Composite resins

Composite resins

Introduction 

The world of dental materials has experienced a series of revolutions rather than an evolution.

 The revolution in materials after dental amalgams is aesthetic thanks to composite resins which appeared in the 1950s. Initially self-polymerizable, composites only became photo-polymerizable at the end of the 1970s and have since gradually become established.

 The constant evolution of their compositions and properties since their advent is considerable. 

By improving their composition, composite resins have gradually become easier to implement, more aesthetic, easier to polish. They have fewer polymerization constraints and have become more resistant to different degradation mechanisms. Microhybrid composite resins (1990s) and then nanohybrid (2000s) are the ultimate products of all the performances acquired by these materials. 

They now exist in a multitude of viscosities (fluid or compactable) to respond to a wide variability of clinical situations. Since the 2010s, so-called “bulk-fill” composites, more sensitive to photopolymerization, can fill cavities of 4 to 5 mm in height in a single increment, with a better controlled setting contraction, thus facilitating their clinical implementation.

The second revolution in dental composites is adhesive. It is this which has enabled the development of more conservative dentistry based on the principle of tissue economy.

1. Definition 

A dental composite is an organo-mineral filling biomaterial consisting of a resin matrix in which mainly mineral fillers are dispersed. The latter are bonded to the matrix by a coupling agent. The composites are fixed to the dental tissues via an adhesive. 

The different components of dental composite and the interface with dental tissues 

2. Composition of composite resins  

2.1 The organic phase 

The organic matrix is ​​also called organic phase, dispersing phase or continuous phase. It constitutes 24 to 50% of the volume of the composite. It includes:

• Matrix resin, 
• Diluents (or viscosity controllers),
• Setting inhibitors, curing agents and pigments

1.  Matrix resin: monomer or oligomer: 

In small proportion compared to mineral fillers, this
essential component is at the origin of the degradation of certain mechanical properties of the composite (resistance to wear for example). It is also at the source of polymerization shrinkage and the constraints that it generates. is a chemically active component of the composite. They are all monomers “R – di methacrylates”, thus making all composite resins compatible with each other and with adhesives. The presence of two polymerizable functions carried by the same molecule makes it possible to generate an organic network with a high crosslinking density.

Example of resin: The Bis-GMA molecule It decomposes :

•  Two aromatic cycles which stiffen the molecule, ü A phenol cycle which reduces setting retraction but increases viscosity,
• Two hydroxyl radicals offering the possibility of obtaining hydrogen bonds which results in a significant viscosity of the unpolymerized matrix, 
•  Two functional methacrylate groups allowing the development of the polymer structure, 
• Two ester bonds (causing potential hydrolysis). Polyurethanes 

Its main advantage: low viscosity, allowing the incorporation of a greater percentage of fillers without adding low molecular weight diluent. 

2.1.2 The different roles of the matrix

Gives the material its plastic consistency before polymerization (+/- liquid or pasty) 

-Allows hardening (it is the matrix which will polymerize): lots of polymers will intersect reaching several mm in length => polymerization allows hardening

 -Guarantees the cohesion and resistance of the whole after polymerization

 -Responsible for many defects (possible depolymerization) 

2.1.3 Diluents or viscosity controllers

• Bis-GMA and diurethane dimethacrylate monomers are very viscous liquids due to their high molecular weight. Addition of a large amount of fillers will result in the formation of a material with a consistency too thick for clinical use. To counteract this problem,
• low viscosity monomers, known as viscosity controllers or diluents, are added.

2.1.4 Inhibitors of uptake

Also called preservatives, because they help preserve composite resins.

Composite materials must be able to be stored without spontaneous polymerization due to heat or exposure to ambient light.

The most frequently used inhibitors are  phenol derivatives  , oxygen from ambient air. 

2.1.5  Pigments 

They are the origin of the color of the composite. To meet aesthetic needs, the pigments must be as stable as possible over time, so that the restoration keeps its initial color for as long as possible. 

Using a shade guide to best adjust the shade of the composite restoration. Colored composite mimicking dentin for the restoration of a posterior tooth

  2.1.5 The setting reaction of composites

The process by which the composite in pasty form transforms into hard material is the polymerization of the resin matrix.

Polymerization of the monomer or oligomer involves:

•  the release of free radicals which are formed by transformation of the initiator or starter by activators or catalysts.
•  These free radicals cause the opening of the double carbon bond of the monomer and thus allow its activation during the initiation phase and the formation and elongation of the polymer.
•  Once the monomer is activated, it can itself react with another monomer and therefore create a chain crosslink to create polymers. 

