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Prosthodontics, São Paulo State University at São José dos Campos, Brazil. b PhD Student, Department of Dental Materials and Prosthodontics, São Paulo.
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pyrig No Co t fo rP ub The Influence of Cutting Speed and Cutting Initiation lication te ss e n c e Location in Specimen Preparation for the Microtensile

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Bond Strength Test Celina Wanderley Abreua/Jarbas F. F. Santosb/Sheila Pestana Passosb/Silvia Masae A. Michidab/ Fernando Eidi Takahashic /Marco Antonio Bottinod

Purpose: This study evaluated the effect of cutting initiation location and cutting speed on the bond strength between resin cement and feldspathic ceramic. Materials and Methods: Thirty-six blocks (6.4 x 6.4 x 4.8 mm) of ceramic (Vita VM7) were produced. The ceramic surfaces were etched with 10% hydrofluoric acid gel for 60 s and then silanized. Each ceramic block was placed in a silicon mold with the treated surface exposed. A resin cement (Variolink II) was injected into the mold over the treated surface and polymerized. The resin cement-ceramic blocks were divided into two groups according to experimental conditions: a) cutting initiation location – resin cement, ceramic and interface; and b) cutting speed – 10,000, 15,000, and 20,000 rpm. The blocks were sectioned to achieve non-trimmed bar specimens. The microtensile test was performed in a universal testing machine (1 mm/min). The failure modes were examined using an optical light microscope and SEM. Bond strength results were analyzed using one-way ANOVA and Tukey’s test (α = 0.05). Results: Significant influences of cutting speed and initiation location on bond strength (p < 0.05) were observed. The highest mean was achieved for specimens cut at 15,000 rpm at the interface (15.12 ± 5.36 MPa). The lowest means were obtained for specimens cut at the highest cutting speed in resin cement (8.50 ± 3.27 MPa), and cut at the lowest cutting speed in ceramic (8.60 ± 2.65MPa). All groups showed mainly mixed failure (75% to 100%). Conclusion: The cutting speed and initiation location are important factors that should be considered during specimen preparation for microtensile bond strength testing, as both may influence the bond strength results. Keywords: microtensile bond strength test, cutting speed, ceramic. J Adhes Dent 2011; 13: 221–226 doi: 10.3290/j.jad.a21540

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he methodology applied for evaluating the bond between different substrates is one of the basic aspects of guaranteeing the validity of research. Various factors influence bond strength, such as cutting procedure of the specimens,13,16 the structural heterogeneity of the sub-

a

Graduate Student in Prosthodontics, Department of Dental Materials and Prosthodontics, São Paulo State University at São José dos Campos, Brazil.

b

PhD Student, Department of Dental Materials and Prosthodontics, São Paulo State University at São José dos Campos, Brazil.

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Associate Professor, Department of Dental Materials and Prosthodontics, São Paulo State University at São José dos Campos, Brazil.

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Professor and Chair, Department of Dental Materials and Prosthodontics, São Paulo State University at São José dos Campos, Brazil.

Correspondence: Sheila Pestana Passos, São Paulo State University, São José dos Campos, Avenida Engenheiro Francisco José Longo, 777, São Dimas. Postal code: 12245000, São José dos Campos, Brazil. Tel: +55-12-3947-9060, Fax: +55-12-3947-9010. e-mail: [email protected]

Vol 13, No 3, 2011

Submitted for publication: 16.04.08; accepted for publication: 16.11.09.

stratum, material properties, adhesive system used, and variations among operators.20 Many studies have stated that an advantage of microtensile bond strength (μTBS) testing is that it allows the use of innumerable small structure specimens,13,18,24 facilitating real evaluation of the adhesion between different substrates. Additionally, it allows the evaluation of small areas (approximately 1 mm2) of the same adhesive surface. There are a number of internal defects in a normal adhesive zone or superficial imperfections3,5,15 that could mask the actual bond strength values. Stick-shaped samples have been used for μTBS testing in several studies,4,7,17,27 in which the authors verified the versatility, reliability, and efficacy of this shape in relation to the dumbbell or hourglass shapes. Trimming the specimens into hourglass or dumbbell shapes can promote tensions in these areas.13,20 In a review of the literature, Pashley et al13 described the μTBS test and its modifications. The authors concluded that the versatility of this test cannot be achieved by other conventional methods, supporting the use of μTBS testing to 221

