Multiple Coatings can Improve the Bond Durability of One-step Self-etching Adhesive to Primary Dentin

Lin Ma,Jian-feng Zhou,Quan Jing,Ji-zhi Zhao,and Kuo Wan＊

1Department of Stomatology,Peking Union Medical College Hospital,Chinese Academy of Medical Sciences &Peking Union Medical College,Beijing 100730,China

2Department of Prosthodontics,Peking University School &Hospital of Stomatology,Beijing 100081,China

ONE-STEP self-etching adhesives were developed in order to shorten the bonding procedure and reduce the sensitivity of the technique,for all of the components are blended in one solution.However,some previous studies have identified some concerns about the performance of these adhesives such as reduced dentin bond strength and poor bond durability.1,2Several possible reasons are responsible for the poor bonding performance of these simplified adhesives∶(1) the presence of highly hydrophilic monomers that are sensitive to water sorption from the underlying dentin,3increasing hybrid layer permeability and nanoleakage;4(2) a differential infiltration gradient through the dentin related to differences in the molecular weight of the adhesive system compounds;5(3) a high concentration of protic solvents and dissolved molecular oxygen within the adhesive layer contributing to poor evaporation;6(4) the limited thickness of the adhesive layer,which may also magnify the oxygen inhibition effect on adhesive polymerization,and (5) their low degree of cure.7

Interactions of the self-etch agents with dentin are limited by many factors from both the substrate and the adhesive itself.Optimal resin infiltration is important to achieving adequate bond strengths,as the effective resindentin bonds may only be formed if primers and resins are able to penetrate through the smear layer and interact with the underlying dentin.8The infiltration of the adhesive blend,solvent/water evaporation,and thickness of the adhesive layer are directly related to chemical and rheological properties,6but may be influenced by the application mode.9It has been hypothesized that more uniform adhesive infiltration and greater solvent/water evaporation would be achieved when bonding with multiple coatings7or by a prolonged application time of the primers/adhesives onto the dentin surface.10

Although there have been studies on the effects of multiple coatings on bond strength of resin to permanent dentin,11-15so far as we know,there is few studies to evaluate the effects of multiple coatings on the bond strength to primary dentin.16In one of our previous studies,the effect of multiple coatings of a one-step self-etching adhesive system on the immediate bond strength to primary dentin was evaluated.17This study was to further assess the effect of multiple coatings on the bond durability of a one-step self-etching adhesive to primary dentin.

MATERIALS AND METHODS

Tooth preparation

Twelve caries-free second primary molars,which had been extracted because of prolonged retention,were collected after obtaining the informed consent of children’s parents.The study protocol was reviewed and approved by the Ethics Committee for Human Studies,Peking University,Beijing,China.The teeth were stored in 0.9% NaCl containing 0.02% sodium azide at 4°C for no more than one month.

The occlusal enamel of each tooth was removed.The teeth were hand polished on a wet 600-grit silicon carbide paper for 30 seconds to create a realistic smear layer on the surface of the occlusal mid-coronal dentin.

Bonding procedures

The 12 prepared teeth specimens were randomly divided into 2 groups,6 teeth in each group.

In group 1,each tooth was hemisected into 2 halves using a low-speed water-cooled saw equipped with a diamond-impregnated disk (Isomet 1000;Buehler Ltd.,Lake Bluff,IL,USA).One half tooth assigned to control (C1)subgroup was bonded with a one-step self-etching adhesive (Clearfil S3 Bond,lot no.00143A;Kuraray Medical,Osaka,Japan) according to the manufacturer’s instructions.The composition of the one-step self-etching adhesive is shown as follows∶10-methacryloyloxydecyl dihydrogen phosphate (MDP),bis-phenol A diglycidylmethacrylate(Bis-GMA),2-hydroxyethyl methacrylate (HEMA),hydrophobic dimethacrylate,camphorquinon,ethyl alcohol,silanated colloidal silica,and water.After undisturbed for 20 seconds,the adhesive was uniformly spread by a strong air stream for 10 seconds and then light-cured for 10 seconds using a halogen light-curing unit with an output of 700 mW/cm2.The other half tooth assigned to experimental(E1) subgroup was continuously bonded with the adhesive for 3 times prior to light-curing for 10 seconds.After an 8-mm-height composite resin block (Clearfil AP-X,Kuraray,Osaka,Japan) was built up on the bonded surfaces by 5 or 6 sequential increments of resin,each one was light-cured for 20 seconds.

In group 2,the dentin surface of one split half (C2 subgroup) was bonded with the one-step self-etching adhesive according to the manufacturer’s instructions.For the other half (E2 subgroup),adhesive was applied to the dentin surface.After leaving undisturbed for 20 seconds,the adhesive was spread by a strong air stream for 10 seconds and light-cured for 10 seconds.This procedure was repeated for 3 times,followed by composite buildups.

