Composite materials in dentistry: chemical, light, fluid, microfilled

Dental composites are polymeric multiphase compositions of various viscosities used for the treatment and restoration of teeth.

They include an organic matrix, an inorganic filler (it must be at least 50% by weight) and silane (silicon hydride, which acts as a binder between the filler and the matrix).

The matrix is ​​the foundation composite, its skeleton, which houses all the other components. It determines the main properties - biocompatibility, adhesive characteristics, plasticity. Affects color stability, strength, polymerization shrinkage.

The matrix is ​​based on polymer resins - decanediol methacrylate, bisphenol glycidyl methacrylate, urentanedimethyl methacrylate and others. To impart the necessary properties, additives are added to the resin.

1. Polymerization inhibitors. They increase the working time, increase the shelf life.
2. Catalysts. The polymerization process is started. Co-catalysts provide chemical curing. Photoinitiators are responsible for the polymerization of light-curable formulations.
3. UV absorbers (UV stabilizers). Prevents discoloration caused by sunlight.

The filler is present in the substance in the form of particles, evenly distributed in the resin. Their type, size and shape determine water absorption, radiopacity, strength, shrinkage, and abrasion resistance.

The filler is made from the following materials:

• glass;
• silica;
• polymerized crushed;
• titanium and zirconium silicate;
• quartz;
• heavy salts;
• some metal oxides.

Silane is a bifunctional substance that provides a bond between an organic matrix and an inorganic filler. Its presence is a feature of dental substances that distinguish them from plastics.

Classification of composites

Complexity and ramification of the classification dental materials due to a wide assortment, constant updating, a variety of types and forms of its components.

The classification takes into account:

• chemical composition;
• the size of the filler fraction;
• particle composition;
• degree of filling;
• curing method;
• consistency;
• appointment.

Chemical composition

According to the chemical composition of the matrix, composites are subdivided into:

• traditional;
• ormokers.

The latter stands for “organically modified ceramics”. This is a new type of dental formulation that has evolved from improvements and modifications to traditional matrices.

Ormockers have increased biological compatibility (the amount of free monomers in them is reduced to minimum), low shrinkage (1.9%), stronger bond with the filler and high physical and mechanical characteristics.

Filler particle sizes

This parameter affects such important properties as wear resistance and polishability. The smaller the filler grains, the higher the wear resistance and the longer the dry gloss lasts.

Large fractions (more than 0.1 microns) are obtained from metal salts - aluminum, barium, lithium, strontium, titanium, as well as glass and quartz. The nanofiller is made from silicon dioxide. If the material contains filler with different grain sizes, the average value is indicated in the description for it.

There are the following types of substances depending on the size of the filler particles.

• microfilled - grain sizes vary in the range of 0.04-0.4 microns;
• minifilled - 1-5 microns;
• macro-filled - 8 microns and more;
• microhybrid - there are 2 types of filler - with particle sizes of 1-5 microns and 0.04-0.1 microns;
• macrohybrid - 8-12 microns and 0.04-0.1 microns;
• hybrid maximally filled (totally executed) compositions - 0.01-0.1 microns, 1-5 microns, 8-5 microns, 1-5 microns;
• nanofilled (nanocluster) - up to 100 nm;
• nanohybrid - a mixture of sizes 0.004-3 microns.

Particle composition

It has been found that the simultaneous use of coarse and fine filler particles improves abrasion resistance, strength and edge fit. It also brings the value of its thermal expansion closer to the values ​​that the tooth tissues have.

By the type of combination of particle sizes, the following are distinguished:

1. Homogeneous (microfilled, macrofilled, minifilled).
2. Heterogeneous (micro- and macrohybrid, nanohybrid, maximally filled).
3. Totally executed (includes particles of different sizes - micro, macro, mini). The degree of filling of these materials is 80-90%, shrinkage is 1.7-2.0%.

Filling degree

Composites for dentistry are characterized by filling - the weight or volumetric content of the filler in the matrix, expressed as a percentage. The degree of filling determines many properties - shrinkage, radiopacity, optical characteristics, strength. The higher the fullness, the stronger the substance, the lower the shrinkage, the better the radiopacity. According to the degree of fullness, substances are divided into:

• highly filled - above 70% by weight;
• medium filled - 65-75%;
• weakly filled - less than 65%.

Method of curing

The process of polymerization (curing) of the matrix consists in the transformation of low molecular weight compounds (monomers) into large molecular weight compounds (polymers). The reaction takes place due to free radicals formed when the polymerization initiator is activated.

During curing, the composite shrinks in volume, its density increases, which leads to shrinkage of 2-6%. The decrease in volume is due to the decrease in the distance between the monomers. The curing reaction is triggered by a special substance - an initiator, according to the type of activation of which all dental substances are divided into:

• light;
• chemical;
• double curing.

For the polymerization of light-curing materials, camphorquinone, lucerin, phenyl-propanedione are used. In chemically curing substances, benzene peroxide and amines are used.

