Effect of different enamel treatments on bond strength using resin dental adhesives

Transcription

1 Effect of different enamel treatments on bond strength using resin dental adhesives A.C. Armas-Vega, DDS, MS 1 /V.E. Arana-Chavez, DDS, MS, PhD 2 / D.A. Botter, MS, PhD 3 /N. Garone Netto, DDS, MS, PhD 1 / M.A.A.C. Luz, DDS, MS, PhD 1 Objective: The aim of this investigation was to evaluate the effect different cleaning techniques used on enamel surfaces have on the bond strength of a composite resin to the dental enamel. Method and Materials: Eighty-eight bovine enamel fragments were mounted in acrylic resin bases. The enamel surfaces were sanded using 200-, 400-, and 600-grit sandpapers, thus creating a smear layer. These surfaces were then randomly divided in 2 groups: 1 of them received salivary contact for 10 minutes, and the other received it for 60 minutes. Then all surfaces were cleaned with a pumice and water paste applied with a rubber cup, followed by the application of biological detergent, sodium bicarbonate jet, or air/water spray. After cleansing, the enamel surfaces received the application of the Scotchbond Multi-Purpose adhesive system and Z100 composite resin, following the manufacturer s instructions, using an appropriate matrix. After storage at 37 C for 8 days, traction tests were carried out using an Instron machine operating at 0.5 mm/minute. Some fractured specimens (randomly chosen) were analyzed under scanning electron microscopy. Results: The statistical analysis using descriptive and inferential methods did not show significant differences between the different periods of time of salivary contact. The technique of pumice and water paste cleaning followed by the application of biological detergent was significantly more efficient than the others. Scanning electron micrographs of the fragments after traction tests confirmed these results. Conclusion: The technique of pumice and water paste cleaning followed by the application of biological detergent was the treatment that allowed the best results in terms of resin bonding to bovine enamel covered with acquired pellicle, and the sodium bicarbonate jet technique presented the lowest bond strength values and seemed to disturb the acid conditioning of enamel surfaces. (Quintessence Int 2007;38:527.e ) Key words: dental bonding, dental cavity preparation, dental enamel bonding The resin union to demineralized dental structures has been tested since 1955 with Buonocore, 1 who described the application of 85% phosphoric acid as a method that promotes superficial demineralization. The creation of microscopic pores on enamel surfaces increases resin adhesion on this tissue. The enamel structure is represented by 1 Department of Operative Dentistry, School of Dentistry, University of São Paulo, São Paulo, SP, Brazil. 2 Department of Oral Histology, Institute of Biomedical Sciences, University of São Paulo, São Paulo, SP, Brazil. 3 Department of Statistics, Institute of Mathematics and Statistics, University of São Paulo, São Paulo, SP, Brazil. Correspondence: Dr Maria Aparecida Alves de Cerqueira Luz, Rua Duarte de Azevedo, 284, cj. 22, São Paulo, SP, Brazil. Fax: maacluz@usp.br hydroxyapatite crystals arranged in prismatic structures. 2 These prisms are organized in such a way that the acid conditioning produces interprismatic and prismatic dissolution, depending on the degree of demineralization and on the direction and position of enamel crystals in the surface submitted to acid conditioning. 4 The enamel acid conditioning results in a superficial demineralization, forming pores that constitute microretention areas, thus facilitating the penetration of resin components, to a depth of 20 to 30 µm. 3 Sometimes, however, the hydroxyapatite crystals are not arranged in prisms on the enamel surface but rather form a low-energy aprismatic zone. 5 Adhesion was defined by Gwinnett 6 as the force through which a substratum is placed in close contact VOLUME 38 NUMBER 6 JUNE 2007 e321

