Fatigue of enamel bonds with self-etch adhesives

d e n t a l m a t e r i a l s 2 5 ( 2 0 0 9 ) 716–720

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Fatigue of enamel bonds with self-etch adhesives Robert L. Erickson, Wayne W. Barkmeier ∗ , Nicole S. Kimmes Department of General Dentistry, Creighton University School of Dentistry, 2500 California Plaza, Omaha, NE 68178, USA

a r t i c l e

i n f o

a b s t r a c t

Article history:

Objective. Fatigue testing of adhesive bonds to tooth structures in conjunction with bond

Received 22 September 2008

strength testing can provide more useful information for examining the effectiveness of

Received in revised form

dental adhesives. The purpose of this study was to determine the shear bond strength (SBS)

1 December 2008

and shear fatigue limit (SFL) of composite to enamel bonds using modern adhesive systems.

Accepted 8 December 2008

Methods. Twelve specimens each were used to determine 24-h resin composite (Z100-3M ESPE) to enamel shear bond strengths with an etch-and-rinse system (ERA), Adper Single Bond Plus (SB), and four self-etch adhesives (SEA)—Adper Prompt-L-Pop (PLP), Clearfil SE

Keywords:

(CSE), Clearfil S3 (CS3) and Xeno IV (X4). A staircase method of fatigue testing was used

Dental materials

in a four-station fatigue cycler to determine the SFL of composite to enamel bonds with

Enamel

the adhesives (16–20 specimens for each adhesive) at 0.25 Hz for 40,000 cycles. ANOVA and

Adhesion

Tukey’s post hoc test were used for the SBS data and a modified t-test with Bonferroni

Fatigue

correction was used for comparisons of the SFL. Results. The SBS and SFL of the etch-and-rinse system were significantly greater (p < 0.05) than those of the four self-etch adhesives. The SBS and SFL of CSE were also significantly greater than for the other three self-etch systems. The ratio of SFL to SBS was highest with the etch-and-rinse system and the ratio became increasing smaller in the same order that the values for SBS decreased with the self-etch systems. Significance. The lower fatigue limits for composite to enamel bonds obtained with the selfetch adhesive systems may indicate that greater enamel margin breakdown will occur with restorations where these systems are used for bonding. © 2008 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

Laboratory testing is used extensively to evaluate the relative effectiveness of dental adhesive systems used to bond resin-based materials to mineralized tooth structures. This is particularly important when new adhesive approaches are advocated to replace those that have previously demonstrated good clinical success. Such is the case with the introduction of self-etching adhesive (SEA) systems, which utilize acidic monomers to etch tooth structure rather than phosphoric acid, which is used with etch-and-rinse adhesive (ERA) sys-

∗

tems. Enamel bonding is of particular concern as the SEA systems are less effective at etching enamel than typical phosphoric acids (35–40%) used with ERA systems [1–3]. Laboratory measurements of bond strength to enamel have generally shown that the values produced with SEA systems are lower than those found using ERA systems [4–12]. This raises the question of whether SEA systems can perform as well in vivo as their ERA counterparts [13,14]. However, the bond strength test, where a monotonically increasing force is applied until the bond is fractured, is not representative of the mode of failure in vivo, where failures are likely to occur

Corresponding author. Tel.: +1 402 280 5262; fax: +1 402 280 5004. E-mail address: [email protected] (W.W. Barkmeier). 0109-5641/$ – see front matter © 2008 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.dental.2008.12.001

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Table 1 – Materials and procedures. Adhesive Adper Single Bond Plus Etchant (Lot No. 7JL), Adhesive (Lot No. 7KY) Adper Prompt L-Pop (Lot No. 255652) Clearfil SE Primer (Lot No.00688A), Bond (Lot No. 00985B) Clearfil S3 (Lot No. 00069A) Xeno IV (Lot No. 061102)

through repeated loading over long periods of time and at load levels well below the bond strength values. Cyclic loading of specimens to elicit failure is often referred to as fatigue testing. A common method of applying fatigue testing is the staircase method, where the load on a specimen is increased or decreased by a fixed amount depending on whether the preceding specimen survived or failed, respectively. With this kind of test a parameter called the fatigue limit can be calculated [15], which represents the load (stress) at which half the specimens will fail for a set number of cycles. Direct fatigue testing of bonds to enamel using SEA systems has only been examined in a few studies [16–18]. Each of these studies found the SEA system to produce fatigue limits and bond strengths that were lower than the ERA system used for comparison. Ideally, the cyclic loading should be carried out using the same test configuration as the bond strength testing so that comparisons can be made between bond strength and fatigue limit. It was found for one SEA system that the ratio of fatigue limit to bond strength was 77% of that found for the ERA material tested, so not only was the bond strength of the SEA material lower than the ERA material but its fatigue limit was proportionally even lower [16]. This suggests that the SEA system performed worse in fatigue testing than might be projected using the initial bond strength values. Comparisons of bond strengths and fatigue limits may give more insight into the expected or observed clinical behavior of adhesive systems. The purpose of this study was to measure both the bond strengths and fatigue limits of resin-composite bonds to enamel using four SEA systems and one ERA system. Based on prior studies, the alternative hypothesis will be posed, that the SEA systems will have significantly lower bond strengths and fatigue limits than the ERA system.

