Evolution and Composition of Composites
The first polymer fillings appeared in the 1940s. These were methyl methacrylate-based acrylic resins. They had a host of critical drawbacks: massive polymerization shrinkage (around 21%), low strength, excessive water absorption, and the toxic effect of residual monomer on the pulp. Subsequent attempts to use epoxy resins (e.g., Dentoxide) were also not entirely successful due to mixing difficulties and discoloration of the restoration.
A breakthrough occurred in 1962 when R. Bowen synthesized bisphenol A-glycidyl methacrylate (BIS-GMA), known as Bowen's resin. A modern composite is a heterogeneous system consisting of the following components:
- Polymer matrix: the base of the material (BIS-GMA, UDMA, TEGDMA).
- Inorganic filler: crystalline quartz, glass, silicon dioxide, or diamond dust. Particle shapes can be spherical, whisker-like, or shredded. The filler is responsible for strength, wear resistance, and minimizing shrinkage.
- Silanes: organosilicon compounds used to coat filler particles at the factory. They guarantee a monolithic chemical bond between the inorganic phase and the organic matrix.
- Additives: initiators, catalysts, and pigments for shade matching.
Classification by Particle Size
The properties of a filling directly depend on the particle size of the filler. Based on this criterion, composites are divided into four main groups:
- Macrofilled: with large particles ranging from 8 to 12 µm.
- Minifilled: the dispersed phase size ranges from 1 to 5 µm.
- Microfilled: ultrafine particles measuring 0.04–0.4 µm.
- Hybrid: contain a balanced mixture of particles of different sizes (e.g., 0.04, 5, 8 µm), combining strength with good polishability.
Medical Treatment and Acid Etching
Before placing a filling, the cavity is washed with water and air-dried (strictly free of compressor oil impurities). Using alcohol, ether, or hydrogen peroxide is strictly prohibited — they destroy the composite matrix, reduce adhesion, and disrupt the oxygen-inhibited layer.
For moderate caries, modern adhesives eliminate the need for a base/liner. However, in deep caries, a therapeutic liner (based on calcium hydroxide) is applied to the cavity floor, followed by a glass ionomer cement base to prevent adhesive acids from dissolving the therapeutic layer.
Next, total-etch technique is performed using 37% phosphoric acid (it is preferable to use a colored gel for visual control). First, the enamel is etched (15–60 seconds), followed by the dentin (15 seconds). The acid is then rinsed off with a water spray for 10 seconds, avoiding direct water pressure on sensitive dentin.
Drying, Bonding, and Composite Placement
The most critical stage is proper tissue drying:
- Enamel is dried until it has a chalky-matte appearance and completely loses its shine.
- Dentin must never be overdried. It must remain slightly moist, retaining its characteristic sheen. Overdrying leads to the collapse of collagen fibers and impaired adhesion.
Next, the adhesive system (bond) is applied, such as 5th-generation products (e.g., Single Bond). The adhesive is applied twice, actively rubbing it into the dentin with a brush, then gently air-dried for 2–5 seconds to create a thin film, and light-cured.
The composite is placed in horizontal increments. The thickness of each layer must not exceed 2.5 mm. Each layer is light-cured for 20 seconds (at a distance of no more than 5 mm from the material surface). When placing the final increment, the dentist shapes the anatomical structures of the tooth — cusps and fissures.
Briefly: answers to topical questions
What generations of adhesive systems are used in modern dentistry, and what are their differences?
Modern dentistry uses seven generations of adhesive systems, differing in bond strength, number of components, and mechanism of action.
| Generation | Components | Bond strength | Characteristics |
|---|---|---|---|
| I | 1 | 2 MPa | Very weak adhesion |
| II | 2 | 8-15 MPa | Weak adhesion, prone to dissolution |
| III | 2-3 | 8-15 MPa | Primer/adhesive system, metal bonding |
| IV | 2-5 | 17-25 MPa | Total-etch, hybridization |
| V | 1 | 20-24 MPa | Wet bonding, no-mix |
| VI | 2-3 | 18-23 MPa | Multi-step, self-etching |
| VII | 1 | Not specified | Single-step single-bottle systems |
What initiators and catalysts of polymerization are part of light-cured composites?
Light-cured composite materials contain:
- Camphorquinone — polymerization catalyst.
- Tertiary amine — polymerization initiator.
Under the influence of light energy, active radicals are formed, leading to the polymerization and hardening of the material. Special lamps emitting intense blue light with a wavelength of 400–500 nm are used for activation.
What clinical methods exist to reduce polymerization shrinkage of composite materials?
To reduce polymerization stress and control shrinkage, the following clinical methods are used:
- Incremental placement — the material is placed in increments of no more than 2 mm thick, with sequential light-curing of each layer.
- Directed polymerization — used when working with microhybrid composites.
- Layered restoration technique — combined use of different composites:
- Flowable composite — the first adaptive layer lining the cavity floor.
- Packable composite — the main layer restoring the dentin bulk.
- Microhybrid composite — the surface enamel layer restoring the occlusal surface.
Class and exam questions
Why are silanes needed in composite materials?
Silanes act as a chemical "bridge." They provide a strong bond between the organic polymer matrix and the inorganic filler particles, preventing the restoration from failing under load.
Why should dentin not be over-dried after washing away the acid?
Overdrying leads to the collapse of exposed dentin collagen fibers. As a result, the adhesive cannot penetrate the dentinal tubules, and a reliable bond between the filling and the tooth will not occur.
Can the cavity be treated with hydrogen peroxide before placing a composite?
No. Hydrogen peroxide saturates the hard tissues of the tooth with oxygen, which promotes the formation of an oxygen-inhibited layer in the composite and drastically degrades the quality of the restoration.
What else to study on this topic
- Amalgam filling technique and working with amalgamators.
- Properties of endodontic sealers and their chemical composition.
- Lateral compaction of cold gutta-percha in the root canal.
- Errors in wedge matrix placement and their impact on the contact point.
- Instruments for finishing and polishing restorations.
Master the entire topic in the «Propaedeutics of Dentistry» course
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Other topics in "Chapter 7. Endodontics"
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.