Acid-Base Resistant Zone in Teeth with the Direct Restoration Using Different Adhesive System Generations: A Systematic Review

ABSTRACT Objective: To find out what the acid-base resistant zone (ABRZ) is and the mechanism of its formation. Material and Methods: This systematic review was based on the search of laboratory studies in which selfetching adhesive systems were used. The electronic database PubMed was used for the search. The search began on August 2021 and ended on June 2022. We have analyzed the materials and methods of each research and entered them in the appropriate tables to give a clearer assessment of the obtained results. Results: This systematic review included 15 full-text articles published from 2011 to 2019. The ABRZ is formed on both dentine and enamel. On dentine, the ABRZ is formed only when using self-etching adhesive systems; on the enamel, on the contrary, the step of preliminary etch and rinse contributes to the formation of a thicker ABRZ. The functional monomer MDP and fluorine increase the thickness of the ABRZ and provide a hybrid layer /ABRZ boundary without defects and erosions. Conclusion: Self-etching adhesive systems ensure the creation of an ABRZ resistant to acid-base tests. This phenomenon can provide the resistance of tooth tissues to demineralization, and therefore increase their resistance to caries.


Introduction
In more than the last 20 years, adhesive dentistry has made a big step forward.Having achieved a high quality of adhesion, scientists began to develop materials that should facilitate the work of the doctor, reduce the number of stages performed and, consequently, minimize possible errors in each of them.One of the developments was the creation of self-etching adhesive systems (SEA) [1] In comparison with etch and rinse adhesive systems, self-etching systems contain an acid component that plays an important role, acting as a gentle etching agent.Due to demineralization, resin monomers penetrate the matrix of dental tissues and chemically interact with hydroxyapatite (Hap) crystals [2][3][4][5].
In 2004, a zone located directly under the hybrid layer (HL) was described when investigating the dentin/SEA interface [1].Further research works showed that this zone was characterized by its resistance against acids [1]; therefore, it was named the "acid-base resistant zone" (ABRZ) [1].The authors suggested that this zone might play an important role in preventing caries formation [6].In 2009, Waidyasekera et al. [7] used transmission electron microscopy (TEM) to study the structure of the ABRZ after an acid-base test; the results showed that ABRZ has a structure, which is more resistant to caries than ordinary dentin.It is believed that self-etching adhesive systems reduce the post-sealing sensitivity, most likely, this is due to the formation of this zone [8].
The source of ABRZ formation is the hybrid layer of dentin, which releases the functional monomers of the adhesive [9,10]; as a result, a stable, insoluble salt is formed [1].Further studies revealed the presence of this zone in the enamel sections [2].However, due to the structure that differs from dentin, as well as the absence of a hybrid layer, the mechanism of ABRZ formation in this case is different [1].It is interesting to note that the etch and rinse technique excludes the formation of this zone in dentin [5], and on the contrary, increases its width in enamel [1].
There is a wide range of SEA on the market, which differ in the composition of their monomers.Since the formation of ABRZ is determined by the chemical reaction between the monomer and hydroxyapatite crystals, the SEA composition also affects the characteristics of this zone [7,[11][12][13][14].Thus, it is necessary to understand the influence of various adhesive systems and techniques of their application on the formation of ABRZ in enamel and dentin.
The primary goal of this systematic review was to find out what the ABRZ is and the mechanism of its formation.Second, the review aimed at studying the influence of the composition of the adhesive system and the protocol of adhesive preparation on the ABRZ formation.Thirdly, it was necessary to study the features of the ABRZ on the enamel and the dentine.

Material and Methods
The concept of this review is based on the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses).

Register
The review protocol was registered in an international prospective register of systematic reviews (PROSPERO ID CRD42021284756) in which the methodology and inclusion and exclusion criteria were

P (Participants)
Self-etch and etch-and-rinse adhesive systems I (Intervention) Direct restorations with the self-etch and etch-and-rinse protocols C (Comparison) Resistance to acid and base of enamel and dentine in self-etch and etch-and-rinse protocols O (Outcome) An acid-base resistant zone (ABRZ).Formation of nano-layered structures.Adhesive stability.

Selection Criteria
Publications that met the following selection criteria were included: 1) Full-text articles in English, not older than 10 years; 2) The articles should contain detailed information about the results and parameters of the study (samples and tissues.adhesive system, surface treatment, ABRZ description); 3) The articles contain studies conducted in vitro on human teeth; and 4) The articles contain studies conducted using self-etch adhesive systems (SEA).Publications that were not related to the topic of the study, literature reviews, as well as articles that did not have sufficient and specific data for the analysis were excluded.

Information Sources
The electronic databases used for the search were PubMed and EMBASE.It was not necessary to contact the authors to access the articles.The search started in July 2021 and ended in June 2022.