The type of light-curing initiator determines the light source. In particular, materials containing lucerin are poorly polymerized by plasma and diode lamps. Modern substances contain several initiators, which makes it possible to use different light sources for polymerization.

Consistency

Along with pasty mixtures, flowable ones are also used. For their manufacture, modified matrices with high-flow resins are used.

According to the degree of density, they are distinguished:

• normal viscosity;
• fluid (subdivided into low, medium and high fluid);
• packable or condensable (high density).

Appointment

Due to the fact that the anterior and posterior teeth experience different loads, the substances used for their restoration can differ significantly in their characteristics. Depending on the purpose, composites are subdivided into compositions:

• for the treatment of lateral (chewing) teeth;
• for the restoration of anterior teeth;
• versatile materials used to restore both anterior and posterior teeth.

Properties of composites

The composites have a number of technological and operational characteristics laid down in them by the manufacturer. It is impossible to change them, so the only way to find the right material is to be well informed about the parameters of a particular composition.

The main properties of dental substances:

1. Compressive / tensile strength. It varies depending on the fullness and consistency. In the most durable packable compositions, it reaches 450 MPa, in flowable compositions it decreases to 220 MPa.
2. Wear resistance. The following pattern is observed: the finer the filler grains, the higher the wear resistance.
3. Optical properties (opacity, opalescence, etc.). Opacity is the ability to trap visible light, that is, opacity, opacity of the material.
4. Radiopacity. Determined by the type and amount of filler. Expressed as a percentage of the reference value - the radiopacity of a 1 mm thick aluminum plate. For example, the radiopacity of enamel is 230%, dentin is 150%. In general, this parameter ranges from 130% for flowable to 350% for dentin nanocomposites. High radiopacity makes the material well visible on X-ray images, increases the diagnostic accuracy.
5. Polymerization shrinkage. The minimum possible shrinkage is 1.6%, the highest is 5.5%. Most of the substances have a shrinkage of 2-3%. Its value depends mainly on the fullness. For flowable formulations, it averages 3.5–5%, for ormokers and packable formulations - 1.7–2%.
6. Thixotropy - a change in viscosity under a mechanical load, an increase in fluidity when a load is applied, and an increase in viscosity at rest.
7. Thermal expansion. Ideally, it should be equal to the thermal expansion of the tooth tissue.
8. Elasticity. It characterizes the resistance of a material to compression and tension during elastic deformation. All composite substances are more elastic than hard tooth tissues. Flowable and microfilic compositions have a lower modulus of elasticity.
9. Biocompatibility. Depends mainly on the volume of residual (unpolymerized) monomer. Its level is regulated by international standards (ISO). It is impossible to achieve 100% polymerization. Light-curing products have a lower residual monomer volume than chemically-cured ones. After proper polymerization, all modern formulations are non-toxic.
10. Working properties. They are composed of a number of factors - speed and convenience of working with composites, efficiency, versatility. The convenience of work, in turn, depends on the viscosity, the type of packaging and other characteristics that affect the ease of insertion into the cavity of the tooth, its distribution there and modeling.
11. Aesthetics. It is determined by the polishability, the duration of the dry gloss retention, the number of color shades. The most aesthetic are gyomers and nanocompositeswith more than 40 color shades. Thanks to this, it is possible to imitate the color shade of the tooth and its enamel as accurately as possible.

Chemical curing materials

Composites with chemical polymerization are mainly represented by hybrid and microfilled compositions. Release form - "liquid / powder" or "paste / paste".

Advantages of chemically cured formulations:

• soft flowing low shrinkage;
• good appearance;
• short time required for restoration.

Disadvantages:

• the need for accurate dosing;
• limited time for work;
• low polishability and color fastness in comparison with light-cured;
• reduced usability;
• relatively large amount of unreacted monomer.

The chemically curing substance adhesive system is designed to bond the material to the enamel of the tooth rather than to the dentin. To adapt to the latter, either an insulating pad or a universal enamel-dentine adhesive system is used.

Light-curing materials

Light-cured composites are available as a one-component paste or as a flowable substance. The initiator of polymerization is a light-absorbing component, most often camphorquinone. When it is irradiated with light, free radicals are formed, due to which polymerization occurs.

Advantages:

• mixing and ensuring the homogeneity of the mixture is not required;
• the restoration can be modeled before polymerization;
• high aesthetics and color fastness (due to the absence of hardening additives).

The main disadvantage of light-curing mixtures is the inhomogeneity of the degree and depth of polymerization, which depends on the transparency and color shade, as well as the power of the light source.

To improve the quality of polymerization, reduce shrinkage and stresses, layer-by-layer application is used.

Photocurable substances are usually incompatible with chemically curable ones.

Macrofilled

The history of dental composites began with macro-filled mixtures. Therefore, it is quite natural that in some respects they are inferior to their followers. But they also have advantages:

• high strength;
• satisfactory radiopacity;
• good optical properties.

