Influence of cross-head speed on resin-dentin shear bond strength

dental materials Dental Materials 17 (2001) 165±169

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In¯uence of cross-head speed on resin-dentin shear bond strength A.T. Hara a, L.A.F. Pimenta a,*, A.L. Rodrigues Jr. b a

Department of Restorative Dentistry, School of Dentistry of Piracicaba, University of Campinas (UNICAMP), Av. Limeira 901, Caixa Postal 52, Piracicaba, Brazil b Discipline of Bioestatistics and Scienti®c Methodology, School of Dentistry of Araraquara, State University of SaÄo Paulo (UNESP), Araraquara, Brazil Received 18 January 2000; revised 16 April 2000; accepted 1 May 2000

Abstract Objective: To evaluate the in¯uence of different cross-head speeds on shear bond strength test on the dentin surface. Methods: One hundred and twenty extracted bovine incisors were embedded in polystyrene resin. The specimens were prepared by wet grinding with 320-, 400- and 600-grit Al2O3 paper exposing dentin. After the application of the adhesive system Single Bond (3M) to etched dentin, the composite resin Z-100 (3M) was applied and light cured. The specimens were randomly assigned to four groups (n ˆ 30). The shear bond strength tests were performed with an EMIC DL 500 universal testing machine at four different cross-head speeds: 0.50 (A); 0.75 (B); 1.00 (C); and 5.00 mm/min (D). Results: The mean values of shear bond strength in MPa (SD) were: A, 11.78 (3.91); B, 11.82 (4.78); C, 16.32 (6.45); D, 15.46 (5.94). The data were analyzed with one-way ANOVA and Tukey's test (a ˆ 0.05). The results indicated that A ˆ B , C ˆ D. The fracture pattern was evaluated by visual analysis in a stereomicroscope (25 £ ). The percentage of fractures that occurred at the adhesive interface were: A, 92.5%; B, 91.6%; C, 70.0%; D, 47.0%. The Student's t-test to percentages (a ˆ 0.05) indicated that there were no signi®cant differences among A, B and C; A and B differed from D, and there was no signi®cant difference between C and D. Signi®cance: Different cross-head speeds may in¯uence the shear bond strength and the fracture pattern in dentin substrate. Shear bond strength using cross-head speeds of 0.50 and 0.75 mm/min should be preferred. q 2001 Academy of Dental Materials. Published by Elsevier Science Ltd. All rights reserved. Keywords: Shear bond strength; Cross-head speed; Adhesion

1. Introduction The advance in adhesive systems has made it necessary to evaluate their effectiveness in bonding restorative materials to enamel or dentin substrate. Some in vitro methods have been extensively used to do this, including the shear, tensile and micro-tensile bond strength tests [1]. Shear test measurements have been reported as the most prevalent in the literature [2]. However, the stress distribution in such tests can be complex [2] contributing to the nonuniformity of results observed by different investigators in apparently identical shear tests [2]. These non-uniform results may also be related to many bonding variables, such as teeth storage medium, type of dentinÐhuman or bovineÐdentinal surface preparation, depth of dentin, presence of thermal cycling and ®lm thickness [1,2]. Another essential issue is the cross-head speed at which the specimen is loaded to fail [2]. * Corresponding author. Tel.: 155-19-430-5340; fax: 155-19-430-5218. E-mail address: [email protected] (L.A.F. Pimenta).

The ISO has recommended that in shear bond strength (SBS) tests, the load should be applied with a cross-head speed of within 0.45 and 1.05 mm/min (ISO-TR 11405) [4]. However, many studies have used a loading rate of 5.00 mm/min [5±9]. It is not known how this fact could be contributory to the variations observed in the shear test results. But, it was hypothesized that relatively high crosshead speeds may develop abnormal stress distributions during the shear test, inducing cohesive failures in the tooth substrate or in the resin-based composite (RBC), which would in¯uence the bond strength values achieved. The purpose of this study was to evaluate the in¯uence of different cross-head speeds on the SBS of a RBC/hydrophilic adhesive system to bovine dentin surfaces; and to classify the failure mode.

