Universidade de São Paulo

2010-04

Bond strength of dental adhesive systems

irradiated with ionizing radiation Journal of Adhesive Dentistry,Surrey : Quintessence Publishing,v. 12, n. 2, p. 123-129, 2010http://www.producao.usp.br/handle/BDPI/49760

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Departamento de Física e Ciências Materiais - IFSC/FCM Artigos e Materiais de Revistas Científicas - IFSC/FCM

17039

Bond Strength of Dental Adhesive Systems Irradiated with Ionizing Radiation

Adriana Dibo da Cruza/Luciano de Souza Gonçalvesb/Alessandra Nara de Souza Rastellib/ Lorenço Correr-Sobrinhod/Vanderlei Salvador Bagnatoe/ Frab Norberto Bóscolof

Purpose: The aim of the present paper was to determine the effect of different types of ionizing radiation on the bond strength of three different dentin adhesive systems.

Materiais and Methods: One hundred twenty specimens of 60 human teeth (protocol number: 032/2007) sectioned mesiodistally were divided into 3 groups according to the adhesives systems used: SB (Adper Single Bond Plus), CB (Clearfil SE Bond) and AP (Adper Prompt Self-Etch). The adhesives were applied on dentin and photo-activated using LED (Lec 1000, MMoptics, 1000 mW/cm 2). Customized elastomer molds (0.5 mm thickness) with three orifices of 1.2 mm diameter were piaced onto the bonding areas and filled with composite resin (Filtek Z-250), which was photo-activated for 20 s. Each group was subdivided into 4 subgroups for application of the different types of ionizing radia-tion: ultraviolet radiation (UV), diagnostic x-ray radiation (DX), therapeutic x-ray radiation (TX) and without irradiation (control group, CG). Microshear tests were carried out (Instron, model 4411), and afterwards the modes of failure were evaluated by optical and scanning electron microscope and classified using 5 scores: adhesive failure, mixed failures with 3 significance leveis, and cohesive failure. The results of the shear bond strength test were submitted to ANOVA with Tukey's test and Dunnett's test, and the data from the failure pattern evaluation were analyzed with the Mann Whitney test (p = 0.05).

Resuits: No change in bond strength of CB and AP was observed after application of the different radiation types, only SB showed increase in bond strength after UV (p = 0.0267) irradiation. The UV also changed the failure patterns of SB (p = 0.0001).

Conciusion: The radio-induced changes did not cause degradation of the restorations, which means that they can be exposed to these types of ionizing radiation without weakening the bond strength.

Keywords: adhesion, dentin, dentin adhesives, irradiation, degradation.

J Adhes Dent 2010; 12: 123-129. Submitted for publication: 09.12.08; accepted for publication: 07.01.09. doi: 10.3290/j.jad.a17530

The light-curing composite resins are widely used in I restorative dentistry. One of the established factors of

clinicai success for dental restorations is the reliable bond

PhD Student, Department of Oral Diagnosis, Dental School of Piracicaba, State University of Campinas, Piracicaba, SP, Brazil. PhD Student, Department of Dental Materiais, Dental School of Piracicaba, State University of Campinas, Piracicaba, SP, Brazil. Researcher, Physics Institute of São Carlos, University of São Paulo, São Car-los, SP, Brazil.

d Chairman Professor, Department of Dental Materiais, Dental School of Piraci-caba, State University of Campinas, Piracicaba, SP, Brazil.

0 Professor and Chairman, Physics lnstitute of São Carlos, University of São Paulo, São Carlos, SP, Brazil.

f Professor and Chairman, Department of Oral Diagnosis, Dental School of Piracicaba, State University of Campinas, Piracicaba, SP, Brazil.

Correspondence: Adriana Dibo da Cruz, Department of Oral Diagnosis, Oral Radiology, School of Dentistry of Piracicaba, UNICAMP, Post office box 52, Av. Limeira 901, Areião, Piracicaba, SP, Brazil 13414-018. Tel: +55-19-2106-5327, Fax: +55-19-2106-5218. e-mail: cruz_a_d@fop.unicamp.br

strength between the restorative material and dental tis-sues 1- 5 . 9,16-18 that it is obtained by dental adhesives.

