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ORIGINAL RESEARCH Table of Contents   
Year : 2009  |  Volume : 20  |  Issue : 4  |  Page : 431-436
Effect of water storage on resin-dentin bond strengths formed by different bonding approaches


1 Department of Restorative Dentistry, State University of Ponta Grossa, (UEPG) Ponta Grossa, PR, Brazil
2 Department of Restorative Dentistry, Banderante University of São Paulo (UNIBAN), São Paulo, SP, Brazil

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Date of Submission05-Sep-2008
Date of Decision29-Jan-2009
Date of Acceptance23-Apr-2009
Date of Web Publication29-Jan-2010
 

   Abstract 

Objectives: The purpose of this study was to evaluate the influence of water storage on resin-dentin bond strengths [µTBS] using different adhesive bonding approaches.
Materials and Methods: Flat superficial dentin surfaces of 24 extracted human third molars were exposed and polished to create a standardized smear layer. The teeth were randomly distributed into four different groups: Three-step etch-and-rinse (Adper Scotchbond Multi-Purpose, 3M ESPE - SBMP), two-step etch-and-rinse (Adper Single Bond 2, 3 M ESPE - SB); two-step self-etch (AdheSE, Ivoclar/Vivadent - AD); and self-etch 1 step (Adper Prompt L-Pop, 3M ESPE - LP). Following the adhesive application (n = 6), resin composite was incrementally applied (Filtek™ Supreme XT - 3 M ESPE) in order to obtain bonded sticks, with a cross-sectioned area of 0.81 mm 2 . The bonded sticks were randomly divided and assigned to be tested after one day [OD] (n 30) or six months [6 M] of water storage [6 M] (n = 30).
Results: Two-way ANOVA and Tukey's test showed that none of the adhesives showed degradation after 6 M. SB achieved the highest µTBS both in the [OD] (49.13 MPa) and [6M] (40.27 MPa). Despite the highest values in both time evaluations, the µTBS of SB significantly reduced after 6M. LP showed the lowest µTBS in both periods of evaluation (18.35 and 18.34 MPa).
Conclusions: Although a significant degradation was only observed for SB, this was the adhesive that showed the highest µTBS after 6 M of water storage.

Keywords: Composite resins, dentin-bonding agents, tensile strength

How to cite this article:
Martins G C, Calixto A L, Gomes O, Loguercio A D, D'Alpino P, Reis A. Effect of water storage on resin-dentin bond strengths formed by different bonding approaches. Indian J Dent Res 2009;20:431-6

How to cite this URL:
Martins G C, Calixto A L, Gomes O, Loguercio A D, D'Alpino P, Reis A. Effect of water storage on resin-dentin bond strengths formed by different bonding approaches. Indian J Dent Res [serial online] 2009 [cited 2019 Sep 17];20:431-6. Available from: http://www.ijdr.in/text.asp?2009/20/4/431/59445
Several studies have been accomplished to evaluate the resin-dentin bond strengths [µTBS]. µTBS is a property of fundamental importance to evaluate the possible longevity of the resin based dental restorative materials. [1],[2],[3] Three-step adhesive systems, called conventional adhesives, require successive applications of an acid etchant, normally phosphoric acid, an adhesion promoting agent or primer, and a bonding resin or adhesive. [4],[5],[6]

Efforts have been directed toward developing new types of dental adhesives to simplify the numerous application steps. One-step self-etch systems combine the acidic primer with the adhesive resin in one application step. This allows for simultaneous infiltration of adhesive resin to the depth of demineralization, which may reduce post-operative sensitivity. The self-etch adhesive system eliminates the rinsing step after etching by using non-rinse acidic monomers to etch and prime dentin simultaneously. Self-priming adhesives are claimed to completely infiltrate the etched dentin with resin. [6],[7],[8]

However, despite their apparent simplicity, these newer systems may not be successful at bonding to such distinct surfaces as enamel, dentin, adhesive, composite, all of which differ markedly in terms of chemical composition. [8] Simplified adhesive protocols do not always guarantee clinical bonding success; [9] in fact, successful adhesive restorations require meticulous technical considerations and an understanding of the principles involved in each clinical procedure. [10]