2. Material and methods One hundred and twenty bovine incisor teeth were used in this investigation. They were extracted and stored in 10%

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Fig. 1. Diagram of the experimental set-up. (A) Bovine tooth was cut with a diamond disk to obtain a piece from the central area of the facial crown. (B) Embedded tooth fragment after dentin exposure. (C) The dentin surface was demarcated by a piece of vinyl tape with a 3 mm diameter hole. (D) Specimen preparation device. (E) Careful removal of the Te¯on ring mold after specimen preparation. (F) Illustration of the shear test.

formaldehyde buffered solution for three weeks until their preparation; after which they were stored in a humid environment at 378C. Tissue remnants and debris were removed from the teeth. In each tooth the root and the marginal parts of the crown were cut-off with a double faced diamond disk (ref. 070, KG Sorensen, SP, Brazil) to obtain a piece from the central area of the facial crown (Fig. 1A), having 5 mm length and 5 mm width, which was placed in a 3/4 00 diameter PVC ring. The rings were ®lled with self-curing polystyrene resin (Cromex, Piracicaba, SP, Brazil) in which to embed the teeth. The embedded teeth were ground on a watercooled mechanical grinder (Maxigrind, Solotest, SaÄo Paulo, SP, Brazil) using 320, 400 and 600-grit Al2O3 abrasive paper (Carborundum Abrasivos, Recife, PE, Brazil) until a circular area of 4 mm diameter of dentin was exposed, to obtain plain dentin surfacesÐclose to the dentin±enamel junction (Fig. 1B). After polishing, the dentin surfaces were demarcated by placing a piece of vinyl tape, in which a 3 mm diameter hole had been punched (Fig. 1C). After that, acid etching was done with 35% phosphoric acid (3M Co., St. Paul, MN, USA) on the dentin surface. The etchant was rinsed for 15 s under running tap water, and the dentin was gently dried with compressed air for 5 s. Two consecutive coats of adhesive (Single Bond, 3M Co., St. Paul, MN, USA) were applied using a saturated brush tip. After gently air drying for 5 s, the material was light cured for 10 s. A 3 mm diameter Te¯on ring mold, 5 mm high, was placed against the specimen to receive a ®lling material (Z 100, 3M Co., St. Paul, MN, USA) (Fig. 1D). The RBC

was placed in two increments of 2.5 mm height; each one of them was light-cured (Optlux 500, Demetron, Kerr Corp., Danbury, CT, USA) for 40 s in the mold, and after removing the mold (Fig. 1E) for a further 20 s on each of the opposite sides of the RBC cylinder. The light intensity was measured periodically by a radiometer (Optlux 500, Demetron, Kerr Corp., Danbury, CT, USA), which ranged from 550 to 650 mW/cm 2. The specimens were stored in a humid environment at 378C for one week. The specimens were randomly assigned to four groups (n ˆ 30) to perform the SBS test. Each group represented a different cross-head speed. The specimen was positioned in a universal testing machine (DL 500, Emic Ltd., SaÄo Jose dos Pinhais, PR, Brazil) with the dentin surface parallel to the machine's line of travel. A steel knife-edge was placed over the specimen so that the shear force was directed along the bond surface (Fig. 1F). The specimens were loaded to fail at cross-head speeds of 0.50, 0.75, 1.00 and 5.00 mm/ min. SBS: of each specimen were noted in MPa. After that the data were subjected to one-way analysis of variance (ANOVA) and Tukey's multiple comparison test at 5% level of signi®cance (a ˆ 0.05). The fractured surfaces of the specimens were examined visually with a stereomicroscope (EMZ-TR, Meiji Techno Co. Ltd., Tokyo, Japan) at 25 £ magni®cation, by two independent evaluators to classify the type of failure that occurred during the de-bonding procedure. They could be: adhesive (cohesive in adhesive interface failure); cohesive in dentin (dental substrate failure); cohesive in RBC (RBC failure); or mixed (cohesive and adhesive failure). The