The improvement of the adhesive systems caused an increase in their clinicai applications in adhesive dentistry, including pit and fissure sealants, orthodontic brackets, adhesive bridges, laminate veneers, and direct resin restorations. 1 However, in spite of the improvements, un-desirable consequences such as recurrent caries or mar-ginal discoloration are often found in resin restorations following long-term clinicai use, due to exposition to sev-era) externai agents. 7

Restorations in the oral cavity are challenged by different externai agents, including ionizing radiation from radio-graphic diagnosis, radiotherapy, or environmental sunlight. These restorations may be negatively influeneed by the in-teraction of radiation with their atoms and molecules through electrostatic and electromagnetic forces. The inter-action occurs with any atoms and molecules exposed to the radiation source, 4,19 ie, tissue, vital organs, or biomaterials.

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Although some studies have reported on the direct effects of the exposure of ionizing radiation on dental mate-r i a is2,3,5-8,11,12,21 and dental tissues, 5,9,10,13,14,18 the results of such exposure are still unciear.

Some studies used high-energy ionizing radiation, such as ga m m a 6,11,12,21 or electron 2,3,11 radiation, on dental materiais or tooth/restoration complexes. In the case where such radiation was directly applied to dental materi-ais, it was observed that microhardness, fracture tough-ness, and others mechanical properties changed pro-portionally to the increase of the applied radiation dose. When radiation was applied to restored teeth to simulate radiothera py, 5,9 ,10,13 ,14 some changes in bond strength were observed.

Other studies used low-energy ionizing radiation, such as ultraviolet radiation (UV), 8 and in these cases color changes were observed (among other alterations). To our knowledge, nothing exists in the literature about effects of the x-ray exposure used for diagnosis on dental materiais or on dental tissues.

Both high- and low-energy ionizing radiation have also been used to induce accelerated aging by exposure to light to assess the resistance of materiais to the degradation process. 72°

Considering that intraoral adhesive restorations can be exposed to different types of ionizing radiation as they have great energy and capacity of penetration, any nega-tive change in the adhesive capacity of the restoration caused by irradiation will be a contraindication for these adhesive systems. Thus, the aim of the present paper was to determine the effect of different types of ionizing radia-tion on the bond strength of three different dentin adhe-sive systems.

MATERIALS AND METHODS

After approval of this project by the Ethics Committee for Human Research of the Schooi of Dentistry, Piracicaba, State University of Campinas, protocol number 032/2007, 60 noncarious human permanent third molars were se-lected. The teeth were cleaned and stored in distilled water at 4°C for no more than 6 months post-extraction. The crowns were cut off of the roots and then sectioned mesiodistally using a diamond saw (#7020 flexible dia-mond disk, KG Sorensen, Barueri, SP, Brazil) in an Isomet low-speed saw (Buehler, Lake Bluff, IL, USA) with water cooling.

Each half crown was embedded in PVC cylinder rings with epoxy resin to maintain the outside dentin surface levei with the ring base. The dentin surfaces were wet ground with 180-, 220-, 400- and 600-grit SiC abrasive pa-pers, in order to create a smooth, flat surface in the medium dentin. After this, 120 specimens were divided randomly to 3 groups (n = 40). Each group used one of the adhesive systems described in Table 1.

The adhesive systems were applied on dentin as de-scribed in Table 2. Absorbent paper was used to remove the excess dentin moisture, and ali adhesives systems were photopolymerized with an LED light-curing unit

(Lec 1000 MMoptics, São Carlos, SP, Brazil) at 1000 mW/cm 2 .