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Main compositions, manufacturers, and batch numbers of the products used in the current study are presented in Table 1. Thirty-six blocks (6.4 x 6.4 x 4.8 mm) of feldspathic ceramic (VITA VM7, Dentin 2M2, Vita Zanhfabrik; Bad Säckingen, Germany, batch #22920) were produced according to the manufacturer’s instructions. The cementation surface of each ceramic block was polished in a machine using silicon carbide papers in sequence (600, 800, and 1200 grit; 3M; St Paul, MN, USA) under water cooling. Impressions were made from each ceramic block with addition silicon putty (Elite HD, Zhermach; Badia Polesine, Italy, batch #18443). The block was pushed inside the silicon to achieve a 3 mm distance between the upper portion of the mold and the surface of the block. Thereafter, the cement was injected into this space. The specimen preparation has been previously described in detail elsewhere.21 Prior to surface conditioning, all blocks were ultrasonically cleaned (Vitasonic, Vita Zanhfabrik) for 5 min using distilled water. The ceramic surfaces were etched with 10% hydrofluoric acid (HF) acid gel (Dentsply Petropolis, Brazil; batch #L595588) for 60 s, rinsed with air-water spray for 60 s, and air dried. The ceramics were cleaned ultrasonically in distilled water for 5 min. Then silane coupling agent was applied in one layer (Monobond S, Ivoclar Vivadent; Schaan, Liechtenstein, batch #H24764) with a clean brush and allowed to sit for 5 min to ensure completion of the setting reaction. Each ceramic block was then placed in its silicon mold with the treated surface exposed. The dual-curing resin cement (Variolink II, Ivoclar Vivadent; base, batch #G26358; catalyst, batch #G28041) was mixed following the manufacturer’s instructions and injected into the mold onto the treated surface of the ceramic block, using a centrix syringe

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verify the bond strength of restorative materials. In addition, the evaluation of the bond strength between two materials must be followed by failure mode analysis of the specimens, which helps avoid inadequate interpretations of the adhesive behavior and inaccurate conclusions. Therefore, the μTBS test is considered a reliable method for the evaluation of the bond between adhesive systems and hard dental tissue, mainly to dentin, due to the mechanical aspects of tension distribution during the test.25 Reis et al16 examined the influence of storage time and cutting speed during specimen preparation on bond strength values of a single-bottle adhesive system to dentin using the μTBS test. The results showed that storage time and cutting speed affected the μTBS values; thus, these variables should be standardized in μTBS tests. The aims of the current study were therefore to examine: a) the influence of the cutting speed and cutting initiation location (ceramic, interface, or resin cement) on bond strength between a ceramic and a resin cement and, b) this influence on the standard failure of specimens. The hypotheses to be tested were that 1. the cutting speed can affect the μTBS results and 2. cutting initiation location in specimen preparation does not influence the μTBS test results.

pyrig No Co tf (DFL; Rio de Janeiro, Brazil). The cement in o the r Pmold was ub light photoactivated (XL 3000, 3M ESPE; St Paul, MN, USA; lica output: 500 mW/cm2) for 40 s on each side of the speciti men. The intensity of the light was verified to be tegreater than on ss eKerr; 500 mW/cm2 using a radiometer (Demetron LC, n c eOr-

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ange, CA, USA) prior to starting the resin polymerization in each group. After 10 min, the ceramic block/resin cement assembly was removed from the mold and the cement was again light polymerized on the five faces of the block (upper and lateral) for 40 s per side.6 Specimen Preparation for the Microtensile Bond Strength Test (μTBS) Ceramic-cement blocks were sectioned using a diamond disk (Microdont; São Paulo, Brazil, no. 34570) in a lowspeed handpiece mounted in a sectioning machine (Kavo; São Paulo, Brazil) under water cooling. The cutting speeds used were (N = 12): a) 10,000 rpm, b) 15,000 rpm, and c) 20,000 rpm. For each group, the cutting procedure was started at the same location: resin cement, ceramic, or the interface between the two materials. For each group, a new diamond disk was used. Initially, the cemented blocks were fixed with cyanoacrylate adhesive gel (Super Bonder Gel, Loctite; São Paulo, Brazil) on a metallic base that was attached to the sectioning machine. The blocks were positioned as perpendicularly as possible in relation to the diamond disk of the machine. The first section, measuring approximately 1 mm, was discarded due to the possibility of excess or absence of cement at the interface that might alter the results. Thereafter, three sections, measuring 1.0 ± 0.1 mm in thickness, were prepared. Each section was rotated 90 degrees and again affixed to the metallic base. The first section was discarded (1 ± 0.1 mm) as explained earlier. Subsequently, four other sections were prepared, also measuring 1.0 ± 0.1 mm in thickness. This process was followed for the other two sections, and thus only the central specimens were used in the experiment.6 Approximately 10 specimens were obtained from each block. The beam specimens had non-machined (nontrimmed) bonding areas with a bonded area measuring approximately 1.0 ± 0.1 mm2 and a length of 8 mm. Thus, 9 groups were obtained (n = 40), considering the “cutting speed” (3 levels) and “cutting initiation location” (3 levels) (Table 2). Microtensile Bond Strength Test Each specimen was fixed with cyanoacrylate gel (Super Bonder Gel, Loctite), keeping the adhesive zone free of the rods of the device adapted for this test. The specimens were positioned parallel to the long axis of the device in order to reduce the bending stresses. The device was fixed in the universal testing machine (EMIC DL-1000, EMIC; São José dos Pinhais, Brazil), as parallel as possible in relation to the application of the tensile load, and the testing was performed at a crosshead speed of 1 mm/min.6 The bond strength was calculated according to the formula R=F/A, where “R” is the strength (MPa), “F” is the load required for rupture of the specimen (N) and “A” is the interface area of the specimen (mm2), measured with a digital caliper before the test. The Journal of Adhesive Dentistry