Changes in the technical application of adhesives are intended not only to simplify the use of the material, but also to ideally obtain an increased long term performance. As adhesive dentistry relies on restorative material that bond to tooth structure, it is important to investigate whether there is a correlation between the simplification of technical adhesive and the immediate and long term µTBS. [9] The evaluation of a dental adhesive/tooth interface often involves an attempt to determine the interfacial bond strength, claimed to be an important mechanical test to estimate the longevity of a restoration. [1],[2],[3]

Therefore, the purpose of this study was to evaluate the effects of water storage in the µTBS using different adhesive bonding systems (three-step and two-step etch-and- rinse systems as well as two-step and one-step self-etch systems). Specimens were tested after one day and six months of water storage. The null hypothesis tested is that no significant difference will be detected among the different bonding strategies in the one day and six-month water storage period.


   Materials and Methods Top


Twenty-four sound, recently extracted, human third molars were scaled, cleaned with slurry of pumice and water, and stored in a 0.1 % thymol solution at room temperature. Teeth were obtained in accordance to guidelines established and approved by the Human Assurance Committee from the State University of Ponta Grossa under protocol number 04930/06. A flat dentin surface was exposed by means of a diamond wheel (Isomet 1000, Buehler; Lake Bluff, IL, USA) The exposed mid-coronal dentin surfaces were further polished on wet # 600-grit silicon-carbide paper for 60 s to standardize the smear layer. Experimental design is depicted in [Figure 1].

Specimens were randomly divided into four groups according to the different bonding approaches:

  • Three-step etch-and-rinse system (Adper Scotchbond Multi-Purpose, 3M ESPE, St. Paul, MN, USA - SBMP),
  • Two-step etch-and-rinse system (Adper Single Bond 2, 3 M ESPE, St. Paul, MN, USA - SB);
  • Two-step self- etch (AdheSE, Ivoclar/Vivadent, Schaan, Liechtenstein - AD); and
  • One-step self-etch (Adper Prompt L-Pop, 3 M ESPE, St. Paul, MN, USA - LP).
All adhesives were applied according to manufacturer´s instructions and polymerized for 10 seconds using a QTH light-curing unit (Optilux, Demetron Research Corporation, USA) Light intensity was monitored throughout the experiment to ensure that a consistent intensity was maintained (600 mW/ cm 2 ) [Table 1].

After bonding procedures (n = 6 teeth), a six mm-thick layer of resin composite (Filtek™ Supreme XT - 3M ESPE, St. Paul, MN, USA) was added to the dentin surface in four increments of 1.5 mm (± 0.1mm). After the application of the last increment, the resin composite was irradiated for 40s using the same light-curing unit. After the end of the restorative procedures, the teeth were stored in distilled water at 37°C for 24 hours.

The teeth were then bucco-lingually and mesio-distally sectioned through the restoration using a water-cooled rotating diamond wheel (Isomet 1000, Buehler; Lake Bluff, IL, USA) to obtain sticks per tooth, each with a cross-sectional area of approximately 0.81 mm 2 (0.9 mm × 0.9 mm). The bonded sticks were randomly divided and assigned to be tested soon after slicing - one day stored specimens [OD] (n= 30 sticks) or after six months of water storage [6 M] (n =30 sticks). The storage solution was kept over time. Each bonded stick was attached to a modified device for microtensile testing with cyanoacrylate resin (Super Bonder Gel/Locitite, Itapevi, SP, Brazil) and subjected to a tensile force in a universal testing machine (Bisco - Inc., Schaumburg, IL, USA) at a crosshead speed of 0.5 mm/ min. The fractured specimens were carefully removed from the apparatus and the cross-sectional area measured with a digital caliper at the site of failure and the results converted into MPa. The failure modes were evaluated at 40 × magnification using a stereomicroscope (Microscopy, Nikon Eclipse E200, Melville, NY, USA) and were classified as (A) adhesive; (D) cohesive within dentin; (R) cohesive within composite resin and (M) mixed (failure at resin/dentin interface or mixed with cohesive failure of the neighboring substrates).