In order to obtain cylinders for the microshear bond strength test, customized 0.5-mm-thick elãatomer molds, each with three cylinder-shaped orifices (1.2 m' rn•in diarne-ter), were placed on the tooth surfaces, allowing delimita-tion of the bonding area. 18,17 Afterwards, the cylinder-shaped orifices were filled with composite resin (Filtek Z-250 Universal Restorative, 3M ESPE, St Paul, MN, USA), and a mylar strip was placed over the filled orifices. Prior to the composite resin photo-activation procedures, a con-stant and uniform 250 g cementation load was applied for 2 min, using a custom-made device. The composite resins were photo-activated for 20 s with the same LED light-cur-ing unit mentioned above. The specimens were stored at 37°C (± 1°C) and 95% (± 5%) relative humidity during the entire experiment.

Each group (n = 40) was subdivided randomly into 4 subgroups (n = 10 specimens with n = 30 cylinders) ac-cording to the following experimental treatments:

1. Ultraviolet radiation (UV), in light box with relative hu-midity at 50% (± 5%) using a mercury light HN ZN, 15 W (Huaning, Nanjing, China), 253.7 nm, 15 W, for 48 h, resulting in 1157.76 J/cm 2 .

2. Diagnostic x-ray radiation (DX) from a Poli-Tecnica 300/125 x-ray unit (Engeclin, São Paulo, SP, Brazil), using 30-s exposure times, 80 kV tube voltage, 200 mA tube current, 100 cm focus-object distance and 10 cm 2

radiation field. 3. Therapeutic x-ray radiation (TX), from Clinac 600 Linear

Accelerator (Varian Medical Systems, Palo Alto, CA, USA), with a 6 MV x-ray beam, 70 Gy exposition dose, 100 cm focus-object distance, 10 cm 2 radiation field.

4. Control group, no treatment.

Thus, with the end of the experimental treatment appli-cation (48 h after specimen photo-activation), the cus-tomized molds were removed from the specimens. Ali resin cylinders were checked by stereomicroscope (Cari Zeiss do Brasil, São Paulo, Brazil) under 40X magnifica-tion. Twenty-two cylinders (6.1% of the sample) presenting flaws, irregularities or bonding defects were eliminated from the test. Specimens with 2 cylinders were accepted as samples, those which presented only 1 cylinder would be eliminated, but none were found. Therefore, the sample maintained ali 120 specimens.

For the microshear test, a thin steel wire (0.2 mm in di-ameter) was looped around each cylinder and aligned with the bonding interface. The test was conducted in a univer-sal testing machine (Instron, Canton, MA, USA, model 4411), at a crosshead speed of 0.5 mm/min until failure. Bond strength values were calculated in MPa. The average bond strength value of the cylinders was taken as a refer-ence value of the specimens, which then was used for sta-tistical analysis. Bond strength data were submitted to ANOVA by Tukey's test (p = 0.05) for comparison among the radiation types and among the adhesives; each group was independently compared with the control group by Dunnett's test (p = 0.05).

124 The Journal of Adhesive Dentistry

Table 1 Adhesive systems used in the present study and its groups

Groups Commercial name Manufacturer Composition*

Bis-GMA, HEMA, dimethacrylates, ethanol, water, copolymer of polyacrylic, polyitaconic acids and silica nanofiller Primer: MDP, HEMA, dimethacrylate monomer, water, catalyst Bond: MDP, HEMA, dimethacrylate monomer, microfiller, catalyst Liquid 1 (red blister): Methacrylated phosphoric esters, bis-GMA, initiators based on camphorquinone, stabilizers Liquid 2 (yellow blister): water, HEMA, polyalkenoic acid, stabilizers

SB Adper Single Bond Plus Adhesive

CB Clearfil SE Bond

AP Adper Prompt Self-Etch Adhesive

3M/ESPE, Dental Products, St Paul, MN, USA Kuruaray, Kurashiki, Japan

3M/ESPE Dental Products, St Paul, MN, USA

* Information available in the technical product profile provided by the manufacturer.

Table 2 Protocol of application of the adhesive systems used in the present study

Groups Procedure*

SB Application of 35% phosphoric acid (Scotchbond Etchant, 3M/ESPE, Dental Products, St. Paul, MN, USA) on dentin for 15 s. Abundant rinse with distilled water for 10 s, and then dried the water excess with absorbent paper. Application of 3 coats of adhesive using a microbrush applicator (Microbrush International, Grafton, USA) with gentle rubbing for 15 s, and then dried with a soft air flow for 5 s. Photo-activation for 10 s.