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Na: 2.4%; Ca: 0.7%; C: 25.7%. O: 42.2%

Bad Säckingen, Germany

Hydrofluoric acid 10%

Hydrofluoric acid, water, thickener, stain

Dentsply; Petrópolis, RJ, Brazil

L595588

Monobond-S

Ethanol, water, silane, acetic acid

Ivoclar Vivadent; Schaan, Liechtenstein

H24764

Variolink II

Base: bis-GMA, UDMA, TEG-DMA, inorganic filler, ytterbium trifluoride, initiator, stabilizer. Inorganic fillers: 46.7% vol Catalyst: bis-GMA, UDMA, TEG-DMA, inorganic filler, ytterbium trifluoride, benzoyl peroxide, stabilizer. Inorganic fillers: 43.6% vol

Ivoclar Vivadent

Base: G26358 Catalyst: G28041

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opyretigal No CAbreu t fo Table 1 Main components, manufacturers, batch numbers of the products used rP ub lica Brand Name Main Composition Manufacturer Batch number tio n te e s s c en VITA VM7 Si: 19.6%; Al: 4.9%; K: 4.0%; Vita Zahnfabrik; 22920

Bis-GMA = bis-phenol-A-glycidylmethacrylate, UDMA = urethane dimethacrylate, TEG-DMA = triethylene glycol methacrylate, HEMA = 2-hydroxyethyl methacrylate.

Table 2 Experimental groups according to cutting initiation location and the cutting speed Cutting initiation location Cutting speed (rpm)

Groups (n = 40)

Resin cement Ceramic Interface Resin cement Ceramic Interface Resin cement Ceramic Interface

1 2 3 4 5 6 7 8 9

10,000

15,000

20,000

Failure Mode Analysis All specimens (360) submitted to the microtensile test were analyzed by optical light microscope (MP 320, Carl Zeiss; Jena, Germany) at 100X to 5000X magnification, and some specimens were selected for analysis under scanning electron microscopy (SEM) (JEOL JSM T330A; Tokyo, Japan) at 75X and 1000X magnification for observation of the failure type. Failures were classified as follows: adhesive between ceramic and cement (ADHES); cohesive failure of the cement (COHES-cem); cohesive failure of ceramic (COHES-cer); cohesive failure of cement and ceramic (MIX). Statistical Analysis The means of each group were analyzed by one-way ANOVA, with the cutting initiation and the cutting speed as Vol 13, No 3, 2011

the independent factors (Statistix 8.0 for Windows, Analytical Software; Tallahassee, FL, USA). P values less than 0.05 were considered to be statistically significant in all groups. Multiple comparisons were made by Tukey’s adjustment test. Furthermore, one-way ANOVA was used to determine the significant differences between methods. The distribution of the failure types was analyzed using the chi-square test. The beam was used as the experimental unit.

RESULTS The mean and standard deviation of the μTBS values measured for all of the tested groups are given in Table 3. Table 4 shows the number of premature failures in each group. 223

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pyrig No Co t Table 3 Mean values and standard deviations of the μTBS results (MPa) per group for Pu bli cat Cutting initiation location Cutting speed (rpm) ion te 10,000 15,000 20,000 ss e n c e n

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Interface Resin cement Ceramic

13.96 ± 4.53a,A 12.09 ± 4.15ab,AB 8.60 ± 2.65b,A

15.12 ± 5.36a,A 14.27 ± 3.88a,B 10.90 ± 5.66a,A

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9.83 ± 3.31a,A 8.50 ± 3.27a,A 9.99 ± 1.83a,A

*Mean values followed by different small letters in the columns and capital letters in the rows differ statistically among themselves (Tukey’s test, p < 0.05).

