Dentin condition and smear layer
Immediately after instrumental tooth preparation, the exposed dentin surface does not remain clean. Electron microscopy confirms that dentin is always covered by the so-called smear layer (smear layer). This layer blocks the entrances to the dentinal tubules and serves as the primary obstacle—or conversely, substrate—for forming a reliable bond between the composite and tooth tissues.
Depending on the clinical approach, dentists use two main concepts of interacting with this layer:
- Transformation (modification). Self-conditioning primers consisting of an organic acid and hydrophilic monomers are used. The acid dissolves the smear layer, but the preparation is not rinsed off. Upon drying, the dissolved components precipitate. As a result, the primer impregnates the smear layer and demineralized dentin, forming a single hybrid layer. The adhesive is then applied on top of it. Typical representatives: Denthesive II, Etch & Prime 3.0.
- Complete dissolution and removal. This method involves the complete elimination of the smear layer followed by demineralization of the superficial dentin. This technique is recognized as the most effective and forms the basis of most modern systems, such as Gluma and Denthesive.
Mechanism of action of self-etching systems
The principle of action of self-etching adhesives is conveniently analyzed using the example of the two-component system XENO III (manufactured by DENTSPLY). This preparation requires strict proportioning of two liquids (A and B), which together contain the primer, conditioner, and adhesive. Mixing of the components occurs in vitro (outside the oral cavity) immediately before application.
The interaction process takes about 40 seconds and includes the following stages:
- Initial status: dentinal tubules are sealed by an intact smear layer.
- Application: active components react with the smear layer. It does not disappear completely, but is transformed into a modified layer.
- Exposure and solvent removal: after 20 seconds, active demineralization of the underlying dentin takes place.
- Polymerization: a homogeneous hybrid layer is formed (demineralized dentin is impregnated with resin). Inside the dentinal tubules, infiltrated tubules—polymer tags—are formed. On top of this structure, a pure adhesive layer is created, ready to bond with the composite.
Evolution of adhesive systems
Over the past two decades, dental science has developed 7 generations of adhesives, systematically improving bond strength and reducing postoperative sensitivity.
- 1st generation: Exhibited extremely weak adhesion (about 2 MPa). Consisted of a single component (e.g., Cervident, Cosmic Bond).
- 2nd generation: Bond strength increased to 8–15 MPa. Systems became two-component but remained prone to water leaching and required additional retention devices (Bond Lite, Scotch-bond).
- 3rd generation: Two- or three-component systems (primer and adhesive) with a bond strength of 8–15 MPa. Bonding to metal became possible and hypersensitivity decreased (Gluma, Prisma Universal Bond).
- 4th generation: A significant breakthrough due to the hybridization and total-etch concepts. Contained from 2 to 5 components, providing powerful adhesion of 17–25 MPa with low sensitivity (Scotchbond MP, All Bond It).
- 5th generation: Single-component systems requiring no preliminary mixing. The wet-bonding technique was introduced. Adhesion of 20–24 MPa (Single Bond, Prime & Bond NT).
- 6th generation: Multi-component, but easier-to-use preparations. Characterized by self-application of primer and bonding, very low sensitivity. Bond strength of 18–23 MPa (SE Bond, Prompt-L-Pop).
- 7th generation: Single-step single-bottle systems ("all-in-one" systems), the first representative of which was iBond. Etching, priming, bonding, desensitization, and disinfection occur during a single procedure. Bond strength of 18–25 MPa. The system requires no mixing and is independent of over-drying or over-wetting of the tooth cavity.
Microscopic picture of adhesion
Electron microscopy data reveal in detail how structural elements are formed during composite fixation:
- At ×2000 magnification, complete removal of the smear layer and demineralization are observed.
- At ×20000 magnification, the exposure of dentin collagen fibers is visualized.
- In cross-section of the adhesive joint, four zones are clearly differentiated: intact dentin (D), the formed hybrid layer (H), composite polymer tags penetrating deep into the tubules (T), and the restoration composite itself (R).
Additional: dental metals
In addition to polymer resins, metals remain crucial structural materials in prosthodontics, used to create appliances, splints, prostheses, and implants.
Metals are not used in their pure form in dentistry because they are susceptible to corrosion and lack sufficient strength. Instead, various metal alloys are used. Their properties are traditionally classified into three groups: chemical, technological, and physicochemical.
Briefly: answers to topical questions
What chemical compounds are used as solvents in modern adhesive systems?
Alcohol and acetone are used as solvents in modern adhesive systems.
They are part of primers (along with a mixture of hydrophilic compounds) to ensure deep penetration of hydrophilic monomers into dentinal tubules. These solvents are also present in the adhesive systems themselves, which is why they are capable of destroying therapeutic bases upon direct contact. This requires mandatory placement of an insulating base on the bottom of the carious cavity before applying the adhesive.
What is the fundamental difference in the mechanisms of action of 5th and 6th generation adhesive systems?
The fundamental difference lies in the approaches to tooth tissue etching and the number of components used.
| Characteristic | 5th generation | 6th generation |
|---|---|---|
| Number of components | Single component (no mixing required) | Multi-component (2–3 components) |
| Etching mechanism | Wet bonding | Self-etching (questionable for enamel) |
| Staging | Single-step application | Multi-step process (self-application of primer and bonding) |
Both generations provide dentin hybridization and are characterized by low or very low postoperative hypersensitivity.
Through which physicochemical mechanisms do polymer tags form within dentinal tubules?
The formation of polymer tags occurs due to the penetration of hydrophilic monomers of the adhesive system into the dentinal tubules, followed by resin polymerization.
This process includes the following stages:
- Demineralization — demineralization of the underlying dentin.
- Penetration — hydrophilic monomers of the adhesive system penetrate inside the dentinal tubules.
- Infiltration — demineralized dentin is impregnated with polymer resin, forming infiltrated tubules.
- Polymerization — after exposure and solvent removal, polymer tags are formed inside the dentinal tubules.
To remember the zones of the adhesive joint on a microphotograph, use the D-H-T-R acronym: Dentin (D) transitions into the Hybrid layer (H), which is held by Tags (T) and bonded to the Restoration (R - composite).
Class and exam questions
What is the core principle of 7th-generation single-step adhesives?
A conditioner, primer, desensitizer, and bonding agent are combined in a single bottle. The material is applied in one step without prior mixing, saves time, and virtually eliminates hypersensitivity.
What happens to the smear layer when a self-etching primer is used?
The layer is dissolved by acid, but is not washed out of the cavity. During drying, it precipitates and, together with demineralized dentin, forms a durable hybrid layer.
Can you use a primer from one manufacturer and an adhesive from another?
No, the fundamental rule for selecting adhesive materials requires using a system from a single manufacturer only; otherwise, strong adhesion is not guaranteed.
Why are pure metals not used in dentistry?
Pure metals have excessively high corrosion susceptibility and insufficient mechanical strength. Special dental alloys are always used instead.
What else to study on this topic
- Application and mixing technique of components in vitro using the XENO III system as an example
- Comparative characteristics of dentin bond strength across various adhesive generations
- Electron microscopy of adhesive joints: collagen fibers and infiltrated tubules
- Properties of dental alloys: physical, mechanical, chemical, and technological aspects
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