence and a large variability of its saturation related to the age of the patient; conversely, the enamel is translucent, opalescent and very weakly saturated.
The differences in opacity are obtained thanks to the differences in refractive index between the mineral fillers and the matrix; the different levels of saturation are obtained thanks to varying concentrations of metal oxides.
The heavy elements contained in the charges (high atomic number) allow the radiographic visualization.
adhesion:
A composite resin does not adhere spontaneously to the dental tissues. For adhesion to dental tissues, an adhesive system must be used:
- Enamel etching (micro-retentions), dentin (opening of the tubules), conditioning or elimination of the smear layer,
- Amelo-dentine coupling agent: the best results are obtained with loaded adhesives and having a solvent that does not contain acetone (volatile, highly dependent operator).
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