Table 4 Number of premature failures per group Cutting initiation location Interface Resin cement Ceramic

Cutting speed (rpm) – Premature failures 10,000 15,000 20,000 6 18 12

The statistical analysis revealed that the cutting initiation location had a significant influence on bond strength only at 10,000 rpm (p < 0.05), and the cutting speed had a significant influence on bond strength only when cutting was initiated in the resin cement (p < 0.05). The highest mean bond strength was obtained for specimens cut at 15,000 at the interface. The lowest means bond strength were achieved for specimens cut at the highest cutting speed in resin cement and cut at the lowest cutting speed in ceramic. Failure analysis demonstrated that all groups showed mainly MIX failures between the ceramic and the resin cement (75% to 100%) (Fig 1). The group in which 10,000 rpm was used presented the highest incidence of cohesive failure in resin cement (COHES-cem) in comparison to the 15,000 and 20,000 rpm groups. The same group (10,000 rpm) did not present cohesive failures in ceramic (COHEScer) while adhesive failures were not found in the 20,000 rpm group (Table 5). SEM micrographs representing the failure types of the debonded specimens are presented in Figs 1a and 1b.

DISCUSSION According to the results obtained, the first hypothesis was accepted and the second hypothesis rejected. Standardization of experimental conditions and bond strength testing of samples12,25 are discussed in the literature, as standardization is important in the interpretation and comparison of results from μTBS testing. It was determined that the cutting procedure during sample preparation may cause differences in the bond strengths, an observation also made in the studies by Pashley et al13 and Reis et al.16 224

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During the cutting process, friction occurs between the disk and the substrate, generating vibration and heat, even under water cooling. It is possible that these phenomena can induce large mechanical stresses in the involved structures,1,11 and also promote premature fractures. In μTBS testing, the stress induced in obtaining the specimen can lead to the propagation of cracks and fracture, masking the real resistance values of the substrate bonding to the dental structure.17 When comparing the number of failures during the cutting procedure of specimens at different speeds and in different places of cutting initiation (ceramic, cement resin, and interface), the greater occurrence of fractures were found in those cut at 20,000 rpm with the cut initiation in ceramic. This indicates that greater cutting speed, despite reducing the contact time of the disk with specimens, provides greater friction. The occurrence of premature fractures has caused controversy in the literature regarding the delimitation of the sample group for statistical analysis. In this study, samples that failed prematurely were not included in the statistical test, as in studies by Armstrong et al,2 Guzman-Armstrong,8 Uno et al,23 and Xie,26 while other researchers included the failed bonding specimens and assigned them bond strengths of zero.14,22 The cutting speed standardization is difficult to define; however, some authors reported that the specimens should be cut with low speeds.2,9,10,20,28 The highest speeds decrease the contact time of the disk with specimens, but provide more friction.17 In the study by Sadek et al,17 different cutting speeds were shown to influence the values of bond strength when enamel and dentin were compared. However, there was only a significant interaction for the enamel specimens, in which the lower speeds provided a greater The Journal of Adhesive Dentistry

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Fig 1a Representative micrograph of the surface of a debonded specimen: cohesive failure of cement and ceramic (MIX) failure of a specimen from group 1 (75X) (C = ceramic, RC = resin cement).

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Fig 1b Representative micrograph of the surface of a debonded specimen: cohesive failure of cement and ceramic in the same specimen at 1000X (C = ceramic, RC = resin cement).

Table 5 Number of tested specimens per group and percentage of failure types after the microtensile test Group

1 2 3 4 5 6 7 8 9 Total

Total no. (%) of specimens

ADHES

Failure type COHES-cer

COHES-cem

MIX

40 (100%) 40 (100%) 40 (100%) 40 (100%) 40 (100%) 40 (100%) 40 (100%) 40 (100%) 40 (100%) 360 (100%)

10 (25%) 0 (0%) 0 (0%) 4 (10%) 10 (25%) 0 (0%) 0 (0%) 0 (0%) 0 (0%) 24 (6.7%)

0 (0%) 0 (0%) 0 (0%) 2 (5%) 0 (0%) 0 (0%) 4 (5%) 0 (0%) 4 (5%) 10 (2.7%)

0 (0%) 8 (20%) 8 (20%) 0 (0%) 0 (0%) 8 (20%) 0 (0%) 0 (0%) 0 (0%) 24 (6.7%)

30 (75%) 32 (80%) 32 (80%) 34 (85%) 30 (75%) 32 (80%) 36 (95%) 40 (100%) 36 (95%) 302 (83.9%)