CB Application of primer using a microbrush applicator for 20 s and then dried thoroughly with a soft airflow. Application of 1 coat of bond using a microbrush applicator with a gentle rubbing and dried soft airflow for 3 s. Photo-activation for 10 s.

AP Rapid mix of 1 drop of liquid 1 with 1 drop of liquid 2 for 5 s. Application of the mixture with hard rubbing over dentin using microbrush applicator for 15 s and then dried thoroughly with a soft air flow. Photo-activation for 10 s.

* Before starting the described procedures, the polished surface was cleaned with distilled water and dried with absorbent paper.

The fractured specimens were examined under an opti-cal microscope (HMV-2, Shimadzu, Nakagyo-ku, Kyoto, Japan) at 200X magnification by the same appraiser. Modes of failure were classified as follows:

1. Adhesive failure, between dentin and adhesive. 2. Mixed failure, between dentin and adhesive with 75%

dentin exposed. 3. Mixed failure, between dentin and adhesive with 50%

dentin exposed. 4. Mixed failure, between dentin and adhesive with 25%

dentin exposed. 5. Cohesive failure, within adhesive or composite resin.

Additionally, representative fractured specimens were sputter coated with gold and re-examined using SEM (JSM5600LV; JEOL, Peabody, MA, USA) for validation of op-tical microscopic assessment. Failure pattern scores were submitted to the Mann Whitney test (p = 0.05).

R ESU LTS

The results of the microshear bond strength test are shown in Fig 1. There were no differences in microshear bond strength due to exposure to different radiation types. No interaction processes occurred between radiation and ad-hesive systems. Statistically significant differences were only found in the UV-irradiated SB group when compared with the control group (Dunnett's test, p = 0.0267).

Figure 2 shows the failure pattern distributions accord-ing to the scores given in Fig 3. In Table 3, the SB adhesive showed a statistically significant change (p = 0.0000) of failure pattern with exposure to different types of radia-tion.

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Cruz et al

Fig 1 Results of the microshear bond strength test. * The value differed from control according to the Dunnett test (p<0.05). Groups with same letter (capital letter in the columns with same treatment comparing adhesive systems and lower case letters in the joined columns comparing different treatments) are not sig-nificantly different according to ANOVA and Tukey's test (p>0.05).

Fig 2 Distribution of the failure patterns (me-dian values) in specimens (from 1 to 10) with scores used (from 1 to 5). A= Results of the SB adhesive; B= Results of the CB adhesive; C= Results of the AP adhesive. The fractioning of median values can occur in specimens with two cylinders.

DISCUSSION

In the present short-term laboratory study, the tooth/res-toration complex was submitted to different types of ioniz-ing radiation to evaluate their influence on adhesive bond strength. Different adhesive systems were also used to evaluate the possibility of alteration on bond strength after irradiation due to the differences in the mode of applica-tion of the adhesive systems. The SB adhesive is a two-

step system, in which the first step consists of preparing the tooth surface with etching gel followed by rinsing, and the second step of applying the primer/bonding agent. The CB adhesive is also a two-step system, but the first step consists of a self-etching primer systerp without rins-ing to prepare the tooth surface, and the bonding agent is applied in the second step. The AP adhesive is a one-step system where etching, primer and bonding agents are ap-plied together.

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Experimental treatment Material SB adhesive CB adhesive AP adhesive

Control non-experimental 3.0 (2.0 - 5.0) Aa 5.0 (1.0 - 5.0) Ba 5.00 (4.0 - 5.0) Ab Diagnostic X-radiation 4.0 (2.0 - 5.0) Bb 2.0 (1.0 - 5.0) Aa 5.00 (3.0 - 5.0) Ac Therapeutic X-radiation 5.0 (3.0 - 5.0) Cb 3.0 (3.0 - 5.0) ABa 5.00 (3.0 - 5.0) Ab Ultraviolet radiation 5.0 (1.0 - 5.0) BCb 2.0 (2.0 - 5.0) Ba 5.00 (4.0 - 5.0) Ac

Values with the same letters (capital letters in the columns and lower case letters in the rows are not significantly different according to the Mann-Whitney test (p>0.05).