2 with another one, with molecular attraction occurring between both surfaces. However, Krejci et al 7 affirmed that adhesion of dental structures to resins is mainly micromechanical in nature. Adhesion on enamel can be harmed by its heterogeneous surface and its relative humidity, characteristics that make it a low superficial energy tissue. 8 Authors such as Luz et al, 9 while analyzing MOD cavity walls after cavity preparations with a diamond FG bur, found under scanning electron microscopy (SEM) analysis that a smear layer covers enamel as it does on dentin. The presence of dental debris left during cavity preparation associated with blood, saliva, and lubricant oils (smear layer), 10 in addition to the acquired pellicle and/or bacterial plaque, prevents contact of the enamel with the acid conditioner, limiting its effect and thus minimizing adhesive forces. This fact may cause poor marginal sealing and poor esthetic results. After dental eruption, diverse organic materials can be deposited on tooth surfaces, forming a salivary film 11 whose thickness varies with time. This fine film can be removed easily with pumice and water paste, 12 but it has been demonstrated that it adheres again immediately on new contact with saliva. The close contact between the restorative resin and the enamel surface, allowable by acid conditioning, improves adhesion forces, which can be damaged by any contamination. 13 For orthodontic bracket adhesion on enamel surfaces, as well as for pit and fissure sealant techniques, cleaning enamel with a pumice and water paste is important for the elimination of dental plaque and of the organic film that recovers enamel. 14,15 Sodium bicarbonate jets eliminate bacterial plaque, producing a clean surface, 16 and are recommended for bracket adhesion and for the application of pit and fissure sealants. Gwinnett 6 recommended an antibacterial substance such as chlorhexidine mixed with pumice to clean cavity preparations as a step prior to acid conditioning. According to Ewoldsen et al, 17 the Dakin/biological detergent association promotes smear layer modifications without exposure of the dentinal tubule apertures. The use of bovine teeth for in vitro experiments is an efficient way to substitute for human teeth because of their similar characteristics, such as their size and availability. 18 METHOD AND MATERIALS Enamel fragments from bovine teeth with mm of area were used. These fragments were enclosed in acrylic resin in order to leave the buccal surfaces exposed, which were planed using 200-, 400-, and 600-grit sandpaper in a Politriz machine (Ecomet 3). Afterward, the surfaces were examined with a loupe to ascertain the presence of enamel. The sample was divided into 2 groups (n = 44), and each fragment was placed in a plastic pot containing fresh human saliva obtained from a single donor through salivary stimulation by chewing a rubber device. The containers with the specimens were kept under agitation for 10 and 60 minutes. After salivary contact each group was subdivided into 4 groups (n = 11), and each of the 8 groups thus obtained received different treatments on their enamel surfaces. Groups 1 and 5, after 10 and 60 minutes of salivary contact, respectively, were washed with an air/water spray for 15 seconds, treated with a pumice and water paste for 20 seconds (P), then washed again for 15 seconds with an air/water spray and dried for 5 seconds. Groups 2 and 6, after 10 and 60 minutes of salivary contact, respectively, were washed with an air/water spray for 15 seconds and treated with a pumice and water paste for 20 seconds, washed for 15 seconds with an air/water spray, and gently dried for 5 seconds. They then received the application of a biologic detergent, lauryl sodium sulfate (Tergensol; Inodon), with cotton pellets under friction for 20 seconds (T) and were washed again with an air/water spray for 15 seconds and gently dried for 5 seconds. Groups 3 and 7, after 10 and 60 minutes of salivary contact, respectively, were washed for 15 seconds with an air/water spray and treated with a sodium bicarbonate jet (S), applied for 15 seconds at a distance of 10 mm from the surfaces (Profi II Ceramic; Dabi Atlante). They were washed again with an air/water spray for 15 seconds and gently dried for 5 seconds. Groups 4 and 8, after 10 and 60 minutes of salivary contact, respectively, were washed with an air/water spray (C) for 15 seconds and gently dried for 5 seconds. Following each treatment, the speci- e322 VOLUME 38 NUMBER 6 JUNE 2007

