BOND STRENGTH OF SELF-ETCH ADHESIVES WITH PRIMARY AND PERMANENT TEETH DENTIN – IN VITRO STUDY

Objective: The aim of this study was to compare dentin bond strength of primary and permanent teeth with self-etching adhesive systems. Methods: On 40 intact specimens of primary and permanent teeth was created flat dentin surfaces. The patterns were divided in 4 groups. Two different self-etching adhesive systems were used – one two steps (AdheSE, VivaDent) and one one step (AdheSE One, VivaDent). Resin composite build-ups were constructed by means of convetional copper ring after applying the adhesive. The specimens were stored in water for 72 h at room temperature. After that specimens were tested for macrotensile bond strength. Debonded surfaces were analyzed by SEM. Conclusions: The measured values of dentin bond strength after applying self-etching adhesives are statistically significant in group of permanent teeth in comparison with group of primary teeth, and for both adhesive generations. Two steps self-etching adhesive provide significant stronger dentin bond strength with both dentitions in comparison with one step self-etching adhesive.


INTRODUCTION
Current self-priming and latest all-in-one adhesives represent an attractive addition to the day-to-day dental practice.They are easy to use, because they have shortened application protocol, which is an important advantage in pediatric dentistry.
Purpose of the self-etching adhesives systems are: -Simplifying the bonding procedures; -Minimizing the technical sensitivity of the applications protocol through the elimination of the step of etching, rinsing and drying of tooth structures (35).
These goals are obtained through the included in the composition sour monomers, which: -Simultaneous etch and prime dentin; -Facilitate the dissolution of crystals around collagen fibrils and the resin infiltration beyond the covered with smear layer surface in the underlying dentin matrix (31,35).
While the dentine bond strength of permanent teeth is examined quite often, significantly smaller number of researches are aimed at testing the dentine bond strength of primary teeth (10,15,26,35).Studies, comparing similar adhesive systems show results, varying from the absence of any significant differences (7,12,14,20,29,30) to higher (3,15,30) or lower (3,15,28,29,36,43) values of the achieved strength by primary teeth measured up to permanent teeth.It is accepted, that chemical, physiological and micro morphological differences are responsible for the weaker dentine bond strength by primary teeth as a lower degree of mineralization, smaller size and amount of dentine tubules, and the connected dentine permeability, as well as bigger reactivity towards acid conditioners (2,7,10,19,21,23,29,30).
The purpose of this study is to: -compare the dentine bond strength after the application of self-etching adhesive systems by primary and permanent teeth; -compare the failure type trough observation with scanning electron microscope after the macrotensile bond strength test.

METHOD AND MATERIALS
The examination consists of 40 intact teeth (20 primary molars and 20 permanent premolars and molars).The teeth are from healthy adult patients and children, after signed consent from them/their parents for the usage of these teeth in the experiment.Primary teeth are extracted because of physiological resorption.The teeth are divided on random principle (only primary and only permanent) in 4 groups.The division is shown in table 1.