The µTBS data from sticks were submitted to a two-way analysis of variance (Adhesive vs. Storage period) and Tukey's post-hoc test was used for pair-wise comparisons at a pre-set alpha of 0.05. Specimens with cohesive failures were excluded from the data analysis.


   Results Top


The cross-sectional areas of the bonded specimens varied from 0.7 to 0.9 and no significant difference was observed among the groups evaluated (P < 0.05). The overall failure modes of this study are depicted in [Table 2] which shows that majority of failures were due to adhesive failure. The experimental results of the µTBS values are summarized in [Figure 2]. One day results demonstrated that SB presented the highest µTBS values (49.13 ± 11.55) whereas LP the lowest mean (18.35 ± 6.55). After six months, SB mean showed a statistically decrease in the bond µTBS to 40.27 ± 8.73 (P < 0.05). Despite the lowest one day values, SBMP and LP presented similar values (P > 0.05) and showed no degradation of the µTBS after six months of water storage.

The µTBS means were also calculated, including microtensile bond strength values, from specimens that failed cohesively and those that had premature failure ("as zero"). Although the overall means changed slightly, the overall significances remained unaffected.


   Discussion Top


The reduction of steps that occurred in the simplified adhesive techniques did not affect the one-day and long term µTBS. On the other hand, a significant reduction in µTBS was observed for SB adhesive after six months. The etch-and-rinse adhesives usually show the highest values of µTBS, however, such systems can be divided in two categories: The ones which require two or three application steps; always the acid etching as a separate step. [7],[11],[12]

The majority of the etch-and-rinse adhesives usually present high µTBS values under controlled, in vitro studies. [2],[13] However, such systems are technically sensitive, increasing the possibility of errors during the adhesive procedure. [10] Interestingly the present investigation demonstrated that the two-step etch-and-rinse system depicted the highest µTBS mean ([OD] (49.13 MPa) and [6 M] (40.27 MPa); however, while SBMP were stable over time, SB showed a degradation of the resin-dentin bonds losing its effectiveness when subjected to storage. [2],[13] In contrast to studies previously mentioned [2],[13] this study showed that the three-step etch-and-rinse adhesive showed stable values over time (27.82 one day, and 28.63, six months later).

Current adhesive systems are designed to provide dentin adhesion through the interaction of a hydrophilic monomer in an organic solvent with a collagen-rich humid tissue. The application of an adhesive resin ideally would fill up any remaining porosities at the de-mineralized dentin surface, creating the hybrid layer. Resin tags are also formed which may seal the dentinal tubules opened by acid etching. Development of a stronger dentin/resin bonding would be reached not only if resin tags were firmly bonded to the tubule walls, but also by the area occupied by them. [12] The resin/dentin interface formed by etch-and-rinse adhesives is prone to degradation by water, being the two-step systems more susceptible than the three-step etch-and-rinse system. [2] This study supports this statement. The three-step etch-and-rinse system is less vulnerable to water degradation effects; established resin-dentin bonds can only be achieved with two-step etch-and-rinse system as long as there is a bonded enamel border. [14]

The degradation of resin-dentin bonds is usually attributed to the degradation of unprotected collagen fibrils at the base of hybrid layer or to the hydrolytic degradation that polymers are prone to after water sorption. Water can infiltrate in the resin matrix and through swelling can reduce the frictional forces between the polymer chains, in a process known as plasticization. This water-driven process can therefore, decrease the mechanical properties of the polymer matrix [15],[16] and cause elution of uncured monomers and break-down products. [17] More recently, evidence has demonstrated that the breakdown of unprotected collagen fibrils [13],[18],[19] can also occur via activation of host-derived matrix metalloproteinase. [20],[21] Although this process can occur to all adhesives it was already demonstrated that more hydrophilic adhesives tend to absorb more water [22] from the environment and are therefore more prone to degradation.