Cruz et al

Table 3 Median values of the failure patterns (minimum - maximum)

Fig 3 Representative samples of each fail-ure pattern in accordance with used scores, shown in the lower right-hand corner of each image (original magnification: 80x).

The results showed that the different types of radiation did not cause changes in the bond strength of the CB or AP adhesive; only the SB adhesive showed an increased bond strength after exposure to UV when compared with control group. The high deviation pattern values observed in Fig 1 could have occured due to the manufacturing method of the cylinders, which could have internai micro-irregularities imperceptible under the stereomicroscope. The results of the present study agreed with the study of Bulucu et a1, 5 who also did not observe changes in bond strength after exposure to TX. However, they observed that the irradiation with TX changed the bond strength behavior of the different adhesives. In the present study, the same phenomenon was observed: the bond strength behavior of the CB adhesive was similar to the SB adhesive before ex-posure to radiation. However, after exposure to DX and TX, the CB adhesive behaved similar to the AP adhesive. The bond strength behavior of the AP adhesive was different

from the SB and CB adhesives, but after exposure to UV, it was similar to them.

In the study by Ye et a1, 22 changes were observed in the degree of conversion of adhesives when light acti-vated using different curing units. The adhesives were applied using a different method. These differences occur because the adhesives are dissimilar both in the amount of solvent and in the type of photoinitiator used, which induced dissimilar conversion behavior. However, this behavior also depends on the intensity and breadth of the radiation spectrum. In the present study, three dif-ferent adhesive systems were seiected and photo-acti-vated with an LED unit, which has a narrower radiation spectrum than halogen bulbs. Due to its narrower spec-trum, using an LED unit could cause insufficient conver-sion, which might lead to greater changes when later exposured to radiation chiefly UV. For other radiation types, a possible change could only occur after interac-

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tion with dental tissue, because absorbed and scatted radiation produces secondary radiation, with smaller wavelengths, and is then able to interact with the mole-cules of the adhesives.

The present study focused on effects of ionizing radia-tion on dental restoration by examining adhesive bond strength behavior, varying the adhesives and energy of the radiation. Other studies 6,9 focused only on therapeutic-dose ionizing radiation in dental tissue. The study by Bu-lucu et al 5 focussed on the effects of dentin exposure to TX at a dose of 60 Gy, although their study had two dis-tinct groups: one with radiation applied on teeth without restorations and the other with radiation applied on re-stored teeth. Gernhardt et a1 9,10 evaluated the effects of restorations on irradiated dentin; however, in their study the restorations were not irradiated. In one study by Gern-hardt et a1, 9 no differences were found in adhesive bond strength after radiotherapy, while Bulucu et al 5 did find dif-ferences. Thus, the present study agreed with Gernhardt et a1,9 because here as well, no differences caused by TX radiation were found.

The therapeutic radiation doses used in the previously cited studies 6,9 and the present study were similar, and comparabie with radiotherapy of the head and neck re-gion. 15 The exposure doses of UV and DX used in the pre-sent study were based on others studies, 7,8,19 but they did not have any clinicai application. Due to great penetration, TX and DX can reach the dental tissue when applied to a restoration; however, the same did not occur with UV. UV radiation can break chemical bonds, to reactivate or to ionize molecules, but their photons have little energy and little penetration ability. 78 Depending on the restoration thickness, UV cannot reach the dental tissue when applied to the restoration. Thus, the UV radiation can act in a more concentrated manner in the restorative material. There are no studies about the effects of DX or UV on dental tissues, only studies with therapeutic radiation doses. 13,14,18 Con-cerning the radiation effects on dental tissues, there are two opposing views: one indicating that radiotherapy is able to change the dental structures 6,18 and the other that it is not possible. 9,13-16 However, in this study, a change of bond strength was only observed after exposing SB adhe-sive to UV (Dunnett's test).