Search and Selection of Studies
A search in English with no time limit was performed by three independent people.The studies were filtered and selected in several stages.Firstly, they were evaluated by titles.Secondly, individual documents at the first stage were additionally assessed by reading the abstracts and full-text articles.
The difference in the choice was resolved through discussion among the readers.

Risk of Bias
Risk assessment of bias was undertaken during the data extraction process.For the included studies, it was conducted using the Cochrane Collaboration's ROBINS-I tool for assessing the risk of bias [15][16][17].The overall risk of bias was then assigned to each trial, according to Higginset et al. [16].The levels of bias were classified as follows: low risk, if all the criteria were met; moderate risk, when only one criterion was missing; high risk, if two or more criteria were missing; and unclear risk if there were very few details to make a judgment about a certain risk assessment.

Results
A total of 210 articles were identified by keywords and resumes.Duplicate studies were excluded.34 articles were identified as potentially relevant articles by checking the titles and abstracts, then a full text of 30 articles analysis was carried out, including materials and methods, for compliance with the inclusion criteria.
Articles that didn't meet the inclusion criteria were excluded from this review.As a result, after applying the inclusion and exclusion criteria, 15 full-text articles published between 2011 and 2019 were included and analyzed in the systematic review.After evaluating the selection of articles in accordance with the inclusion criteria, a final analysis of individual studies was conducted.The process of sampling and analyzing studies is presented in the block schematic diagram (Figure 1).

Enamel and Dentin
Nikaido et al. [16] Presence: in all groups The studies selected for the review examined the effect of the following 2-SEA on the formation of ABRZ: Clearfil SE Bond [8,14,15,17,18,[20][21][22], Optibond XTR [15] and three other studies used experimental 2-SEA [10,21,23].In all samples of 2-SEA groups, the ABRZ was formed on the dentin and enamel.In all studies where Clearfil SE Bond was used, the adhesion between ABRZ and HL was without erosion and formed a butt joint; sometimes, a slope from the top to the bottom of the outer lesion could be observed.Preliminary dentin etching with orthophosphoric acid led to the absence of ABRZ.

One-Step Self-Etch Adhesive on Dentine
The studies included in the review examined the effect of the following 1-SEA on the formation of ABRZ: Scotchbond Universal adhesive [4,13,15], Clearfil Bond SE One [14], Adper easy bond [18] and M-Bond (II) [24].All the studied samples had KORZ.In all samples with the use of 1-SEA, a funnel-shaped erosion was formed at the ABRZ/HL border.The thickness of the ABRZ ranged from 0.27 to 0.5 microns.Accordingly, preliminary dentin etching with orthophosphoric acid also led to the absence of ABRZ.

Two-Step Self-Etch Adhesive on Enamel
The studies included in the review examined the effect of the following 2-SEA on the formation of ABRZ in the enamel: Clearfil SE Bond [3] and experimental adhesive [23].In the enamel samples after application of 2-SEA, there was an ABRZ with a thickness of 0,5-3 µm; however, when performing the stage of and rinse with orthophosphoric acid, the thickness was increased and was about 5 µm.Moreover, the stage of etch and rinse before applying 2-SEA contributed to the formation of a butt joint, while its absence led to extremely small funnel-shaped erosion.

One-Step Self-Etch Adhesive on Enamel
The studies included in the review examined the effect of the following 1-SEA on the formation of ABRZ in enamel: Clearfil Universal Bond Quick [19], G-Premio Bond [19], and experimental adhesive [16].When using 1-SEA, the ABRZ was formed on the enamel with the funnel-shaped erosion at the joint with HL; its thickness was from 0.5 to 1.0 depending on the type of adhesive; however, as in the case of 2-SEA, with the preliminary etching stage with orthophosphoric acid, there was no gaps or defects at the ABRZ/HL border.
Comparison of 1-SEA and 2-SEA A comparative assessment of 1-SEA and 2-SEA was carried out in three studies included in our review; all tests were performed on dentin [14,15,18].The formation of ABRZ was observed in all experimental groups.
2-SEA showed the widest crust, the fit of which to the hybrid layer had no erosion.There was funnel-shaped erosion in the 1-SEA samples at the HL/ ABRZ boundary.
In studies included in the review following combinations of two types of ions were considered: adhesive systems with the presence of fluoride [10,14,15,21,23], preconditioning procedure with fluoride [17], adhesive systems with the presence of calcium [21] and adhesive systems with the presence both calcium and fluoride [21].The of fluorine ions provided a butt-joint at the ABRZ/HL boundary, without erosion and defects; the presence of both calcium and fluorine ions also formed a compound without erosion; however, the presence of calcium ions alone in the adhesive led to the formation of funnel-shaped erosion.Adhesives with the presence of fluorine ions created a thicker ABRZ, which was 1.0 μm, while calcium ions did not affect the thickness of the layer in any way.