DOI: 10.5272/jimab.2012182.168
Preparation of teeth surface.A section is made on the central occlusal fissure of every teeth, in medio-distal direction with turbine round borer (ISO 806 314 001534 012 for primary teeth and 806 314 001534 014 for permanent teeth), under water-air cooling.The depth of the section is dependant on the diameter of the borer.In order to remove the occlusal enamel and parts of dentine, another section is made, parallel to the occlusal surface, in the controlled depth, determined by the initial section of the borer.This manipulation is made with turbine diamond fissure borer (ISO 806 204 108 524 835 010) and water-air cooling.Afterwards the surface is leveled with abrasive disc, which is being replaced after every tooth.The models are observed with optical microscope OLYMPUS VANOX-T, under 25x-100x magnification, so that the complete removal of the enamel from the occlusal surface can be verified.
Perform of the restoration.A copper ring (!15) with height 5 mm and diameter 5mm is used for the manufacture of comparable and predictable surfaces of the restoration.Factory-made standard copper rings are cut with the help of two-side diamond disks, in order to achieve the desired height.
Adhesive systems were applied according to the manufactute×s instructions.By all patterns the application of the adhesive system is made directly on the occlusal dentine surface with a diameter, comparable to the one of the copper ring.On the arranged dentine surface is placed the beforehand prepared copper ring.Within this ring is placed one layer light cure composite (Tetric EvoCeram, Ivoclar Vivadent, A3 shade) with thickness 2mm, which is polymerized with UV light for 40 s and a lamp (Coltolux 75, Curing Light, Coltène Whaledent).The next step consists of setting a metal loop of orthodontic wire (!0.8), perpendicular to the section of the occlusal surface with a length of about 10mm in the center of the ring, and a fresh amount of composit, which stretches the metal ring and clamps the metal loop.Up next comes light polymerization for 40 s.The two free ends of every metal loop, which is placed in the copper ring and is covered with composite, are finished with "rotational" loop.
From the side of the pulp camber, on the level of the cut roots, the preparations of the specimens consists of etching of the entire pulp camber for 15 s, washing -15 s, air-drying and application of 3-steps adhesive system.A layer of composite is placed, along with light polymerization for 40 s.After that a second metal loop is being placed, alignment of the one, which is already on top of the occlusal surface in the metal ring.The final step is addition of composite untill the whole pulp camber is filled, along with light polymerization for 40s.
The prepared specimens are stored in water at room temperature for 72 hours, before the test conduct.
Testing the bond strength.The measurement of the achieved bond strength is conducted on stand for physicalmechanical examination type INSTRON -1185.Metal loops similar to the ones built in the studied teeth are rigid standard in the grips of the machine and at the other endhinged metal loops to the model.The loading bar is moving at a steady speed of 1 mm/min.Registered the maximum force of resistance, causing debonding (in MPa).The test is terminated after the final destruction of the test specimen.
Determining the failure type.After the macrotensile bond strength test specimens were dehydrated in ascending concentrations of ethanol -75%, 95% and 100% for 1 h at each concentration.After dehydration the samples were placed on filter paper, covered with a glass lid for 24 h.Both halves of each specimen were observed in the SEM from 18x to 1500x magnification to determine the failure type.SEM observation allows more precise determination of the place of failure.The failure type for each sample is classified into one of the following types: Type 1: Adhesive failure mode -the fracture line is located in the adhesive layer of the borderzone dentineadhesive or composite -adhesive -this is a failure in the adhesion.
Type 2: Cohesive failure mode -fracture line passes only in the volume of the composite.
Type 3: Mixed failure mode -the specimens show both types of fracture -the adhesive and cohesive destruction -dentin-adhesive-composite.
The prepared samples are placed onto aluminum discs.Cover with gold dust vacuum in an environment of argon -cathode sputtering apparatus using a JEOL JFC -1200 Fine coater.Studies were made with a SEM JEOL JSM -5510 SEM

RESULTS
Table 2 presents the values of minimum, maximum and average bond strength after a macrotensile bond strength test, measured in MPa.There was a statistically significant difference in the bond strength between the specimens from two dentitions (p<0.000).Greater bond strength is established between the groups with samples of permanent teeth (mean -13.06 MPa) compared with speciemens of primary teeth (mean -8.84 MPa) when tested by us selfetch adhesive systems (table 3).This indicates that the type of teeth (primary or permanent) effect the bond strength.In application of self-etch adhesive systems in both dentitions are registered higher average values of bond strength formations of permanent teeth (group 2 and 4) compared with those of primary dentition (group 1 and 3, table 2).Registered differences were statistically significant for both self-etch adhesive systems (p<0.05,table 4).This indicates that the type of adhesive system influence the results obtained for the bond strength.

Tab
The results show that application of two-and onestep self-etch adhesive systems led to reliably greater bond strength by permanent teeth compared with the primary teeth (group 2 permanent teeth -14,40 MPa to a group 1, primary teeth -10.88 MPa; group 4 permanent teeth -11.72 MPa to group 3 primary teeth -6.88 MPa, Table 3  and 4).
Registered average values of bond strength between group 1 and group 3 showed a statistically significant difference in favour of greater strength and applicated to the two-step self-etch adhesive system (Table 4) (p <0.0001).Statistically significant difference was found between samples from group 2 and group 4 (Table 4) (p <0.05).Greater average bond strength is taken into consideration when applicated to 2-steps self-etch adhesive (Table 4).Two-steps self-etch adhesive reliably create greater bond strength in both dentitions than all in one self-etch adhesive.All in one self-etch adhesive showed  2 and 4).

