Explanations: A. The fourth-generation adhesive system. SYNTAC Primer/Adhesive, 3 ml.; B. The fifth-generation adhesive system. ENA Bond Light Curing Bonding 5 ml.; C. the seventh-generation adhesive system. Bond Force II Einzelpackung. KANIEDENTA

The first phase of action of adhesives used during total etching is acid etching and thorough flushing of acid to ensure complete removal of the smear layer and its plugs in dentin tubules.12 Due to enamel etching, the prisms open up randomly, resulting in micro- and macroporosity. Due to capillary attraction, this makes the enamel easily permeable even to the penetration of conventional hydrophobic resin.

Because of its different structure, the micromechanical bonding of rubber bands to etched enamel is much stronger and simpler than dentin. At the same time, the depth of dentin demineralization by acid etching is 3-5 nm, which damages the hydroxyapatite supports of collagen fibers.

Acid etching is followed by applying a primer containing specific monomers with hydrophilic properties. Its main chemical component is 2-hydroxyethyl methacrylate (HEMA). HEMA is soluble in organic solvents, such as acetone and ethanol (alcohol) or water. HEMA is "responsible" for better moisture content and reexpansion of the unsupported collagen network. The solvents ensure water displacement from the dentin surface, preparing the collagen network for further infiltration. The solvent-free bond (resin) is applied to the surface prepared by the primer, which is accompanied by the penetration of hydrophobic monomer not only in the interfibrillar spaces but also between the dentinal tubules. Polymerization helps form a hybrid layer, which, together with the rubber bands infiltrated into dental tubules, provides reliable micromechanical retention of the composite with dentin.

The fifth-generation adhesives (Fig.1) have less ability to infiltrate in demineralized dentin and result in suboptimal hybridization compared to the adhesives of earlier generations. Moreover, due to their hydrophilic properties, the fifth-generation systems are more prone to water absorption, which makes them more susceptible to hydrolytic degradation. The solvent evaporates significantly more slowly in this generation of adhesives but remains in the adhesive layer after polymerization.13 It should be noted that this technique is quite sensitive in case dentin is over-dried, as the collapse of collagen fibers makes it difficult for monomers to infiltrate between the fibers, which prevents the formation of a functionally stable hybrid layer. Excess moisture after acid etching causes the separation of adhesive hydrophilic and hydrophobic phases and forms vacuoles on the adhesive-dentin interface. At the same time, excess moisture leads to insufficient monomer polymerization and water absorption in a hybrid layer, which may cause premature degradation of this layer. Overdying and excess moisture are the main issues with fifth-generation adhesives.

Adhesive systems used with total etching techniques are considered classic in dentistry, but their nano-permeability is still an issue and affects the durability of the adhesive layer.

Moisture-sensitivity of adhesive systems has necessitated synthesizing self-etch adhesive systems with one or two stages. Their component is an aqueous solution of acid functional monomers with high (pH<1), medium (pH=1.5), or low (pH>2) acidity (pH). Less acid adhesives cause demineralization of the dentin surface due to chemical interaction with hydroxyapatite crystals around the collagen fibers. As a rule, most plugs cannot be removed from dental tubules, forming a shallow hybrid layer of submicron sizes. Adhesives with high acidity cause the type of dentin demineralization achievable with the total etching technique.14 Water in self-etch adhesives as a solvent is essential for ionizing functional monomers, while organic solvents are added to facilitate the mixing of hydrophilic and hydrophobic components. However, the presence of water and acid monomers can affect the durability of adhesives, partly due to their hydrophilicity. They can bind water from a wet substrate. In this case, the adhesive acts as a semi-permeable membrane, where liquid migration from the substrate through the adhesive layer may occur after polymerization. As a result, tiny droplets may appear between the composite and the tooth, especially after delayed polymerization. This phenomenon may result in a degraded resin bond over time.

With weak self-etch adhesives, the risk of postoperative sensitivity is minimal because smear layer plugs are used as a bonding substrate. Leaving the smear layer plugs prevents postoperative sensitivity, related to preserving the liquor flow in dentin tubules.

Like the fourth- and fifth-generation adhesives, the self-etch adhesive system contains Hema (2-hydroxyethyl methacrylate) hydrophilic monomer. Due to its low molecular weight, it acts as a co-solvent. It helps mix hydrophilic and hydrophobic components in the solution, thus increasing the moisture content of the dentin surface. Self-etch adhesive systems do not require an additional step in dentin conditioning because it contains acid monomers. The so-called 'self-etch' is viewed as a simplified, time-saving technique. Self-etch adhesives modify a "smear layer" to the depth of 0.5-1.2 nm. (15)

It should be noted that the bands formed due to adhesive polymerization are short and narrow. On the other hand, due to low acidity, layer particles must obliterate the tubules, which limits the hybridization of peritubular dentin and reduces postoperative hyperesthesia. Despite the thin hybridized layer, this system creates a chemical bond with dentin. Due to their low acidity and minimal invasiveness, these systems do not sufficiently demineralize enamel, thus creating the need for additional etching (so-called selective etching) of this tissue.

A distinction is made between one- and two-component self-etch adhesives. The two components of two-component self-etch adhesives are (i) acid and primer in one system, and (ii) hydrophobic bond, as a separate component.

Self-etch primer and hydrophobic bond in one-step systems (SEP) combine the application of acid functional monomers. It combines acid functional monomers, hydrophilic and hydrophobic monomers, fillers, water, solvents (acetone, ethanol, butanol), resin components, and photo inhibitors. They are called seventh-generation adhesives and are comfortable, convenient, and fast.

Following the abovementioned, our study aimed to identify carious and non-carious diseases in human teeth' cervical area and evaluate the clinical efficiency of restorations made with adhesive systems of different generations to treat them.

To achieve this goal, we conducted research in following two directions: (i) Registration of cervical lesions in the study population, and (ii) Restoration of class 5 defects and assessment of their clinical efficiency.