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Table 1 Shear bond strength testÐmeans and standard deviation (* ˆ statistical differences expressed by different letters (p,0.05)) Groups

Mean (SD)*

A (0.50 mm/min)

B (0.75 mm/min)

C (1.00 mm/min)

D (5.00 mm/min)

11.78 (3.91) a

11.82 (4.78) a

16.32 (6.45) b

15.46 (5.94) b

adhesive failure scores that were coincident among the evaluators were statistically analyzed by Student's t-test to percentages [10] at 5% level of signi®cance (a ˆ 0.05). 3. Results The means and the standard deviations obtained with the SBS test for each group evaluated are listed in Table 1. The one-way ANOVA showed signi®cant differences among groups (pvalue ˆ 0.0013), that were highlighted by Tukey's test (p , 0.05). No differences were observed between groups A (0.50 mm/min) and B (0.75 mm/min), neither between groups C (1.00 mm/min) and D (5.00 mm/min). The SBS values of groups C and D were statistically higher than groups A and B. The coincident data obtained from the examiners' evaluations were considered for the failure mode statistical analysis. (Table 2) The Student's t-test to percentages (a ˆ 0.05) for adhesive failures indicated that there were no signi®cant differences among groups A, B and C; groups A and B differed from group D, and there was no signi®cant difference between groups C and D. 4. Discussion There is a consensus that in vivo trials are imperative for evaluating the performance of dentin bonding agents in the oral environment [11±13]. However, due to the rapid development and introduction of these materials on the market, it has become necessary to ®nd simple and fast methods for evaluating their effectiveness, since clinical trials are timeconsuming [14] and too costly [15]. Therefore, in vitro bond strength tests have been done, the most popular being the shear method [2]. The SBS test is de®ned as a test in which two materials are connected via an adhesive agent and loaded in shear

until fracture occurs; the nominal bond strength is calculated by dividing the maximum applied force by the bonded cross-sectional area [16]. This test is relatively simple and easy to do, allowing rapid results to be obtained. However some critical aspects must be considered in using this in vitro method to predict the clinical performance of dentin bond adhesives. Firstly, in vitro information cannot be extrapolated directly to clinical situations as, together with other evaluations, it is important in predicting the performance of the materials tested. Secondly, the great variations in SBS test results have made the test questionable and, sometimes, futile [17]. An effort must therefore be made to standardize SBS test methods in order to improve the usefulness of this in vitro test. Some important aspects should be considered, such as storage conditions, type of substrateÐhuman or bovine tooth, tooth age, dentinal depthÐspecimen preparation, rate of load application, presence of thermal cycling, ®lm thickness and cross-sectional surface area [1,2]. The establishment of parameters for some of these issues was proposed in 1994 by an ISO standard norm [4], and in addition, studies have been made to evaluate each one of these. This study speci®cally tested the in¯uence of cross-head speeds on the SBS of RBC cylinders/adhesive system to a dentin surface. The ISO standards recommend that the rate of loading for a bonded specimen should be 0.75(^0.30) mm/ min [4]. Therefore, the reason for choosing the 0.50, 0.75 and 1.00 mm/min speeds was based on this ISO standard, while the choice of 5.00 mm/min was made because this loading rate is the most prevalent in works cited in the literature [4]. Higher bond strength values could be observed in this study in groups with 1.00 and 5.00 mm/min cross-head speeds, denoting statistical differences from the 0.50 and 0.75 mm/min groups. These differences may be explained as follows: when the load was applied to the base of a RBC cylinder at a relatively high cross-head speed, the generated stress deviated from the adhesive interface to other

Table 2 Percentages of coincident failure mode data as evaluated by two independent evaluators (* ˆ statistical differences expressed by different letters (p,0.05)) Groups (mm/min)