3 Table 1 Mean values (M) and standard deviation (SD) of the bond strength values of each group according to the cleaning techniques Group N M SD Table 2 Difference Confidence intervals for comparison between treatment effects (Tukey test) Interval µ T µ C [0.311; 8.643] µ T µ P [ 1.807; 6.525] µ T µ S [0.761; 9.093] µ S µ C [ 4.616; 3.716] µ S µ P [ 6.734; 1.598] µ P µ C [ 2.048; 6.284] Obs.: µ i = population mean of the breaking load of the sample following the i cleaning technique, where i = P, T, S e C. mens immediately received acid conditioning before the application of the primer and bond of the Scotchbond Multipurpose system (3M) following the manufacturer s instructions. For composite resin insertion, the specimens were adapted in a dismountable metallic table that holds in position a 3- mm-diameter conical Teflon matrix, and the Z100 (3M) composite resin insertion was then carried out through small increments, photocured for 40 seconds. The samples were kept in distilled water at 37 C for 7 to 10 days. After this period, traction tests were conducted using an Instron machine (Model 4442) set to operate at a 0.5 mm/minute speed. The parts obtained from the traction tests were identified and stored in distilled water at 37 C. Two pairs of fragments obtained from the specimen traction tests of each experimental group were randomly chosen and prepared for analysis under SEM (JEOL 6100; operating at 10 to 15 kv). The bond strength values were transformed into MPa and submitted to statistical tests, appropriated for the experiment. RESULTS Fig 1 Profiles of mean bond strength values for both times of salivary contamination. C: control groups; P: pumice and water paste group; S: sodium/bicarbonate jet group; T: pumice and water paste plus biologic detergent group. The descriptive analysis of the data and the profile analysis of mean values of bond strength (Table 1) showed that they were similar for all cleaning techniques applied for both salivary contact periods. The lowest mean value of bond strength was observed for treatment C and the highest one for treatments P and T, for both periods of salivary contact (Fig 1). The similar behavior of both periods was confirmed through the inferential statistical analysis applying 2 fixed crossed factors (period of contact and cleaning technique). A diagnostic analysis of the residues 19 demonstrated that all the suppositions enforced for the model were satisfied. Then a model of analysis of variance with a fixed factor (cleaning technique) revealed that bond strength values differed for at least 2 of the cleaning techniques (P =.010). The Tukey analysis was used to compare bond strength values, 2 by 2, through confidence intervals with a 0.95 global confidence coefficient, 19 and showed that the mean values of VOLUME 38 NUMBER 6 JUNE 2007 e323

4 Fig 2a Scanning electron micrography image of a specimen of group P showing an enamel surface covered with composite resin (CR) and some areas of adhesive resin (AR) after traction test (magnification 600). Fig 2b Scanning electron micrography image of a specimen of group P showing the surface of the composite resin fragment correspondent to Fig 2a specimen. Areas compatible with adhesive resin (AR) and some structures compatible with enamel fragments (arrows) may be observed (magnification 600). Fig 3a Scanning electron micrography image of a specimen of group T showing an enamel surface, after traction test, covered with composite resin (CR). Some small bubbles may be seen (arrows) and may indicate the presence of adhesive resin over this area (magnification 600). Fig 3b Scanning electron micrography image of a specimen of group T showing the surface of the composite resin fragment correspondent to Fig 3a specimen. Two different areas can be seen: a dark one at the upper side of the image suggesting adhesive resin (AR) and one below it (CR) in an area compatible with composite resin (magnification 600). bond strength were similar for treatments C, P, and S and that the bond strength value obtained with treatment T was higher than those obtained with treatments S and C (Table 2). The analysis of Scheffé through a confidence interval with a 0.95 global confidence coefficient was applied to identify the best cleaning technique, and it demonstrated that the mean value for treatment T was higher than the mean values for the other treatments. These results were confirmed by means of the agglomerative and hierarchic analysis, 20 applying a significance level of 10%, where 2 groups were considered: the first represented by treatment T and the second represented by treatments P, S, and C, confirming the highest performance of treatment T. Analysis of the scanning electron micrographs demonstrated that the surfaces of group P showed rough areas suggesting e324 VOLUME 38 NUMBER 6 JUNE 2007