2.

Materials and methods

2.1.

Bonded enamel specimen preparation

A new model was developed to bond resin composite to enamel surfaces for shear bond strength (SBS) and shear fatigue limit (SFL). Metal rings with an inner diameter of 2.4 mm, an outer diameter of 4.8 mm and a thickness of 2.6 mm, were machined from 304-stainless steel. The rings were then used to bond a resin-composite to flat ground enamel surfaces resulting in a bonded cylinder inside the ring 2.4 mm in diameter and approximately 2.5 mm in length. The ring was left in place for the tests. The enamel bonding sites were prepared by sectioning extracted human molar teeth mesio-distally and then removing approximately two-thirds of the apical root structure. The buccal and lingual tooth sections were mounted with Triad

Code SB PLP CSE SC3 X4

System/etchant Etch-and-rinse/35 wt.% phosphoric acid Self-etch/Two components mixed—one-step system Self-etch/Primer and adhesive—two step system Self-etch/One component—one step system Self-etch/One component—one step system

DuaLine (DENTSPLY International, York, PA, USA) in 25 mm phenolic ring forms (Buehler, Lake Bluff, IL, USA). The enamel was ground flat to 4000 grit using a water coolant and a sequence of carbide polishing papers (Struers Inc., Cleveland, OH, USA). An etch-and-rinse adhesive system, Adper Single Bond Plus (3M ESPE, St. Paul, MN, USA) (SB) and four self-etch adhesives Adper Prompt L-Pop (3M ESPE, St. Paul, MN, USA) (PLP), Clearfil SE (Kuraray Medical Inc., Okayama, Japan) (CSE), Clearfil S3 (Kuraray Medical Inc., Okayama, Japan) (CS3) and Xeno IV (DENTSPLY Caulk, Milford, DE, USA) (X4) were used in this study (Table 1). Twelve specimens each were prepared for SBS testing and 20 specimens each were made for the SFL tests. The adhesive agents were used according to manufacturers’ directions to bond Z100 [Lot No. 7PP] (3M ESPE, St. Paul, MN, USA) resin composite to the enamel bonding sites (Table 2). Following the treatment of the enamel surface with the adhesive agent, the metal ring was positioned over the bonding site and secured in place by clamping in a custom fixture. The resin composite was condensed into the ring and polymerized for 40 s with a Spectrum 800 Curing Unit (DENTSPLY Caulk, Milford, DE, USA) set at 600 mW/cm2 . The bonded specimens (Fig. 1) were stored for 24 h in distilled water at 37 ◦ C before testing.

Table 2 – Bonding procedures. Adhesive

Procedure

SB

1. Etch enamel 15 s, water rinse, dry thoroughly with air 2. Apply 2 coats of adhesive 15 s with gentle agitation, air thin 3. Light cure 10 s

PLP

1. Mix adhesive according to directions 2. Apply adhesive with rubbing action for 15 s 3. Gentle air stream to thoroughly air dry 4. Apply second coat of adhesive and thoroughly air dry 5. Light cure 10 s

CSE

1. Apply primer 20 s, dry with mild air flow 2. Apply adhesive followed by gentle air flow 3. Light cure 10 s

CS3

1. Apply adhesive 20 s, dry with high pressure sir flow 2. Light cure 10 s

X4

1. Apply adhesive and actively scrub surface for 15 s 2. Repeat adhesive application with scrubbing 15 s, gentle air stream to yield smooth glossy surface 3. Light cure 10 s

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Table 3 – Shear bond strength, shear fatigue limit and ratio of SFL to SBS. Adhesive SB CSE PLP CS3 X4

SBS MPa (S.D.)

SFL MPa (S.D.)

Ratio of SFL to SBS

55.9 (7.5) a 47.5 (4.2) b 37.7 (8.7) c 32.2 (2.7) c 31.8 (8.2) c

27.8 (2.8) a 22.1 (4.1) b 15.0 (1.4) c 12.2 (1.5) c 11.9 (2.6) c

.50 .47 .40 .38 .37

Different letters in columns indicate differences at the 5% significance level.

Fig. 1 – Bonded resin composite inside metal ring.

2.2.