Microtensile bond strength test

After stored in distilled water at 37°C for a week,the bonded teeth were longitudinally sectioned across the bonded interface into sections with a diamond saw (Isomet 1000),to produce a series of 0.8 mm×1.5 mm×8.0 mm beams.Four to six beams were obtained from each specimen.These beams were then trimmed to form hourglass shape specimen with a cross-sectional area of approximate 0.6 mm2.Then specimens were stored in 0.9% NaCl containing 0.02% sodium azide at 37°C for 18 months before bond strength test.The storage solution was changed each month.

After 18 months of water storage,the specimens were individually fixed to a custom-made testing jig with cyanoacrylate glue (Universal Instant Adhesive,Henkel Adhesives Co.Ltd.,Shantou,China).The specimen was then subjected to tensile load at a crosshead speed of 1.0 mm per minute until failure (Autograph DCS-5000,Shimadzu,Japan).The bond strength values were recorded as Newton (N) and later converted to megapascals by dividing the loads in Newtons by the total bonded surface area(MPa=N/mm2).

Debond pathway determination

Both surfaces of each fractured specimen were observed under a stereomicroscope (Olympus 220670,Tokyo,Japan)with 40× magnification to record the failure modes.The fracture modes were classified as follows∶(1) cohesive failures in the composite resin were defined as the fracture occurred exclusively within the resin composite;(2) failures in the adhesive joint were confirmed if the fracture site was entirely within the adhesive;(3) cohesive failures in dentin were the fracture occurring exclusively within the dentin;and (4) mixed failures were verified if the fracture site continued from the adhesive into either the resin composite or dentin.

Statistical analysis

We tested all data using SPSS statistical software (version 16.0).The microtensile bond strength was expressed as mean±SD.Two-way analysis of variance (ANOVA) andpost hoc Tukeytests were used to compare the effects of the dentin treatments on bond strength.Monte Carlo method was used to analyze the distribution of the fracture mode.The statistical unit was beams,not teeth.Statistical significance was pre-set at α=0.05.

RESULTS

Of a total of 123 specimens,only 4 specimens failed,and 1 was from subgroup C1,1 from subgroup E1,and 2 from subgroup E2.All failure occurred during specimen preparation for bond strength test.Finally,the sample size of C1,E1,C2 and E2 subgroups was 31,30,28,and 30,respectively.

Comparison of bond strength

In group 1,the bond strength of the E1 subgroup was significantly higher than that of the C1 subgroup(47.46±13.91vs.38.12±11.21 MPa,P＜0.05).In group 2,there was no difference in bond strength between the C2 and E2 subgroups (39.40±8.87vs.40.87±9.33 MPa,P＞0.05).

Distribution of the fracture mode

Table 1 summarizes the distribution of the fracture modes.Adhesive joint failures were the most common fracture pattern observed in both the control and experimental subgroups.The incidence of cohesive failure in the composite resin in E1 subgroup was significantly higher than that in the other three subgroups (allP＜0.05).

DISCUSSION

As shown in this study,after 18 months of water storage,the bond strength of the E1 subgroup was higher than all the other three subgroups,indicating that applying 3 layers of adhesive before light curing can improve the bond durability of the one-step self-etching adhesive to primary dentin.No significant difference was observed between the C2 subgroup and E2 subgroup,indicating that the technique of applying 3 layers of adhesive with curing each successive layer can not improve the bond durability of the one-step self-etching adhesive to primary dentin.However,compared with the immediate bond strengthreported in our previous study,17the bond strength decreased in all subgroups after 18 months water storage,indicating that the technique of multiple coatings can not totally prevent the bond aging of the one-step self-etching adhesive.

Table 1.Distribution of the fracture mode of the four study groups

There are minor differences between primary and permanent teeth in terms of dentin composition and morphology.18,19It has been pointed out that the concentration and diameter of dentinal tubules is higher in primary dentin than that in permanent dentin.The amount of surface moisture was also found to be higher in primary dentin.20,21All these factors alter the effectiveness of the dentin conditioners,hence affecting the bond strength.20,21However,people often transfer the knowledge about permanent teeth to primary teeth,regardless of these microstructural differences between permanent and primary dentin.Therefore,the value of the studies on permanent teeth,such as the technique of multiple coatings which might alter the conditions of bond surface and the quality of the hybrid layers,is worthy to be ascertained on primary teeth.