Discussion
Currently, there is a wide selection of adhesive systems on the market, and despite the existing "gold standard", there are systems that have certain advantages.
Self-etching adhesive systems, thanks to their etching component, provide a "soft" gradual demineralization of the underlying layer of hard tooth tissues, creating a certain matrix.At the same time, hydrophobic adhesive monomers are infiltrated into this matrix in parallel, which interact with hydroxyapatite crystals, thereby creating a stronger zone of dental tissue.This zone was called ABRZ.Сompared to HL, the remaining apatite crystals in ABRZ were relatively denser [14].
The formation of this zone on the dentin is observed only when using self-etching adhesive systems, according to the mechanism described above, and does not apply to etch and rinse adhesive systems [14].
Moreover, it is interesting to note that the degradation of adhesion between these systems also differs [14].
There are two phases of dentin bonding degradation: hydrolytic degradation of the collagen matrix and hydrolytic degradation of the bonding resin within the HL [25,26].For a etch and rinse system, hydrolysis of unprotected collagen networks is typical, because of incomplete penetration of the resin to the entire etching depth; however, for SEA, this type of degradation is minimal [27].Hydrolysis of polymer components occurs from the HLs of both etch and rinse and self-etching adhesives since water can remain and penetrate the interface as a result of diffusion from wet dentin during adhesive polymerization [28].
The formation of the enamel and dentin ABRZ has certain differences that are associated with the different structures of these tissues.Enamel has a significantly higher concentration of the mineral component and differs from dentin in the absence of a collagen network [16].In this regard, demineralization occurs to a lower depth, which causes a smaller thickness of the enamel ABRZ.In studies [16,22], it was noted that preliminary etching of enamel with orthophosphoric acid increases the thickness of the ABRZ.In contrast with dentin, where the source of functional monomers is a hybrid layer, in enamel, these molecules are directly penetrated from the adhesive layer into the etched matrix.Due to the high percentage of the mineral component, soft etching of SAE demineralizes the enamel to a lower depth in comparison with etch and rinse with orthophosphoric acid; therefore, when using the latter, the ABRZ increases.
Comparing two SEA systems, both one-step and two-step, it can be concluded that the latter ensures the creation of a more reliable ABRZ dentin since no undercuts in the form of funnel-shaped erosion are formed at the HL/ABRZ interface [13][14][15]18].However, this defect depends not only on the number of steps but also on the components of adhesive systems.Thus, the content of MDP and fluoride provided a connection at the HL/ABRZ boundary by the type of joint; sometimes, a slope was formed, i.e., an increase in the thickness of the ABRZ [10,13,15,16,23].Regarding enamel, the results largely depended on preliminary etching with orthophosphoric acid, which provided a butt joint [8,19].On the contrary, the etching of dentin excluded the formation of ABRZ [4,15,18].
SEA containing the MDP monomer has shown good results with respect to the formation of ABRZ.
MDP has two functional groups; one of them is methacrylate, and the other is phosphate.The phosphate groups interact with the calcium ions of hydroxyapatite crystals and form an insoluble, strong salt [1].When MDP was included in the adhesive system, ABRO was clearly identified not only on the dentine but also on the enamel.
Thus, it can be noted that functional monomers in adhesive systems strongly influence the morphology of the hard tooth tissues ABRZ [16].
Adhesive systems, which included fluoride, created a slope-like increase in the thickness of the ABRZ from the border of the sample to the bottom of the outer lesion, which was confirmed by a few studies [7,[29][30][31].
On the contrary, systems without fluorine did not lead to such a phenomenon [16].
The main limitation of this review is that the same group has done all of the work on this topic, so there is no real independent confirmation or otherwise of the formation of the ABRZ and the differences noted with Etch and Rinse systems.In addition, an overall limitation is that a number of the conclusions about restoration longevity, recurrent caries resistance, etc., have not been fully tested in a laboratory setting or clinically.Much of what has been described is a supposition with some evidence for the longevity of some restorations in clinical trials, but the actual effect of the ABRZ cannot be easily tested clinically.

Conclusion
Self-etching adhesive systems ensure the creation of an ABRZ resistant to acid-base tests.This phenomenon can provide the resistance of tooth tissues to demineralization and therefore increase their resistance to caries.
specified and documented.The research strategy of the present work was formulated according to PICO (Participant, Intervention, Comparison, Outcome), as seen in

Table 2 . Characteristics of the studies included in this analysis. Author Year Number of Specimens Research Material Examined Tooth Tissue
SEM: Scanning Electron Microscopy; TEM: Transmission Electron Microscopy.