But there are still more disadvantages:

• poor polishability, lack of dry shine;
• large roughness of the surface of the seal;
• plaque formation;
• low color fastness.

All this leads to a decrease in the aesthetics of the restoration and a relatively rapid wear of the matrix, from which individual particles are exfoliated, leaving behind craters. Accelerated wear of the filling causes a change in the occlusal plane and displacement (migration) of the teeth.

Microfilled

Microfilled (microfilic) composites were developed almost 50 years ago. For their time, they represented a real breakthrough in restoration technology, as they ensured excellent polishability and high aesthetics of the restoration.

Initially, the microfilled substances had a particle size of about 1 micron. It is currently only 0.04 microns. Microfilic compositions are used mainly for the restoration of anterior dental units and the manufacture of direct veneers.

Advantages:

• high color fastness, polishability and wear resistance;
• long-lasting glossy surface;
• good aesthetics.

Disadvantages:

• relatively low strength;
• significant polymerization shrinkage and thermal expansion.

Flowable materials

Flowable are used mainly for filling small carious cavities, as well as where high-quality marginal adhesion and compensation of polymerization shrinkage are required.

Benefits of flowable composites:

• small modulus of elasticity;
• ease of use;
• good polishability and aesthetics.

Disadvantages:

• insufficient strength;
• significant shrinkage:
• low radiopacity.

Hybrid

Hybrid formulations are the most widely used today dental material. Largely due to its versatility. The limitation in use exists only for carious cavities, to which access is difficult, and therefore a substance of a different consistency is required.

Advantages:

• versatility;
• the convenience of use;
• high strength;
• increased aesthetics;
• sufficient radiopacity.

Disadvantages:

• average or above average shrinkage;
• significant modulus of elasticity;
• not always affordable price.

Nanocomposites

Nanocluster compositions are considered the most promising group of restorative materials. Their feature is the use of a filler made of nanoparticles (nanomers and nanoclusters), which ensure homogeneity and high filling of the matrix.

Advantages of nanocomposites:

• high aesthetics, provided by excellent polishability and long-lasting dry gloss;
• acceptable strength properties;
• low shrinkage.

Disadvantages:

• significant price;
• insufficient knowledge of the results of restoration.

Ormockers

The emergence of organically modified ceramics is a result of the search for materials with low polymerization shrinkage and long service life. The modification of the matrix made it possible to increase the density of the composite, reduce its shrinkage (less than 2%), and achieve the minimum amount of residual monomer. In terms of other characteristics, ormokers are close to hybrid ones.

Advantages of Ormokers:

• low shrinkage;
• practical absence of residual monomer;
• high strength;
• good aesthetics.

Disadvantages:

• low level aesthetics;
• high price;
• insufficient knowledge.

Requirements for composites

The possibility of direct filling using dental composites has significantly expanded the possibilities dental restoration. Modern polymer substances have high adhesion to teeth, which is not inferior to the connection of tooth tissues with each other.

Polymer composites are inert and non-toxic, which allows them to be used without insulating spacers. An important advantage of materials is the ability of a non-polymerized (viscous) form to combine with a polymerized (hardened) one.

The main characteristics of substances are constantly improving - thixotropy increases, polymerization shrinkage, new color shades are added, compressive strength, tensile strength and abrasion.

However, despite all these advances in development, the ideal material has not yet been created. To successfully solve the problems of dental restoration, the composites used must have the following properties.

1. High radiopacity of materials used for filling the chewing teeth.
2. Good adhesion to tooth tissues, ensuring complete tightness of the internal cavities of the restored teeth.
3. High compressive and tensile strength, abrasion resistance. These properties are especially important for the materials used for filling the chewing teeth, since during the chewing process very large loads are created on the filling, reaching 70 kg.
4. Ease and ease of use. The substance should be easily introduced into the carious cavity and not create problems during the formation of the filling.
5. Biocompatibility with the oral cavity and tooth tissues. The substances must not contain substances that irritate the mucous membrane and pulp.
6. Possibility of long-term storage without deterioration of properties.
7. Lack of sensitizing effects on the doctor and patient.
8. Maximum correspondence of color, shine, transparency of the polymerized material to natural tooth tissues. Preservation of color stability.
9. The proximity of physical characteristics (thermal conductivity, thermal expansion, etc.) to those of the tooth tissue.
10. Versatility. The ability to use the same substance in different clinical settings. Today, the most versatile are ormokers and hybrid composites.
11. Availability.

Dental polymer composites compete successfully with other filling compounds. Their advantages include high strength, wear resistance, good aesthetic qualities, low polymerization shrinkage, versatility, allowing them to be used in various clinical situations, on the frontal and chewing teeth.

There is no doubt that in the near future there will be new materials that will meet the requirements for an “ideal” composite to the maximum extent.