Shear bond strength tests

The 12 bonded assemblies were individually mounted in a custom fixture for debonding on an Instron (Model 1123) test frame (Instron, Norwood, MA, USA), with an MTS ReNew Upgrade Package and TestWorks software (MTS Systems Corporation, Eden Prairie, MN, USA). The specimens were loaded to failure at 1 mm per minute using a chisel-shaped device applied to the metal ring immediately adjacent to the flat ground surface.

2.3.

Shear fatigue limit testing

A four-station fatigue cycler (Proto-tech, Portland, OR, USA) was used for SFL testing. Specimens were mounted into a custom fixture inside a cylinder-shaped water bath (Fig. 2). A chisel-shaped bar was positioned on the metal ring immediately adjacent to the flat-ground tooth surface. Each of the four stations has a load cell positioned under the fixture to measure the load being applied. The load for each station was adjusted using DASYLab software (DASYLab, Norton, PA, USA). A staircase method of fatigue testing and analysis described by Draughn [15] was used to determine SFL values. The lower load limit was set at zero and the initial maximum loads applied were 40–50% of the SBS determined for each of the adhesive

Fig. 2 – Specimen fixture for fatigue cycling.

systems tested. The load was applied at a rate of 0.25 Hz using a sine wave for 40,000 cycles or until failure occurred. For each adhesive system, 16–20 specimens were used to determine the SFL.

2.4.

Data analysis

Shear bond strength data were tested for conformance to a normal distribution using a Shapiro-Wilk test. ANOVA and Tukey’s post hoc test were used for the SBS data and a modified t-test with Bonferroni correction was used for comparison of the SFL.

3.

Results

The results for the SBS and SFL are shown in Table 3. There was a significant difference (p < 0.05) in the SBS and SFL between the etch-and-rinse adhesive and the four self-etch systems. The SBS and SFL of CSE were also significantly greater than the other three self-etch systems. The ratio of SFL/SBS was less for the self-etch systems when compared to the etch-andrinse system, and the ratio decreased as their bonds strengths decreased.

4.

Discussion

The four SEA systems used in this study to form bonds to enamel had significantly lower SBS and SFL than the ERA system, confirming the alternative hypothesis. This is in agreement with the findings of previous studies [16–18]. While the general findings are similar, it is not reasonable to compare the numerical values found in this study with the others as the methodologies differed among the studies. However, on a relative basis, the SFL ratio between SB and PLP was 0.54 in this study and 0.58 in a similar study [16], so despite some differences in methodologies, there is a reasonable correspondence in findings. In a previous study [16] it was found that the ratio of SFL to SBS was lower for the SEA system than for the ERA system. This suggests that the ability to resist fracture under fatigue conditions is worse on a relative basis than for the ERA system, meaning that the SFL is not only lower for the SEA system but as a percentage of its bond strength it is lower than is the case for the ERA system. Similar results were found for the SEA system in the present study. Further, it was found that the lower the bond strength of the SEA adhesive, the lower the

d e n t a l m a t e r i a l s 2 5 ( 2 0 0 9 ) 716–720

ratio of SFL to SBS. A power law fit to this ratio as a function of bond strength gave the relationship: SFL/SBS = 0.058SBS0.537 with a correlation of R2 = 0.986. Fatigue failure is characterized by a process of crack initiation, often at defects such as scratches, voids or inclusions, followed by progressive crack growth until eventually unstable crack growth leads to total failure [19]. The most likely indication of fatigue failure of enamel bonds in vivo would be enamel margin breakdown. For some types of restorations this might lead to catastrophic failure of the restoration but in most cases this would be the precursor to failure through marginal defect problems such as caries, exposure of dentin or unacceptable staining of esthetic restorations. Larger fatigue limits in theory would reduce the probability of marginal breakdown. The results of this study suggest that restorations placed using SEA systems should show greater degradation of enamel margins than those placed with ERA systems. It was of interest to see if the fatigue limits obtained in this study could be correlated with data from a study where restorations were subjected to fatigue conditions. One such study used cyclic loading with a 50 N load applied for 105 cycles [20]. Ten adhesive systems ranging from three-step ERA systems to one-step SEA systems were examined for the percentage of gap-free margins in enamel and dentin following the cyclic loading. The general result was that the percentage of gap-free margin declined when going from the ERA systems through the SEA systems. Three adhesives of that study overlap with the present study: SB, CSE and PLP. A comparison of the ratios of the gap-free margin percentages provides the following: CSE/SB-0.75 and PLP/SB-0.59. Ratios of the SFL values from the present study yields: CSE/SB-0.79 and PLP/SB-0.54. This degree of correspondence may simply be coincidental, but it is not unreasonable to expect that the differences in fatigue limits should be manifested in the degree of marginal breakdown. Laboratory testing can give some insight into how various adhesives might perform clinically, however, the best determination of performance is to be found from well-designed, controlled clinical trials. Most clinical trials of adhesive systems examine restoration of the non-carious cervical lesion (NCCL) as the model for observing clinical performance. The NCCL restoration may not be the best model for correlating results with enamel fatigue data, as this is primarily a dentin lesion with a smaller amount of enamel, and the stress on the restoration margins may not be as great as for Class I/II restorations. There are not many controlled studies available in the literature that compare SEA systems with ERA systems, and most are relatively short in duration (1–3 years) [21–25]. Of these only one is a study of posterior restorations and the others are studies of NCCL restorations. Other than some losses of retention in the NCCL studies, the common finding in all studies was the difference in marginal defects and staining, which was considerably greater for the SEA systems than for the ERA systems. This could be in accordance with lower fatigue limits for the SEA systems. From these NCCL studies there was only one study that examined adhesive systems matching those used in the present study. This study was reported after 18 months [22] and 36 months [23] and compared PLP as the SEA system with SB as the ERA control system, with 30 restorations for each