1. Chemical initiation (chemo- or autopolymerization)

In the first composites (chemo- or self-polymerizable), this result was obtained by mixing two pastes containing the components necessary for the induction of polymerization: one paste contained an activator (tertiary amine), while the other paste contained an initiator, generally benzoyl peroxide.

  a) Advantages

– photopolymerization of all materials currently on the market 

– low cost;

– possibility to choose among various systems intended to control the polymerization kinetics

b) Disadvantage

-progressive loss of power of the light source requiring monitoring by the practitioner

-This heating of the lamp requiring a fan cooling system makes the gun heavy, noisy  

2.1.5.2 Photopolymerization

Instead of the amine-accelerated benzoyl peroxide initiation system, a polymerization reaction can be initiated by exposure to electromagnetic radiation such as UV light (wavelength 365 nm) or visible light (in the region 420 to 470 nm). It is the photons that serve as activators by acting on the photoinitiators to form free radicals. 

        a)  Advantages

-photopolymerization of all materials currently on the market 

– low cost;

– possibility to choose among the various systems intended to control the kinetics of polymerization.

  b) Disadvantages 

– Progressive loss of power of the light source requiring monitoring by the practitioner

– This heating of the lamp requiring a fan cooling system makes the gun heavy, noisy  

2.1 Fillers
Composites currently available on the market are mainly differentiated by
the characteristics of the fillers they contain. The role of these fillers is to compensate for the inadequacies (poor mechanical and thermal properties for example) of the host matrix to which they are bound (chemically and/or physically). Furthermore, fillers greatly influence the polymerization contraction and the water absorption of composites. Thus the composition, size, size distribution and mass or volume percentage of the fillers within the organic matrix will define a wide range of composites.

2.2.1 Main effects of increased load

They increase:

– compressive strength,

-tensile strength,

– bending resistance,

-radiopacity.

2.2.2. Main effects of particle size reduction 

• the surface condition is improved by reducing the particle size. This provides an aesthetic advantage and reduces the aggressiveness of the material towards opposing teeth 
•  Wear resistance is improved as particle size decreases.

2.2.3 The nature of the charges

2.2.4 The size of the loads: 

• Macrocharges: initially 1 to 50 μm, composed of large particles of glass or quartz. 
• Microcharges: approximately 0.04μm (silica, SiO2) 
• Nanofillers: Today, the trend is towards the commercialization of composites based on nanotechnology and containing, among other things, nanoparticles of 2 to 70 nm. 

2.2.5 The shape of the charges

It varies depending on the method of preparation:

• angular: obtained by grinding and attrition,

• rounded: result from sintering,

• spherical: sol-gel process 

2.3 The interfacial phase corresponds to the coupling agent between the matrix and the fillers. These are organosilane derivatives that will chemically bind the matrix and the fillers. The most recent developments aim to improve this bond in order to reduce solubility and thus extend the life of the restorations.

3. Classification of composite resins

3.1 Depending on the size of the loads

Although no classification is universal, this one seems to be the most consistent, since the size of the fillers determines many properties of the material. 

1. Macrocharged They were the first on the market. They came in the form of two pastes to mix. They were a combination of macroparticles (quartz, ceramic, glass) obtained by crushing and a matrix resin. The size of the charges varied from 1 to 40µm. 

3.1.2 Microcharged In order to overcome the shortcomings of macrocharged composites, microcharged composites have appeared on the market, which, as their name indicates, were composed of silica microcharges of 0.04µm on average. Can be homogeneous or heterogeneous 

3.1.3 Hybrid Composites They currently represent the largest family of composites. Characterized by a mixture of fillers of different sizes and composition of variable nature, shape and size. We can find micro-fillers of silica (0.04µm), macro- (10 to 100µm), midi- (1 to 10µm), mini– (0.1 to 1µm) and microfillers (0.01 to 0.1µm) of glass….

Group together hybrid-microhybrid-nanofilled composites

3.2 classification according to viscosity

The consistency of a composite is one of the practitioner’s selection criteria for a given restoration. During the same procedure, the practitioner may use several composites with very different consistencies.

1. Medium viscosity composites: 

Called “universal”. Their viscosity is suitable for a large number of indications, both anterior and posterior. Their opacity is adjusted, depending on the clinical indication targeted and/or the complexity of the shades to be reproduced (enamel, dentine and intermediate opacity). The filler rate of these composites is of the order of 78% by weight; 60% by volume.

2. Fluid composites

Their clinical indications are specific (occlusal microcavities, slot cavities, cervical cavities or dentin substitute, for example). 

Due to their fluidity, they show easy spreading, associated with good adaptation to cavity walls. 

 They are preferred in the case of covering the bottom of a cavity before adding a more viscous composite.

They exhibit significant polymerization shrinkage (up to 5% by volume) and reduced mechanical properties (compared to universal composites) due to their low filler content (between 50 and 70% by weight and less
than 50% by volume) 

3. Compactable or condensable composites These composites were developed in the 1990s for posterior restorations in order to replace amalgams while trying to maintain their conditions of use (ease, speed of handling). 

Due to a high failure rate during their clinical use, this type of composite has been gradually
abandoned.

3. Classification according to the mode of polymerization of the resin matrix
   1. Chemo-polymerizable composites Chemo-polymerizable matrix composites come in the form of two components (two pastes or a powder and a liquid), one containing the initiator, the other containing the co-initiator. The practitioner mixes them at the time of his
      intervention. 