Coincident data

Failure mode Adhesive*

A (0.50) B (0.75) C (1.00) D (5.00)

27 24 20 17

a

25/27 (92.5%) 22/24 (91.6%) a 14/20 (70.0%) ab 8/17 (47.0%) b

Cohesive in dentin

Cohesive in RBC

Mixed

± ± ± ±

± 1/24 (4.2%) ± 5/17 (29.4%)

2/27 (7.5%) 1/24 (4.2%) 6/20 (30.0%) 4/17 (23.6%)

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components of the specimenÐRBC cylinder or dentin substrateÐallowing high bond strength values to be shown. This suggests that the SBS of the adhesive interface could not essentially be evaluated in groups at higher speeds, and the higher bond strength values obtained may be related to the higher cohesive strength of RBC and/or dentin substrate. Sano et al. [18] obtained an ultimate bovine dentin tensile strength of 90.6 (18.9) MPa. The increase in the SBS value with the increase in cross-head speed has already been demonstrated in another study [19]. Since the shear test was criticized [3,16] by its own biomechanics, it is essential that a failure mode analysis be done to evaluate if the fracture occurred at the adhesive interface [2]. Therefore, in this study, the percentages of adhesive failures were analyzed. The results showed that in the group at 5.00 mm/min only 47.0% of adhesive failures occurred. In the 1.00 mm/min group this percentage was 70.0%, and in the 0.75 and 0.50 mm/min groups these were 91.6 and 92.5%, respectively. No cohesive in dentin failure alone was found, but occurred together with cohesive in RBC failure when it was classi®ed as mixed. Although the correlation between the SBS values and failure patterns was not statistically evaluated, the data obtained suggest that the higher the cross-head speed, the higher the bond strength value and the lower the percentages of adhesive failures. This could mean, the higher the crosshead speed, the lower the sensitivity of this test in measuring the SBS of the adhesive interface between the RBC and dentin. Some authors have pointed out the increasing quality of adhesive materials as the reason for the occurrence of dental cohesive failures. This can be con®rmed by our previous study (unpublished data), in which we used the same materials and methods to evaluate the in¯uence of different cross-head speeds on a bovine enamel surface. No statistically signi®cant differences among groups were found by the SBS test nor by fracture analysis. Mixed failures were predominant in all of groups. Considering that the adhesion of RBC to enamel is accepted as good, it was concluded that the better the adhesion, the more sensitive would be the tests needed to measure it [20]. Thus, other kinds of adhesion tests should be considered. The micro-tensile test appears to be a good option, since adhesive failures are predominant in this kind of test [1]. In this context, the interfacial fracture toughness test should also be considered [20]. However, for bovine dentin substrate and for the use of a hydrophilic adhesive system, the SBS test at 0.50 or 0.75 mm/min, as was demonstrated in this study, was capable of adequately measuring the bond strength. Although the cross-head speed seems to interfere in the SBS, some studies using 1.00 or 5.00 mm/min, which evaluate the failure mode visually, also observed a predominance of adhesive failures, indicating the ef®cacy of the method employed in measuring the SBS. It is important to clarify that in evaluating the in¯uence of the results

achieved in the present study, compared to works reported in the literature, the variations must be related not only to the cross-head speed, but also to other method variables, as explained before. Therefore, care must be taken when comparing results of different studies with different methodologies. The differences among groups observed in this studyÐ including the shear values in MPa and the fracture patternsÐshowed that the standardization of the SBS test is of great importance and efforts should be made for doing this. However, as has been demonstrated by the literature, sometimes standardization among studies is dif®cult to achieve, since few speci®cations are available [4] and, moreover, they are not well accepted by researchers [2,15]. It can be concluded that in the SBS test the variation of cross-head speed may in¯uence the bond strength values and the fracture pattern obtained. Cross-head speeds of 0.50 and 0.75 mm/min result in more adhesive failures, and are therefore preferable in SBS tests.

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