5 Fig 4a Scanning electron micrography image of a specimen of group S showing enamel surface, after traction test, covered with composite resin (CR) and adhesive resin (AR) (magnification 600). Fig 4b Scanning electron micrography image of a specimen of group S showing the surface of the composite resin fragment correspondent to Fig 4a specimen. Structures (arrows) can be seen that are compatible with enamel fragments (magnification 600). Fig 5a Scanning electron micrography image of a specimen of group C showing enamel surface after bond strength test. Parallel bands compatible with sound enamel image (E) and areas suggesting composite resin (CR) can be observed (magnification 600). Fig 5b Scanning electron micrography image of a specimen of group C showing the surface of the composite resin fragment corresponding to Fig 5a specimen. Some small smooth and dark areas compatible with adhesive resin can be seen (magnification 600). Fig 5c Scanning electron micrography image of a specimen of group C showing the surface of the composite resin fragment corresponding to Fig 5a specimen. Some small smooth and dark areas compatible with adhesive resin can be seen (magnification 600). VOLUME 38 NUMBER 6 JUNE 2007 e325

6 composite resin images and dark and smooth areas compatible with adhesive resin images. On the correspondent resin fragment, some areas compatible with adhesive resin images were observed (Figs 2a and 2b). These images suggest a cohesive fracture in composite resin and adhesive resin. Scanning electron micrograph examination of the specimens that received treatment T demonstrated, on enamel fragments, images similar to adhesive resin bubbles covering the enamel surface. In the correspondent resin fragments, composite resin images were observed on all surfaces, as were small dark and smooth areas suggesting adhesive resin images (Figs 3a and 3b), indicating also a cohesive fracture in composite resin and in adhesive resin. The specimens of group S showed, on enamel fragments, areas suggesting conditioned enamel with penetration of adhesive resin and areas of a thicker adhesive layer that appear smooth and dark covering the enamel surface, like the other images; the remainder of the composite resin could also be seen. In the correspondent composite resin pair, areas compatible with adhesive resin and small enamel fragments were observed (Figs 4a and 4b). These images may indicate predominantly adhesive breakage, although irregular. Examination of the control group specimens revealed adhesive resin residues with bacterial plaque elements on the enamel surface and areas of sound enamel as well as the remainder of the composite resin. In the corresponding composite resin pair, a visibly uniform surface was observed, with few adhesive resin areas (Figs 5a, 5b, and 5c). These images also suggest predominantly adhesive breakage, although irregular. DISCUSSION Cavity walls must be cleaned to finish cavity preparation well, as any kind of exogenous component may interfere with the contact of restorative material and dental tissues. 2 This care can improve the performance of the adhesive restorations, as it facilitates the mechanical interlocking between adhesive resin and enamel. 17 Since Buonocore, 1 studies have been conducted on the action of acid conditioners on dental surfaces and their influence on the adhesion process. It has been observed that acid conditioning creates enamel porosity forming salience and re-entrance, which constitute microretention points, increasing the surface area that will interlock with the resin. 15 This irregularity on the enamel surface enhances the retention mechanism, allowing the resin molecules to penetrate the enamel surface. To increase the effectiveness of the acid conditioning, it becomes necessary to expose the internal enamel layer, 2 thus justifying the bevel that regularizes the enamel surface besides eliminating the external low-energy enamel surface, characterized by a nonprismatic enamel zone. 7 Dental surface cleaning, using biocompatible agents, 2,6 increases the superficial energy of dental tissues. The acquired pellicle is an acid-resistant layer that minimizes the superficial energy, making the decalcification process difficult 6,21 and acting as a mechanical barrier of selective permeability. The salivary contact period of 10 minutes to simulate salivary contact during cavity preparation and the salivary contact period of 60 minutes are necessary to form a thick acquired pellicle that remains unchanged after this. 22,23 This fact justifies the cleaning of surface enamel prior to acid conditioning, as the results of this study indicate, based mainly on the SEM images of the control group. The periods of time of salivary contact used in this study (10 and 60 minutes) possibly allowed a thickness of acquired pellicle that did not differ between them. The superficial enamel energy can also be affected by the presence of the smear layer that may contain blood, motor oil lubricant, residues of restorative materials, fluoride ions, and bacteria from the cavity preparation or from the oral environment. 12 Cleaning with a pumice and water paste is considered to be necessary as the last step in cavity preparation or the first one in the restorative process. In addition to the elimination of residues, cavity cleaning may become the smear layer thinner, thus facilitating acid conditioning on the underlying enamel. It is known that the pumice and water paste using a rubber cup e326 VOLUME 38 NUMBER 6 JUNE 2007

