Influence of ultrasound and diamond burs treatments on microtensile bond strength

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1 Original Research Influence of ultrasound and diamond burs treatments on microtensile bond strength Alexandre Conde, Vivian Mainieri, Eduardo Gonçalves Mota, Hugo Mitsuo Oshima Post-Graduate Program in Dentistry, Pontifical Catholic University of Rio Grande do Sul, Porto Alegre, Brazil Received : Review completed : Accepted : ABSTRACT Objective: To compare surface treatments with CVDentUS ultrasound tips (UT) and KGSorensen diamond burs (DB) on etched (e) and non-etched (n/e) dentin. The microtensile bond strength (μtbs) was measured and fractography was assessed by scanning electron microscope (SEM). Materials and Methods: Sixteen molars were divided into four groups of four teeth each according to treatment (DB-n/e; DB-e; UT-n/e; UT-e). The teeth were restored, sectioned into samples for μtbs (n=40) and tested on a EMIC DL-2000 universal machine (0.5 mm/min) and analyzed by SEM for fracture classification. Statistical Analysis: For analysis of the data on μtbs, the two-way ANOVA, using treatment and acid etching as fixed factor, and the Tukey test were used (α=0.05). To failures classification in cohesive in dentin (CD); cohesive in composite resin (CC); cohesive interfacial on base or top of hybrid layer (CBT); cohesive in adhesive (CA); mixed (M); interfacial on smear layer (S) the Fisher s exact test (α=0.05) was performed. Results: The mean values of μtbs (in MPa) in the different groups were as follows: UT-e: 45.31±8.16; DB-e: 34.04±9.29; UT-n/e: 15.17±3.71; and DB-n/e: 9.86±3.80. On analysis of the SEM micrographs, the DB-n/e group showed total obstruction of dentinal tubules; the UT-n/e group showed partial desobstruction of dentinal tubules and irregular surface; the DB-e group showed complete desobstruction of dentinal tubules; and the UT-e group showed complete desobstruction of dentinal tubules and irregular surface. Conclusion: The combination of ultrasound treatment and acid etching provides high values of μtbs. An association exists between CA/CC failures and the UT method, CBT failure and the DB method, CBT/CC failures and etching, S failure and non-etching. Key words: Dentin, diamond bur, microtensile bond strength, ultrasound Ultrasonic cavity preparation is not recent [1,2] and a few researchers [3 7] have currently resumed its concept. In restorative dentistry and pediatric dentistry ultrasonic cavity preparation is used for sub- and supra-gingival removal of decayed tissue; marginal repairs of esthetic restorations; removal of esthetic restorations, amalgam, provisory restorations, and cements of prostheses and temporary restorations. It is also useful in patients who do not tolerate anesthesia. [8] Address for correspondence: Dr Eduardo Mota eduardo.mota@pucrs.br 373 Quick Response Code: Access this article online Website: PMID: *** DOI: / The use of ultrasound diamond tips has been reported to increase roughness of the dentinal surface. [9,10] Although there has been some research on this type of preparation, the very small amount of published work is mainly related to bond strength of composite resin on dentinal walls using, for instance, microtensile bond strength test (μtbs). [11 13] The aim of this study was compare treatments with CVDentUS ultrasound tips and KGSorensen diamond burs at etched and non-etched dentin surfaces. In this study, the null hypothesis was that there is no significant difference between ultrasonic treatment and high-speed treatment with regard to μtbs and fractography (as assessed by scanning electron microscopy). MATERIALS AND METHODS This study had the approval of the Ethics Committee of the Pontifical Catholic University of Rio Grande do Sul (PUCRS) and the Ethics Committee of the School