This can explain the lack of degradation observed for SBMP and AD as both adhesives, although they rely on different strategies, employ a more hydrophobic adhesive coat over the primer turning the adhesive more resistant to water sorption. It is known that simplified adhesives, such as two-step etch-and-rinse and one-step self-etch systems behave as permeable membranes even after polymerization. [23],[24] The lack of additional coat of a hydrophobic bonding resin and the presence of highly hydrophilic groups draw water from the underlying hydrated dentin [25] and from the oral environment after polymerization. This study detected a significant decrease in the µTBS for the two-step etch-and-rinse SB, however it failed to demonstrate the same degradation effect to the other simplified adhesive. However, one can observe that the one day µTBS observed for LP were rather low, which may be the reason of why the micro-tensile test was not sensitive enough to detect such degradation.

Resin-dentin bonds degrade over time. [2] Many factors can cause the degradation over time, among them, an exposure to water, incomplete hybridization and residual solvent of the adhesive. [2],[26],[27] The polymerization of the adhesive in the presence of water prevents the attainment of a highly cross-linked polymer. [28] The previous authors demonstrated that the addition of as little as 20 microliters of water per mL of adhesive to water-free HEMA-based adhesive is capable to reduce its degree of conversion by 50%. Studies corroborate with this research showing that there is a decrease of µTBS of adhesives after a short period of storage and an increase in degradation. [1],[2],[13],[26],[27],[29]

Self-etching primers and adhesives are composed of aqueous solutions of acidic functional monomers with a pH relatively higher than phosphoric acid etchants. [29] Water is necessary to provide the medium for ionization and action of these acidic resin monomers. HEMA monomer is added because most acidic monomers have a low solubility in water, while bi-or multi-functional monomers are important to provide strength to the cross-linking at the formed polymer matrix. [30] The self-etching adhesives offer some advantages over conventional etch-and-rinse systems such as reduction of postoperative sensitivity, less sensitive technique, and simplification of bonding procedures because they do not require a separate acid conditioning step and moist post-rinse control and infiltration of the adhesive resin occurring simultaneously with the self-etching process. [31] (The bonding mechanism of self-etching adhesives has been intensely investigated and described). However, the durability of composite restorations bonded with self-etching adhesives still remains questionable. [8] The long-term effects of incorporating dissolved hydroxyapatite crystals and residual smear layer remnants within the bond are still unknown, as a few studies have been evaluating this issue under in vitro and in vivo conditions. The rapid progression of adhesive material during the recent 20 years has resulted in a situation where many adhesive systems have been replaced by modified successors, which claimed to be better, without clinical validation.

The micro-tensile bond test utilized has been reported to be well suited for the evaluation of bond strengths to enamel and dentin. [32],[33] This helps to reflect higher regional differences in bond strengths within the same tooth. Another important advantage is that adhesive pattern of failure is mostly seen, meaning that the failure occurs at the bonding area itself. It is somewhat true that most of the failure modes observed in the present study were adhesive failure mode.


   Conclusions Top


Based on the limitations of this study, it can be concluded that:

  • Not all adhesives showed a decrease in µTBS values with the exception of the two-step etch-and-rinse adhesive.
  • The two-step etch-and-rinse adhesive is more susceptible to water degradation.
  • The two-step self-etch adhesive obtained similar results compared to that obtained with the etch-and-rinse adhesive but no significant decline was seen after storage.


 
   References Top

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32.Sano H, Ciucchi B, Matthews WG, Pashley DH. Tensile properties of mineralized and demineralized human and bovine dentin. J Dent Res 1994;73:1205-11.  Back to cited text no. 32      
33.Pashley DH, Carvalho RM, Sano H, Nakajima M, Yoshiyama M, Shono Y, et al. The microtensile bond test: A review. J Adhes Dent 1999;1:299-309.  Back to cited text no. 33      

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Correspondence Address:
G C Martins
Department of Restorative Dentistry, State University of Ponta Grossa, (UEPG) Ponta Grossa, PR
Brazil
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0970-9290.59445

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    Figures

  [Figure 1], [Figure 2]
 
 
    Tables

  [Table 1], [Table 2]

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