Failure type
Figure1 presents photographs depicting various failure types in samples from both dentitions and used in our experimental setup self-etch adhesive systems.By application of self-etching adhesives the percentage of adhesive failure mode is highest -95% for primary and 70% for permanent teeth (Table 5).

DISCUSION
The obtained results from the macrotensile bond strength test by primary and permanent teeth after application self-etch adhesive systems showed statistically significantly lower values of the bond strength to dentin of deciduous teeth compared to those in permanent (Table 3 and 4).Our results are consistent with results of other researchers who reported a weaker dentin bond strength of deciduous teeth due to micromorfological and chemical differences between the both dentitions (2,8,21,29).On the other hand, the specific characteristics of different adhesive systems that determine the degree of smear layer removal, demineralization of the underlying dentin, ability of the adhesive wets and penetrates the dentine are also important to achieve sufficient bond strength (4,11,16,17,18,21,23,29,38).These results indicate that the type of tooth and type of used self-etch adhesive system affect the achieved dentin bond strength.They also show that there is no adhesive systems that are equally effective in achieved bond strength in the primary and permanent teeth.
Our SEM observation of the resin-dentin interdiffusion zone after application of various aggressive self-etch adhesives in both dentition also showed significant differences in morphological characteristics of this zone for the observed hybrid and adhesive layer thickness.These layers were thicker in primary teeth and indicate the presence of microcracks in them.Differences were observed in the characteristics of the formed resin tags and in the depth at which they penetrates into dentinal tubules (1).These descriptive characteristics probably have also relation to the explanation for the weaker results achieved in bond strength in primary teeth.In the literature results in terms of self-etch adhesives are contradictory.Some suggest that their application results in a not enough good and long-term bond strength (8,22,24,32,33), other studies are in support of their use (5,9,19).The tendency of simplifying and shortening the adhesive application protocol is associated with loss of bonding efficiency to tooth structure, so that their benefits should be linked with their disadvantages (6, 11, 25, 37, 39).Our study found that more frequently observed failure type by self-etch adhesives is adhesive type for both dentitions (95% for primary and 70% for permanent teeth).At the higher dentin bond strength would be observed a higher frequency of mixed or cohesive type of failure.The study of the type of failure provides important information when analyzing the test results of bond strength and the classification of the type of failure is important observation (8).In vitro tests are useful and important for predicting the effect of new adhesive systems and their possible correlation with clinical practice, although in vitro studies are not able to fully predict clinical success.Must be taken into account the influence of other parameters of the study -the design of the experimental setup and experimental conditions, the operator skills, storage of samples until the experiment, the storage time of samples in water until testing.These are all factors which significantly influence the values obtained for the bond strength and type of failure (10,13,34,40).Although not directly predict the clinical performance of adhesives the comparison between different generations of adhesives is valid and can be used in clinical decision making (41).

CONCLUSION:
1.In our study all in one adhesives showed lower values of achieved bond strength, compared with selfpriming adhesives for the samples from both dentitions.
2. Lower values of the bond strength are registered in samples of deciduous teeth compared to permanent studied by us self-etching adhesives 3. None of the tested adhesive systems showed the same values of the dentin bond strength of the teeth from both dentitions.

Table 5 .
Distribution by failure mode in patterns of primary and permanent teeth and self-etch adhesive systems

Table 1 .
Grouping of experimental samples

Table 2 .
Values of the bond strength in MPa, as measured by macrotensile bond strength test.
. 3. Bond strength after macrotensile test in samples of both dentitions.
*The difference is statistically significant

Table 4 .
Bond strength in both dentitions with self-etch adhesive systems.
*The difference is statistically significant / JofIMAB; Issue: 2012, vol.18, book 2 / 171 lower average values of the bond strength in both samples dentitions (group 3 and group 4, Table