2 of Dentistry. Sixteen extracted sound third molars were collected, cleaned, and stored in 4 C distilled water till use (within days). Each tooth was embedded in self-polymerizing acrylic resin (Jet Clássico, São Paulo, SP, Brazil) by means of a polytetrafluorethylene tube to obtain cylinders and facilitate subsequent sectioning procedures. The occlusal surface of the cylinder was flattened using a double-faced diamond disk (Buehler, Illinois, EUA) mounted in a Labcut 1010 sectioning machine (Extec Technologies, Enfield, CT, USA) at 500 rpm under constant water cooling to expose the occlusal middle dentin. The dentinal surface was ground using 600- and 1200-grit sandpaper with a Struers DPU-10 polishing machine (Panambra, São Paulo, SP, Brazil) to remove enamel completely, which was subsequently confirmed with a 30 stereoscopic magnifying glass (Olympus, Tokyo, Japan). The 16 cylinders with polished dentin were randomly divided into four groups of four samples each, according to the type of dentinal surface preparation [Figure 1]. For all groups, a positioning device was used, adapted to a microscope base to ensure a standard position to the Roll Air high-speed turbine (KaVo Do Brasil SA Ind. e Com., Joinville, SC, Brasil), and the Nac Plus Hz ultrasound (Adiel, Ind. e Com., Ribeirão Preto, SP, Brazil). The DB-n/e and DB-e groups were roughened with KGSorensen cylindrical diamond bur (model 1094; KGSorensen Ind. e Com. LTDA., Barueri, SP, Brazil), and the UT-n/e and UT-e groups were roughened using CVDentUS cylindrical ultrasound tip (model ; Clorovale, São José dos Campos, SP, Brazil), in both cases using abundant water irrigation. The DB-e and UT-e groups were etched using 37% orthophosphoric acid with 2% chlorhexidine (Acid Gel ; Dentalville do Brasil LTDA., Joinville, SC, Brazil) for 15 seconds, rinsed with Figure 1: Schematic view of group distribution. water stream for 30 seconds, and then dried using an oilfree air stream. Two layers of adhesive system (Single Bond 2 ; 3M ESPE, St. Paul, MN, USA) were applied, and light-cured for 10 seconds using an XL 3000 light-curing unit (3M ESPE, St. Paul, MN, USA) with a curing intensity of 400±20 mw/cm 2, which was checked with an analogical radiometer (Demetron ; Kerr Corporation, Orange, CA, USA). Composite resin (Filtek Z-250 ; 3M ESPE, St. Paul, MN, USA) was used at increments of approximately 2 mm to obtain a 6-mm high cylinder on the exposed dentinal surface only. Each increment was light-cured for 20 seconds using the light-curing unit described above. Subsequently, composite resin restorations were stabilized by a second embedment with self-polymerizing acrylic resin, using the aid of the polytetrafluorethylene tube to complete the 6 mm height. Microtensile test After storage in distilled water at 37 C for 24 hours, the embedded samples were sectioned longitudinally using a double-faced diamond disk mounted in a Labcut 1010 sectioning machine at 500 rpm under constant water cooling, so as to obtain 40 small stick-shaped specimens with 0.49±0.08 mm 2 adhesive area for each group (n=40). The exact adhesive area of each stick was measured with 0.01-mm precision using a digital caliper (Mitutoyo, Tokyo, Japan), and its ends were fixed with a cyanoacrylate-based glue (Super Bonder gel; Loctite, São Paulo, SP, Brazil) to a μtbs device, which was subjected to a tensile stress in an EMIC DL-2000 universal testing machine (EMIC, São José dos Pinhais, PR, Brazil). The microtensile force was applied parallel to the long axis of each sample. The test was run at a crosshead speed of 0.5 mm/minute until specimen failure occurred. Bond strength (in MPa) was obtained by dividing the ultimate bond strength (N) by the sectional area (mm 2 ). Hybrid layer analysis The hybrid layer analysis was performed on one nonfractured specimen from each group. Each one was examined at 1000 magnification using a scanning electron microscope (SEM) to assess tags (absence or presence) and adhesive/dentin contact form. Fractographic analysis All fractured specimens were examined at 200 magnification (increasing up to in case of doubt), using a SEM Phillips model XL30 (Philips Electron Optics B.V., Eindhoven, Holland). Those with 60% of fractured area were classified as interfacial failure (CBT = cohesive on base or top of hybrid layer; CA = cohesive in adhesive; S = in smear layer) or cohesive failure (CD = in dentin; CC = in composite resin). It was considered mixed failure (M) when no failure occurred over 60% or more of 374