7 at a low speed, before acid conditioning, constitutes an adequate cleaning technique that eliminates bacterial plaque and minimizes the smear layer without totally eliminating it. 8 This procedure may thus facilitate acid conditioning of enamel surfaces, increasing adhesive resin penetration, as was demonstrated in this study through the results of groups P and T. The detergent solution, composed mainly of lauryl sodium sulfate, facilitates lubricant oil removal and removal of the pumice paste residue. Thus, the detergent application after pumice and water paste can improve the cleaning action of the pumice and water paste, as demonstrated by specimens of group T in this study, which presented statistically higher values of bond strength than the other groups. The SEM images of group T suggested cohesive and not adhesive breakage, thus demonstrating that this cleaning technique makes the enamel surface receptive to adhesive resin. The sodium bicarbonate jet, in turn, is a very common technique in dental prophylaxis for bacterial film elimination. 16 However, in this study, the sodium bicarbonate/air/water jet cleaning technique did not provide good results in bond strength tests, as the enamel surfaces treated with this cleaning process presented lower bond strength values than the control group. Despite its efficiency in cleaning pits and fissures, 16 because of its high degree of penetration in these areas, the sodium bicarbonate jet has drawbacks, as its residue or the ph surface alterations caused by it may be responsible for harming phosphoric acid action and for its poor performance on smooth surfaces. For these reasons, the phosphoric acid did not produce sufficient demineralization on the surface enamel treated with a sodium bicarbonate jet, as suggested by the SEM images of group S. The sodium bicarbonate jet causes poor resin and enamel interlocking, resulting predominantly in adhesive breakage in the bond strength tests, and this was also suggested by the SEM images. The statistical results of the control group were also confirmed by its images since they show mainly adhesive breakage, besides images suggesting dental plaque and sound enamel, with few areas suggesting composite resin adhesion. CONCLUSIONS Under the conditions of this investigation and as observed by SEM images, the cleaning technique using biologic detergent following the application of a pumice and water paste was the treatment that allowed the best results in terms of resin bonding to bovine enamel. The sodium bicarbonate jet technique presented the lowest bond strength values and seemed to disturb the acid conditioning of enamel surfaces. The salivary contamination periods of 10 and 60 minutes were statistically similar regarding acquired pellicle formation, as they did not differ in bond strength. REFERENCES 1. Buonocore MG. A simple method of increasing the adhesion of acrylic filling materials to enamel surfaces. J Dent Res 1955;34: Orban BJ, Bhaskar SN. Orban s Oral Histology and Embriology, ed 8. St Louis: Mosby, Silverstone LM, Saxton CA, Dogon JB, Fejerskov O. Variation in the pattern of acid etching of human dental enamel examined by scanning electron microscopy. Caries Res 1975;9: Anderson P, Elliott JC. Rates of mineral loss in human enamel during in vitro demineralization perpendicular and parallel to the natural surface. Caries Res 2000;34: Ten Cate AR. Oral Histology: Development, Structure, and Function, ed. 5. St Louis: Mosby, Gwinnett AJ. Smear layer: morphological considerations. Oper Dent 1984;3(suppl): Kreijci I, Schupbach P, Balmelli F, Lutz F. The ultrastructure of a compomer adhesive interface in enamel and dentin, and its marginal adaptation under dentinal fluid as compared to that of a composite. Dent Mater 1999;15: Gwinnett AJ. Structure and composition of enamel. Oper Dent 1992;5(suppl): Luz MAAC, Garone Netto N, Arana-Chavez VE, Sobral MAP, Singer JM.Chemical and/or machanical treatmants of smear layer as revealed by scanning electron microscopy: a blind comparative study. Pesq Odontol Bras 2000;14: McInnes-Ledoux P, Austin JC, Cleaton-Jones PE. Effect of citric acid pretreatment on dentinal cavity walls. J Prosthet Dent 1984;52: Sanz M, Newman MG. Dental plaque and calculus. In: Nisengard RJ (ed). Oral Microbiology and Imunology, ed 2. Philadelphia: Saunders, 1994: VOLUME 38 NUMBER 6 JUNE 2007 e327

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