Failure between ceramic and cement (ADHES); cohesive failure of cement and ceramic (MIX); cohesive failure of the cement (COHES-cem).

mean bond strength. The current study did not confirm this. In this study, the strongest bond strength was associated with the intermediate cutting speed (15,000 rpm). According to Sadek et al,17 the higher cutting speed (20,000 rpm) can generate more structural defects, resulting in lower bond strength values and premature failures (Table 4). Those authors observed this only for enamel. According to Reis et al,16 increasing the cutting speed probably reduces the magnitude of the disk’s oscillatory movements. In relation to the cut initiation location, higher speeds (15,000 rpm and 20,000 rpm) showed no difference between the interface, resin cement, and ceramic, as can be seen in Table 3. For the lowest speed (10,000 rpm), cutting initiation in ceramic yielded lower bond strength values. The premature debonding of specimens occurred when greater cutting speed was used for all groups, and when the initiation of the cut was in the ceramic, for all speeds studied (Table 4). The effect of the materials’ different moduli of elasticity on Vol 13, No 3, 2011

stress distribution at the interface could be an important factor. The modulus of elasticity of the ceramic is higher than that of the resin cement. Thus, the resin cement can absorb more stress, and this may explain why initiating cutting from this face could result in greater loss of samples. The interface seems to be the best option for initiating the cut. The analysis of the debonded surfaces under optical light microscope and SEM demonstrated mainly a mixed type of failure between the ceramic and resin cement. The μTBS test promotes a greater incidence of adhesive or mixed failures than conventional bond strength testing, which produces a greater incidence of cohesive failures.4,18-20 While several factors inherent in specimen preparation for testing bond strength on the various substrates have been reported,13,16,20 the variables involved in the manufacture of the specimens for mechanical tests should be continuously monitored to better suit the methodology, thus providing more reliable results. 225

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1. Arikawa H. Dynamic shear modulus in torsion of human dentin and enamel. Dent Mater J 1989;8:223-235. 2. Armstrong SR, Vargas MA, Fang Q, Lafoon JE. Microtensile bond strength of a total-etch 3-step, total etch 2-step, self etch 2-step, and a self-etch 2step, and a self-etch 1-step dentin bonding system through 15-month water storage. J Adhes Dent 2003;5:47-56. 3. Cardoso PEC, Braga RR, Carrilho MRO. Evaluation of micro-tensile, shear and tensile tests determining the bond strength of three adhesive systems. Dent Mater 1998;14:394-398. 4. Dündar M, Özcan M, Gökçe B, Cömleko l u , Leite F, Valandro LF. Comparison of two bond strength testing methodologies for bilayered all-ceramics. Dent Mater 2007;23:630-636. 5. El Zohairy AA, de Gee AJ, de Jager N, van Ruijven, Feilzer AJ. The influence of specimen attachment and dimension on microtensile strength. J Dent Res 2004;83:420-424. 6. Foxton RM, Pereira PNR, Nakajima M, Tagami J, Miura H. Durability of the dual-cure resin cement / ceramic bond with different curing strategies. J Adhes Dent 2002;4:49-59. 7. Goracci C, Sadek FT, Monticelli F, Cardoso PEC, Ferrari M. Influence of substrate, shape, and thickness on microtensile specimens’ structural integrity and their measured bond strengths. Dent Mater 2004;20:643-654. 8. Guzman-Armstrong S, Armstrong SR, Qian F. Relationship between nanoleakage and microtensile bond strength at the resin-dentin interface. Oper Dent 2003;28:60-66. 9. Inoue S, Vargas MA, Abe Y, Yoshida Y, Lambrechts P, Vanherle G, Sano H, Van Meerbeek B. Microtensile bond strength of eleven contemporary adhesives to dentin. J Adhes Dent 2001;3:237-245. 10. Inoue S, Vargas MA, Abe Y, Yoshida Y, Lambrechts P, Vanherle G, Sano H, Van Meerbeek B. Microtensile bond strength of eleven contemporary adhesives to enamel. Am J Dent 2003;16:329-334. 11. Mahoney E, Holt A, Swain M, Kilpatrick N. The hardness and modulus of elasticity of primary molar teeth: an ultra-micro-indentation study. J Dent 2000;28:589-594.

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Cutting speed and initiation location influence the bond strength during specimen preparation for μTBS testing. Cutting initiation in the ceramic and resin cement are not recommended, whereas cutting initiation at the interface showed the best results. Cutting speeds of 10,000 and 15,000 rpm are adequate for specimen production.

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CONCLUSION

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