When dental materiais alone, such as photoactivated composite resin, were irradiated with a therapeutic radia-tion dose, as in the study by von Fraunhofer et a1, 21 chan-ges were observed in some mechanical properties of some materiais. However, Curtis et a1 6 did not observe any change after using the same restorative materiais and the same radiation exposures, but they evaluated other me-chanical properties that did von Fraunhofer et a1. 21 Thus, according to the cited authors, 6,21 therapeutic radiation doses are able to cause some changes to dental materi-ais; however, the response of each mechanical property is independent. Other previous studies 11,12 corroborate the cited authors, affirming that the same exposure to radia-tion may induce different responses of different materiais, due each material possessing either a larger or a smaller amount of radiosensitive chemical groups. 11-12 The pre-sent study also corroborated these previously cited studies

because each one of the three studied materiais showed a different response to exposure to the same radiation. How-ever, this may have occurred because of different applica-tion modes of each adhesive system.

The different types of radiation changed the failure pat-terns of SB adhesive, and UV induced greater changes when compared with the control group. Under the influ-ence of the same radiation, the failure patterns also varied between the different adhesive systems. For SB adhesive, the application of x-ray radiation resulted in a siight in-crease in mixed failures, and UV radiation resuited in changes from adhesive failure to cohesive failure. The oc-currence of cohesive failure could be an indication that the bond strength of the dentin to the adhesive equals or exceeds the resistance of the composite resin used.' The reduction in incidence of adhesive failure due to exposure to radiation and the increased bond strength may indicate an adhesion improvement in the dentin/adhesive inter-face. 1 Because in the present study, the results of failure patterns are linked with the increased bond strength of the SB adhesive, we may affirm that radiation improved this adhesive. When the increase of cohesive failure was added to the decreased bond strength, it suggests a degradation of the material, which did not occur in the present study. The results of the present study are in agreement with results of other studies 6,11,21 which indi-cated that the therapeutic ionizing radiation dose does not cause detrimental effects on photoactivated composite materiais, but it is able to improve the materiais studied.

The use of different types of ionizing radiation to im-prove dental materiais is known, 2,3,12 however, the radia-tion doses applied in these cases in nonclinical situation are much higher. Regarding UV in the present study, a dis-agreement exists with previous studies 7,8 which indicated that this radiation type has detrimental effects on dental materiais. However, in these previous studies, 7,8 the action of UV exposure on adhesive bond strength or failure pat-tern was not evaluated. The UV was used to cause aging in terms of color changes, microhardness, and degree of conversion, among others. Regarding DX, there are no studies in the literature about its interaction with dental materiais. Therefore, further research should attempt to explain the interaction of different types of ionizing radia-tion with dental materiais by use of other mechanical tests, in order to extend the lifetime of the restoration.

CONCLUSIONS

The types of ionizing radiation used in this study induced changes in bond strength and failure patterns of the stud-ied adhesives. However, the changes depended on the ra-diation energy and type of adhesive materiais evaluated. Considering the experimental model, it is possible to af-firm that the radiation types applied do not cause degra-dation of adhesive materiais by weakeri,ng the bond strength.

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ACKNOWLEDGMENTS

This study is part of a PhD degree thesis in dental radiology at the Oral Diagnosis Department (FOP-UNICAMP). The authors thank "Servidores de Cana de Piracicaba" Hospital for authorizing the use of the linear accelerator, and for placing their radiotherapy techni-cians at the authors' disposal for this study. They also thank Marcos Blanco Cangiani of the Dental Materiais Department (FOP-UNICAMP) for his technical support during experimental procedures, and the FUNCAMP for providing financial support for this study.

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Clinicai relevance: Different externai agents, including ionizing radiation from radiographic diagnosis, radio-therapy, or environmental sunlight, challenge restora-tions in the oral cavity. Thus, it is necessary to evaluate whether such radiation in different doses can induce changes in dental materiais.

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