Basics of classification and polymerization
Polymerization (curing) of composite filling materials is always initiated by free radicals. Depending on how exactly these radicals are formed, the following types of reactions are distinguished: thermal, chemical, and photochemical (light-activated).
In addition to the curing mechanism, composites are classified according to several basic parameters:
- By consistency: traditional (regular viscosity), flowable (low-modulus), and packable (condensable).
- By indication: for posterior teeth (where mechanical strength is essential), for anterior teeth (where aesthetics take precedence), and universal.
Thermal activation is not used in clinical dental practice. It is employed exclusively in laboratory settings when a dental technician fabricates indirect restoration elements: inlays, onlays, or veneers.
Chemically activated composites
Chemically cured composite materials (also known as self-curing, or self-curing) are available as two-component systems. This can be a paste-paste or powder-liquid format.
Mechanism of action: One of the system components contains a chemical activator (tertiary aromatic amines), and the other contains a chemical reaction initiator (benzoyl peroxide). At the moment of manual mixing of these components, a reaction begins with the release of free radicals, which leads to the hardening of the mass.
Advantages of chemical composites:
- Guaranteed and uniform polymerization of the entire portion, regardless of the depth of the carious cavity and the thickness of the applied layer.
- Ease of use and minimal time spent on placing the filling (about 20–30 minutes).
Clinical disadvantages: Materials of this group are toxic, characterized by low wear resistance and unsatisfactory aesthetics. During the mixing process, air bubbles inevitably get trapped in the mass, leading to porosity of the finished filling. In addition, they are prone to specific "amine discoloration" — darkening of the restoration over time due to unreacted activator remaining in the structure.
Light-cured materials and curing lights
The advent of photopolymers was a true revolution, displacing silicate cements and unfilled resins from practice. These are single-paste systems where the reaction is activated by photon energy. The initiator here is a tertiary amine, and the catalyst is camphorquinone.
Evolution of curing lights:
- 1970s: ultraviolet light was used.
- 1977: halogen lamps emitting visible light (blue spectrum) were introduced.
- Modern standard: cordless light-emitting diode (LED) lamps. Unlike halogen alternatives, they do not emit heat when generating light, which completely eliminates the risk of thermal overheating of vital tooth tissues.
Pros and cons of photopolymers: The dentist gets the opportunity to contour the restoration for an unlimited time, since polymerization occurs strictly "on command" — when the lamp is turned on. The materials do not require mixing (no porosity), do not darken, and provide a magnificent aesthetic result. However, they are significantly more expensive, require a long working time (40–60 minutes per restoration), and necessitate the use of eye protection equipment against the lamp's harmful radiation.
Classification by filler particle size
The physical and mechanical characteristics of a composite (polishability, strength, shrinkage) depend on the shape and size of the inorganic filler particles. In 1983, F. Lutz and R. Phillips proposed a classification that remains relevant today:
- Macrofilled (8–12 µm and larger):
The first composites to replace acrylic resins. They are strong, have lower shrinkage and water sorption. However, due to the huge particle size, they polish catastrophically poorly. The soft organic matrix wears away quickly, exposing large particles — the surface becomes rough, accumulates dental plaque and pigments, and darkens. They are used only where aesthetics are not critical (Class I, II, V cavities). A typical representative is Evicrol.
- Minifilled (1–5 µm):
A transitional group. They polish better than macrofilled ones, but have lower hardness. They have not found wide clinical application due to weak color stability.
- Microfilled (0.04–0.4 µm):
Created in 1977. Characterized by the smallest particle size (filler volume content is about 37%). They polish beautifully to a "dry gloss" and maintain aesthetics for a long time.
- Hybrid (0.04–8.0 µm):
Contain a mixture of particles of different sizes, attempting to combine the strength of macrofilled and the polishability of microfilled composites.
Briefly: answers to topical questions
What generations are dental adhesive systems divided into?
Dental adhesive systems are divided into seven generations, differing in bond strength and number of components.
- 1st generation — single-component, have very weak adhesion to dentin (2 MPa).
- 2nd generation — two-component, with weak adhesion (8–15 MPa), prone to water dissolution.
- 3rd generation — primer/adhesive system, provides bonding to metal.
- 4th generation — characterized by total-etch technique and hybridization (17–25 MPa).
- 5th generation — single component, uses wet bonding and does not require mixing.
- 6th generation — multi-step systems with self-etching and self-priming.
- 7th generation — single-step, single-bottle systems combining all components.
What main components does a modern adhesive system consist of?
The basic composition of a modern adhesive system includes three main components, which in the latest generations can be combined in a single bottle.
- Conditioner — most often 37% phosphoric acid, providing demineralization of enamel and dentin.
- Primer — a mixture of hydrophilic compounds, alcohol, or acetone, necessary for the formation of the hybrid layer.
- Adhesive (bond agent) — unfilled resins that provide the bond between the composite, the hybrid layer, and the enamel.
In 7th generation systems, a desensitizer is also additionally included in the single formulation.
What are the physical and mechanical features of hybrid composites, and into what subtypes are they divided?
The physical and mechanical features of hybrid composites are associated with the inclusion of large, small, and extra-small filler particles in their composition.
- The introduction of large and small particles provides increased strength and abrasive resistance, bringing the coefficient of thermal expansion closer to that of hard tooth tissues, and improving marginal adaptation.
- The introduction of extra-small particles provides improved aesthetic properties and polishability, as well as reduced polymerization shrinkage.
Hybrid composites are divided into the following subtypes:
- Macrohybrid composites — created by introducing large particles; particle size: 0.04–0.1 (8–12) µm.
- Microhybrid composites — created by introducing small particles; particle size: from 0.04–0.1 to 1–5 µm.
- Fully filled composites — contain both large and small particles.
- Nanohybrid composites — contain extra-small particles, up to 0.0004 µm.
What mechanisms ensure the micromechanical retention of the adhesive to tooth enamel?
Micromechanical retention of the adhesive to tooth enamel is provided by preliminary demineralization followed by polymer penetration into the formed micropores. The mechanism includes the following stages:
- Conditioning — treatment with 37% phosphoric acid partially dissolves enamel prisms and the interprismatic substance.
- Microrelief formation — etched enamel acquires a characteristic porous structure resembling a honeycomb.
- Bonding — enamel adhesives penetrate into the created micropores.
- Polymerization — the adhesive hardens and forms rigid polymer tags that securely bond to the enamel and ensure composite fixation.
To remember the degradation mechanism of macrofilled composites: "Matrix wears away — stone is exposed — plaque adheres — color changes".
Class and exam questions
Why do chemical composites inevitably darken over time?
This phenomenon is called "amine discoloration". It occurs because a portion of unreacted chemical activator — a tertiary amine — always remains within the structure of the polymerized filling, changing color over time.
What is the main clinical advantage of LED lamps over halogen lamps?
Light-emitting diode (LED) lamps convert electron energy into light without generating thermal energy. This eliminates the risk of pulp burn and overheating of the surrounding hard dental tissues.
Why are macrofilled materials not used for aesthetic restorations?
Due to their large particle size (over 8 µm), they cannot be polished to a smooth finish. The rough surface of the filling actively accumulates food pigments and dental plaque, rapidly leading to a loss of aesthetics.
What else to study on this topic
- Chemical composition of composites: organic matrix, silane coupling agents, and types of inorganic fillers
- Properties and clinical application of hybrid composite materials
- Mechanism of free radical initiation during the interaction of camphorquinone and photon energy
- Russian and international experience with Evicrol
- Principles of eye protection for dentists working with photopolymerization curing lights
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