3 fractured area. The visual method to examine failure areas used two software programs simultaneously (Microsoft Paint 5.1 was used to divide the SEM digital image in 4 lines and 5 columns, thus obtaining 20 small areas, each one representing 5% of the total area. Windows Picture and Fax Viewer were used to locate the fraction area and to identify and visually measure failure area percentage) [Figure 2]. Statistical analysis The μtbs data was first checked for normality with the Kolmogorov-Smirnov test. The two-way ANOVA and Tukey multiple comparison tests were used for μtbs analysis at a 95% confidence level, and Fisher s exact test was used for the fractographic analysis at the same confidence level. Figure 2: Example of applied method of fractography to classification of failure mode. RESULTS Two-way ANOVA revealed that the factors surface treatment (diamond bur or ultrasound tip) (F=60.66; P=.000) and acid etching (with or without) (F=651.64; P=.000) and their interaction (F=7.85; P=.006) had a significant effect on μtbs. Table 1 shows the results of Tukey multiple comparisons. a b Comparisons of dentin surface subjected to different treatments and the subsequent hybrid layer of each group are presented in Figures 3 and 4. Fractography results are summarized in Table 2. Fisher s exact test showed that there is an association between failure and treatment; CBT failure is associated with the diamond bur method, and CA and CM failures are associated with the ultrasound tips method (c 2 =18.08; P=.003). There is an association between failure and etching; CBT and CM failures are associated with the etching method, and S failure is associated with the non-etching method (c 2 =110.87; P=.000). DISCUSSION The null hypothesis of the study was rejected because the ultrasonic treatment, in comparison to diamond bur, produced higher values of μtbs (P<.05). One of the factors that may have contributed to these findings was the choice of the µtbs test. Among the simulation tests, µtbs has gained popularity with researchers, [14 17] because of its simplicity and rapidity. The advantages of the test are the possibility to determine high values of bond strength; little number of cohesive failures in dentin and, consequently, higher number of interfacial failures; possibility of multiple evaluations in one tooth; low dentin variation coefficient; capacity to measure regional bond strength; facility to study failure by SEM; the lack of need for a large number of teeth; and better distribution of forces during the test. [18 20] 375 c Figure 3: (a) DB-n/e - total obstruction of dentinal tubules; (b) DB-e - complete desobstruction of dentinal tubules; (c) UT-n/e - partial desobstruction of dentinal tubules and irregular surface appearance; (d) UT-e - complete desobstruction of dentinal tubules and irregular surface appearance. a c Figure 4: Examples of hybrid layer of each group: (a) DB-n/e, rectilinear hybrid layer occurred without penetration of tags; (b) DB-e, rectilinear hybrid layer occurred with penetration of tags; (c) UTn/e, curvilinear hybrid layer occurred with little penetration of tags; (d) UT-e, curvilinear hybrid layer occurred with penetration of tags. Bond strength of the adhesive to the dentin can be affected by the type of instrument used to prepare the tooth (#600 sandpaper, diamond burs, or carbide drill). [13] d b d

4 Table 1: Microtensile bond strength results and multiple comparisons Tukey test Group Mean (MPa) Minimum Maximum SD Variation coefficient UT-e (ultrasound tips DB-e (diamond burs UT-n/e (ultrasound tips and non-etched dentin) DB-n/e (diamond burs and non-etched dentin) a MPa % b MPa % c MPa % 9.86 d MPa % Different superscript letters indicate statistically significant differences among groups (P<.05). Table 2: Failure mode distribution after fractography Failure mode DB-e (diamond burs DB-n/e (diamond burs and non-etched dentin) UT-e (ultrasound tips and etched dentin) UT-n/e (ultrasound tips and non-etched dentin) CBT (cohesive interfacial on 26 (65.0%) - 15 (37.5%) - 41 (25.6%) base or top of hybrid layer) CA (cohesive in adhesive) (2.5%) 3 (7.5%) 4 (2.5%) CC (cohesive in composite 3 (7.5%) - 14 (35.0%) 2 (5.0%) 19 (11.9%) resin) CD (cohesive in dentin) 1 (2.5%) (0.6%) M (mixed) 10 (25.0%) 13 (32.5%) 10 (25.0%) 6 (15.0%) 39 (24.4%) S (interfacial on smear layer) - 27 (67.5%) - 29(72.5%) 56 (35.0%) Total Even different sizes of diamond into the burs can affect adhesion. Koase et al. [16] recorded a significant increasing in μtbs when superfine diamond burs were used compared to regular-grit burs. Meerbeek et al. [15] reported that the bond strengths of enamel and dentin treated by sonoabrasion and air abrasion were similar to that obtained by treatment with diamond burs, whereas Çehreli et