Although the effects of multiple coatings on the immediate bond strength were well investigated,there are only a few studies evaluating the effect of multiple coatings on the bond durability of dental adhesives to permanent teeth and to primary teeth.Reiset al22evaluated the immediate and 6-month resin-dentin bond strength of one-step self-etch systems under double application mode.They concluded that double application of two one-step self-etch adhesive (Xeno III and iBond) could improve the bond durability.Similar results were obtained in this study,although different adhesive and different adhesive layers were used.For the teeth bonded with one-step self-etching adhesives,penetration of the adhesive into dentin occurs simultaneously with demineralization of the dentin.When the resin fails to completely infiltrate the deeper portions of the demineralized dentin,the bond between resin and dentin might be weakened.23Applying multiple consecutive coatings prolongs adhesive application time,which might allow resin monomer to penetrate into the total depth of the demineralized dentin.24,25Besides,Clearfil S3 Bond contains a significant amount of water and solvent which are expected to be removed by air blowing after application of the adhesive.11Multiple coatings also prolong air-drying time,therefore can remove residual water and solvent more effectively.13This has been confirmed that the prolonged application and subsequent solvent evaporation may improve resin infiltration within the exposed collagen fibers.13,26Therefore,the use of multiple layers of adhesive before curing,as done in this study,allows more time for adhesive diffusion and removal of residual water,thereby,improving resin infiltration and crosslinking of the adhesive comonomers within the hybrid layer.

When each successive layer is light cured,the adhesive layer becomes thicker without changing the quality of the hybrid layer,which can create a relatively thick intermediate layer with low elastic modulus between dentin and composite.It has been speculated that as the component overlying the hybrid layer,the adhesive layer might help to preserve the integrity of the hybridized dentin,protect it from polymerization shrinkage stresses,act as a stress absorbing layer,and contribute to improving and maintaining bond strength.27,28However,each adhesive system has an ideal thickness to reduce interface stress while preserving the interface integrity.And quite different results were achieved in different studies∶D'Arcangeloet al12reported an excess of adhesive layer thickness could negatively influence the strength.Pashleyet al7and de Silvaet al29demonstrated an increased bond strength using the technique of applying multiple layers of adhesive then curing each successive layer.This is probably due to the differences in procedures,application modalities,and compositions (solvent agents and filling materials) of the various commercially available bonding systems used.30It is worth emphasizing that,to date,no study has successfully established a positive correlation between the thickness of the resin-infiltrated layer and bond strength,suggesting that the quality,rather than the thickness of the resin-infiltrated layer,is more important for bond strength measurements.27,31

Analyzing the failure modes can help to compare the bond strength.Microtensile bond strength test is considered the most credible method for evaluating dentin adhesion.With using small sized specimens (1 mm2),it offers several advantages including optimal stress distribution at the resin/dentin interface,more adhesive failures (that is,true bond strength values),fewer cohesive failures,and a higher interface bond strength.32In the E1 subgroup,the incidence of cohesive failures in the composite resin was significantly higher than that in the other three subgroups.This result indicated higher bond strength in the E1 subgroup.

In conclusion,applying 3 layers of one-step selfetching adhesive before light curing might be able to improve the bond durability of resin to primary dentin.

1.Chersoni S,Suppa P,Grandini S,et al.In vivoandin vitropermeability of one-step self-etch adhesives.J Dent Res 2004;83∶459-64.

2.Bouillaguet S,Gysi P,Wataha JC,et al.Bond strength of composite to dentin using conventional,one-step,and self-etching adhesive systems.J Dent 2001;29∶55-61.

3.Wang Y,Spencer P.Continuing etching of an all-in-one adhesive in wet dentin tubules.J Dent Res 2005;84∶350-4.

4.Carvalho RM,Chersoni S,Frankenberger R,et al.A challenge to the conventional wisdom that simultaneous etching and resin infiltration always occurs in self-etch adhesives.Biomaterials 2005;26∶1035-42.

5.Eliades G,Vougiouklakis G,Palaghias G.Heterogeneous distribution of single-bottle adhesive monomers in the resin-dentin interdiffusion zone.Dent Mater 2001;17∶277-83.

6.Nunes TG,Garcia FC,Osorio R,et al.Polymerization efficacy of simplified adhesive systems studied by NMR and MRI techniques.Dent Mater 2006;22∶963-72.

7.Pashley EL,Agee KA,Pashley DH,et al.Effects of oneversustwo applications of an unfilled,all-in-one adhesive on dentine bonding.J Dent 2002;30∶83-90.

8.Duarte S Jr.,Perdigao J,Lopes MM.Effect of dentin conditioning time on nanoleakage.Oper Dent 2006;31∶500-11.