3.3.2 Photopolymerizable composites In these composites the generation of primary radicals is initiated photochemically. The photochemical initiation resulting from the sole activation of the monomers by photons

3.3.3. Dual composites In these composites the generation of primary radicals is initiated photochemically and chemically

3.4 Classification according to clinical indication

4. Properties of composite resins 
5. Mechanical properties

• Compressive strength Comparing the compressive strengths of various composites and amalgams with those of enamel and dentine, it is possible at first sight to conclude that these materials have satisfactory values.

unfilled resins	conventional composites	Microfines	universal hybrids
80 MPa	300 MPa (240-350)	350 MPa (300-400)	a (350-450)

• Tensile strength This is the resistance of the material to lateral forces. Composites have a higher tensile strength than amalgam (≈ 48 MPa). There is a high sensitivity to internal defects or small surface micro-fractures, which cannot be eliminated, therefore the tensile strength of composites also depends on the quality of surface finish.   

microfiller composites	fluid composites	macrocharge composites	hybrids
≈ 40 MPa	≈ 35 MPa	53.4 MPa	52 to 72 MPa

• YOUNG’s modulus of elasticity

It characterizes the material subjected to constraints and determines its rigidity by measuring the forces from which the material will be deformed reversibly then irreversibly. The higher the modulus of elasticity, the more rigid the material and therefore the less it will deform under the constraint 

The composite should have a Young’s modulus close to that of dentin.

 If the Young’s modulus is low, the material will deform and the occlusal stresses will be exerted directly on the tooth walls. 

• Hardness 

Is a surface mechanical property. It defines the resistance to penetration of a material or to permanent deformation per unit area

The least hard are fluid and microfine composites. The highest values ​​are recorded for universal microhybrids. 

The hardness of enamel is significantly higher than all composite materials

• Fatigue resistance: 

 Cyclic mechanical stresses cause microcracks that lead to fatigue fractures, particularly in occlusal contact areas. They also play an unfavorable role in marginal adaptation, particularly in posterior composites with deep gingival extensions.

• Wear resistance: 

  Wear has long been considered the weak point of posterior composites. However, significant advances in their composition and in filler technology have largely contributed to improving wear resistance, through different, more numerous and smaller fillers.

4.2 Physical properties 

• Thermal expansion:

Another factor affecting the integrity of the peripheral seal is the difference between the coefficient of thermal expansion of the composite and that of the dental tissues, which is approximately 3 to 4 times smaller than that of composites.

The ideal coefficient of thermal expansion should be around 10 since the coefficient of thermal expansion of enamel is 11.4 and that of dentin is 8.3.

• Setting shrinkage after polymerization 

The major drawback of composites has been and remains setting retraction. The significant development of aesthetic materials has had as a determining objective the control of setting retraction. The clinical consequences are important. Retraction allows the appearance of a peripheral hiatus which can lead to postoperative pain, discoloration and secondary caries.

Tooth-filling joint infiltrations

• Water absorption and solubility: 

Composites absorb significant amounts of water, about 2% by weight, water absorption is a progressive process that increases in composites with lower filler concentration (microfines).

The water absorption and solubility of different composite resins depend on:

• From the proportion of charge to resin. The less the resin is charged, the greater the proportion of matrix, and thus the greater the absorption.
• Degree of polymerization. The increase in absorption will be X 2 and that of solubility X 4 or 6 when the polymerization time is reduced by 25%. Inadequate polymerization will seriously affect both the longevity of the compost and the color stability.

S olubility of a composite resin

• Radio opacity 

The radiopacity of composites is influenced by the percentage and type of fillers used

 With the exception of microfine composites which have almost zero radiopacity because SiO2 is not radiopaque, the majority of composites currently available on the market have a radiopacity greater than that of enamel. 

4.3 Optical properties 

The differences in opacity in composite resins are obtained by differences in refractive indices between the mineral fillers and the matrix. The different saturation levels are obtained by varying concentrations of metal oxides.

The colorimetric logic of current systems is based on the histological structure of the tooth by proposing “enamel” or “dentine” composites; this is the notion of stratification.

4.4. Biological properties

• Biocompatibility: The biocompatibility of composite resins remains a poorly defined problem. It is mainly the free monomers contained in the resin matrix that are likely to cause cellular damage.

• Gingival tissue reaction It has been shown that gingival tissue cells react less favorably to composite resins than to CVI, macrocharged resins appear to be irritating to tissues and that roughness or porosity will tend to promote the accumulation of dental plaque. 
• Post-operative sensitivities It is not so much the type of composite resin used as the thickness of residual dentin that is potentially the cause of post-operative pain.
• The persistence of the dentinal smear ( self-etching adhesive) in the tubules preventing variations in intra-tubular pressure , the main source of suffering in the dentino-pulpal system

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Composite resins