al. [12] found that the bond strength of the dentinal adhesive is not influenced by different techniques of caries removal (their study examined four different preparation systems: conventional drill, chemo-mechanical removal, sonoabrasion, and air abrasion). The reason the present study showed significant differences in µtbs for all the groups (UT-e: MPa, DBe: MPa, UT-n/e: MPa, and DB-n/e: 9.86 MPa; P<.05) are obvious in the SEM study, which showed the removal standard of smear layer and the roughness pattern that increased surface area for bonding purposes promoted by the cutting instruments. Figures 3 and 4 show two distinct surfaces: flattened for diamond bur and irregular and rounded for ultrasound tip, which is concordant with previous published studies. [3,9,10,13,16] However, the findings of this study are not concordant with another earlier study [11] that did not record improvements in µtbs with ultrasound treatment. The usage of a self-etch bonding system might explain such difference from our findings. Due to the type of cleanness standard promoted by the ultrasound, in the group UT-n/e we opted not to use etching so that it would be possible to compare the type of result that would appear with this procedure. In group DB-n/e the option was the same in order to verify comparisons, even if this is not a clinical practice. Smear layer removal increases adhesive penetration in dentin, thus promoting better interlocks micromechanically. [21] It is believed that removal of the smear layer in the groups treated with ultrasonic tips increased bond strength, and certainly because the ultrasonic vibration promotes overpressure resulting from the implosion of the microbubbles of cavitation at water, in addition to other phenomena associated to this fact such as acoustic microcurrent, impact waves, temperature rise, chemical effects of oxidation/ reduction and emulsification. Ultrasonic removal of smear layer does not occur in the same way as the removal by etching, which is evidenced by the images of the substrate obtained in this study. Our findings support that of other researchers who have also shown that the ultrasonic treatment alone does not promote the total removal of smear layer. It is believed that both the dentinal cleanliness and the roughness influence the μtbs. In this study we noticed in SEM substrate images that the superficial roughness standard produced by the ultrasonic tip is different from the standard produced by the diamond bur. It is believed that the ultrasonic tip standard promotes a greater area of superficial contact than the standard promoted by the diamond bur and, consequently, a bigger interlocks micromechanically for the ultrasonic tip than the diamond bur. This factor certainly contributed to the increased μtbs values. SEM images of the nontested samples treated with ultrasonic tips showed that the roughness produced by this treatment caused uneven adhesive thickness, showing that in some points of a dentinal surface it is possible to have thicker adhesive layer. This fact enables the occurrence of the 376

5 elastic adhesive layer concept, [22] which is a thick adhesive layer capable of absorbing and dissipating the light-curing contractile force generated in the restoring process of composite resins, thus preventing failures in the adhesive interface, where adhesive nanoparticles are also involved in the concept of elastic adhesive layer, contributing for a thicker adhesive layer. This study was based on the fractography concept described previously; [14] however, the emergence of a failure pattern different from the one in the cited study originated the addition of another type of failure that had occurred in the smear layer due to the adhesive application on the substrate without etching. This failure was included as belonging to interfacial failure, thus appearing failure S. Within the limitations of this in vitro study, the rougher aspect recorded associated to smear layer removal pattern promoted by the ultrasonic tip can induce a higher clinical longevity. 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Oper Dent 2001;Suppl 6: How to cite this article: Conde A, Mainieri V, Mota EG, Oshima HM. Influence of ultrasound and diamond burs treatments on microtensile bond strength. Indian J Dent Res 2012;23: Source of Support: Nil, Conflict of Interest: None declared. 377