9.Toledano M,Proenca JP,Erhardt MC,et al.Increases in dentin-bond strength if doubling application time of an acetone-containing one-step adhesive.Oper Dent 2007;32∶133-7.

10.Moszner N,Salz U,Zimmermann J.Chemical aspects of self-etching enamel-dentin adhesives∶a systematic review.Dent Mater 2005;21∶895-910.

11.Arisu HD,Eliguzeloglu E,Uctasli MB,et al.Effect of multiple consecutive applications of one-step self-etch adhesive on microtensile bond strength.J Contemp Dent Pract 2009;10∶67-74.

12.D'Arcangelo C,Vanini L,Prosperi GD,et al.The influence of adhesive thickness on the microtensile bond strength of three adhesive systems.J Adhes Dent 2009;11∶109-15.

13.Erhardt MC,Osorio R,Pisani-Proenca J,et al.Effect of double layering and prolonged application time on MTBS of water/ethanol-based self-etch adhesives to dentin.Oper Dent 2009;34∶571-7.

14.Belli R,Sartori N,Peruchi LD,et al.Effect of multiple coats of ultra-mild all-in-one adhesives on bond strength to dentin covered with two different smear layer thicknesses.J Adhes Dent 2011;13∶507-16.

15.Soares CG,Carracho HG,Braun AP,et al.Evaluation of bond strength and internal adaptation between the dental cavity and adhesives applied in one and two layers.Oper Dent 2010;35∶69-76.

16.Bansal S,Pandit IK,Srivastava N,et al.Technique-sensitivity of dentin-bonding agent application∶the effect on shear bond strength using one-step self-etch adhesive in primary molars∶anin vitrostudy.J Indian Soc Pedod Prev Dent 2010;28∶183-8.

17.Ma L,Zhou JF,Tan JG,et al.Effect of multiple coatings of one-step self-etching adhesive on microtensile bond strength to primary dentin.Chin Med Sci J 2011;26∶146-51.

18.Baygin O,Korkmaz FM,Arslan I.Effects of different types of adhesive systems on the microleakage of compomer restorations in Class V cavities prepared by Er,Cr∶YSGG laser in primary teeth.Dent Mater J 2012;31∶206-14.

19.Manfro AR,Reis A,Loguercio AD,et al.Effect of different concentrations of chlorhexidine on bond strength of primary dentin.Pediatr Dent 2012;34∶11-5.

20.Sumikawa DA,Marshall GW,Gee L,et al.Microstructure of primary tooth dentin.Pediatr Dent 1999;21∶439-44.

21.Nor JE,Feigal RJ,Dennison JB,et al.Dentin bonding∶SEM comparison of the dentin surface in primary and permanent teeth.Pediatr Dent 1997;19∶246-52.

22.Reis A,Albuquerque M,Pegoraro M,et al.Can the durability of one-step self-etch adhesives be improved by double application or by an extra layer of hydrophobic resin? J Dent 2008;36∶309-15.

23.Sano H,Yoshikawa T,Pereira PN,et al.Long-term durability of dentin bonds made with a self-etching primer,in vivo.J Dent Res 1999;78∶906-11.

24.Ito S,Tay FR,Hashimoto M,et al.Effects of multiple coatings of two all-in-one adhesives on dentin bonding.J Adhes Dent 2005;7∶133-41.

25.el-Din AK,Abd el-Mohsen MM.Effect of changing application times on adhesive systems bond strengths.Am J Dent 2002;15∶321-4.

26.Hashimoto M,Sano H,Yoshida E,et al.Effects of multiple adhesive coatings on dentin bonding.Oper Dent 2004;29∶416-23.

27.Lodovici E,Reis A,Geraldeli S,et al.Does adhesive thickness affect resin-dentin bond strength after thermal/load cycling? Oper Dent 2009;34∶58-64.

28.Choi KK,Condon JR,Ferracane JL.The effects of adhesive thickness on polymerization contraction stress of composite.J Dent Res 2000;79∶812-7.

29.de Silva AL,Lima DA,de Souza GM,et al.Influence of additional adhesive application on the microtensile bond strength of adhesive systems.Oper Dent 2006;31∶562-8.

30.Zheng L,Pereira PN,Nakajima M,et al.Relationship between adhesive thickness and microtensile bond strength.Oper Dent 2001;26∶97-104.

31.do Amaral RC,Stanislawczuk R,Zander-Grande C,et al.Bond strength and quality of the hybrid layer of one-step self-etch adhesives applied with agitation on dentin.Oper Dent 2010;35∶211-9.

32.Sano H,Ciucchi B,Matthews WG,et al.Tensile properties of mineralized and demineralized human and bovine dentin.J Dent Res 1994;73∶1205-11.