719

group. After 18 months there were 2 restorations lost from the PLP group and none from the SB group. There were 19 restorations having Alfa ratings for enamel margins (Alfa rating: continuous margin with not defects) compared with 26 restorations for the SB group, giving a ratio: PLP/SB-0.73. At the 36-month examination, five restorations were lost from the PLP group (17%) and one was lost from the SB group (3%). There were 15 restorations with Alfa ratings for the PLP group and 24 for the SB group, giving a ratio: PLP/SB-0.62. Therefore, in this study there were more marginal defects when PLP was used compared to SB and the difference increased over time, which might be consistent with the SFL being lower for PLP. A 5-year study was conducted using the SEA system, CSE, in a NCCL model study [26]. Instead of a ERA system as a control group there was comparison to a group where the enamel was pre-etched with phosphoric acid prior to applying the CSE system. At the end of 5 years 36% of the restorations had no enamel margin defects in the CSE group compared with 66% for the CSE-etch group, giving a ratio: CSE/CSE-etch-0.54. It has been found that pre-etching enamel before application of CSE increases the bond strength to levels comparable with ERA systems [27,28]. It is likely that the fatigue limits would also be increased under these conditions and the results of this study could be consistent with the differences in fatigue limit for the two groups. One clinical study involved posterior Class I/II restorations [25] and could provide a better test of fatigue failure. This study included two SEA systems that were examined in the present fatigue study, these being PLP and CS3. The ERA control system was One-Step Plus (Bisco Inc., Schaumburg, IL, USA) (OSP). After 1 year the percentage of restorations receiving Alfa ratings for each group was: PLP-31%, CS3-59% and OSP-93%. The differences between these three groups were statistically significant (p < 0.05). Two restorations (7%) of the PLP group were rated clinically unacceptable with marginal gaps extending to dentin. These results demonstrate a more severe degradation of enamel margins for the SEA systems than for the ERA system, as might be expected, assuming that the OSP system has a fatigue limit similar to SB, however, the ranking of PLP and CS3 is the reverse of what might be expected from the fatigue limits found in the present study. A third adhesive system, iBond (Heraeus Kulzer Inc., Armonk, NY, USA), had only 7% of the restorations rated Alfa and 11% were unacceptable due to marginal gaps extending to dentin. The results of this study appear to show that failures, potentially related to fatigue, show up more dramatically and in a shorter time for posterior restorations than NCCL restorations. It is clear that the SEA systems did not perform as well as the ERA system in this study and it might be expected that with longer time periods more of the restorations would be found to be unacceptable as fatigue enlarges the marginal gaps. While there is a clear qualitative agreement between the lower SFL values found for SEA systems and marginal degradation found clinically, quantitative comparisons are more problematic than might be the case for restorations tested in vitro [20]. For in vitro testing the dimensions of restorations are standardized, as is the load application, whereas in vivo the restorations are of varied size, shape and location; with variable loading. Further, the time scales and aging of specimens are quite different. Laboratory specimens are, generally, not

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aged for long periods of time, and the testing phase is relatively brief, whereas in vivo restorations are subjected to the oral environment for years. This concern was also raised when a simulated attempt to reproduce clinical failures in the laboratory was unsuccessful [29]. Other factors that can interfere with laboratory/clinical comparisons involve substrate differences. Most in vitro studies use ground or prepared enamel to provide a reliable substrate, but aprismatic enamel may be involved in vivo and could adversely affect the performance of SEA systems [30,31].

5.

Conclusions

The four self-etching adhesive systems produced significantly lower (p < 0.05) composite to enamel shear bond strengths and shear fatigue limits than the etch-and-rinse system.

[14]

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[17]

[18]

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