Stability of wet versus dry bonding with different solvent-based adhesives

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1 dental materials 24 (2008) available at journal homepage: Stability of wet versus dry bonding with different solvent-based adhesives Adriana P. Manso a, Luiz Marquezini Jr. a, Safira M.A. Silva a, David H. Pashley b, Franklyn R. Tay c, Ricardo M. Carvalho d, a Department of Operative Dentistry, Endodontics and Dental Materials, Bauru School of Dentistry, University of São Paulo, Bauru, Brazil b Department of Oral Biology and Maxillofacial Pathology, School of Dentistry, Medical College of Georgia, Augusta, GA, USA c Department of Conservative Dentistry, Faculty of Dentistry, University of Hong Kong, Hong Kong, SAR, China d Department of Prosthodontics, Integrated Center for Research CIP-I, Bauru School of Dentistry, University of São Paulo, Bauru, Brazil article info abstract Article history: Received 9 August 2006 Accepted 3 April 2007 Keywords: Adhesive Dentin Bonding Storage Durability Objective. To evaluate the effects of surface moisture (wet or dry) and storage (24 h or 3 months) on the microtensile bond strength (BS) of resin/dentin bonds mediated by two water/ethanol based adhesives Single Bond, 3M-ESPE, (SB) and Opti Bond Solo Plus, Kerr, (OB), and two acetone-based adhesives, One Step, Bisco, (OS) and Prime&Bond NT, Caulk/Dentsply, (PB). Materials and methods. Flat dentin surfaces were polished with 600-grit SiC paper, etched with 35% phosphoric acid for 15 s and rinsed for 20 s. Half the surface was maintained moist and the other half was air-dried for 30 s. Each adhesive was applied simultaneously to both halves, left undisturbed for 30 s and light-cured. Four-mm resin build-ups were constructed incrementally. After storage in water at 37 C for 24 h, slabs were produced by transversal sectioning and trimmed to an hourglass shape (0.8 mm 2 ). Half of the specimens were tested in tension at 0.6 mm/min immediately after trimming and the other half after 3 months of water storage. Data were analyzed by two-way ANOVA and SNK for each material. Results. Both moisture and storage affected BS to dentin, and was material-dependent. Dry bonding affected mostly the acetone-based adhesives. Larger reductions in bond strength were associated with dry bonding after 3 months of water storage. Significance. Wet bonding resulted in more stable bonds over 3 months of water storage for most of the materials tested Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. 1. Introduction The durability of the adhesive dentin interface is related to its quality, based upon the ability of adhesive monomers to occupy the spaces created by the removal of apatitic mineral by acid-etching, and their ability to envelop the exposed collagen fibrils [1]. After the dentin surface is conditioned, it is recommended that it should be maintained in a moist state prior to bonding, commonly referred to as wet bonding [2,3]. Thus, the demineralization of intertubular dentin and maintenance of interfibrillar porosity is required for monomer penetration into the prepared dentin. Air drying of the conditioned dentin surface has been shown to cause the unsupported collagen web to shrink and collapse, preventing monomers of the adhesive resin from efficiently wetting and infiltrating the conditioned surface [4]. The dry bonding tech- Corresponding author at: Al. Octávio Pinheiro Brizola, 9-75, Bauru, SP, Brazil. Tel.: ; fax: address: ricfob@fob.usp.br (R.M. Carvalho) /$ see front matter 2007 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. doi: /j.dental

2 dental materials 24 (2008) nique has been shown to cause a marked reduction in the bond strengths to dentin with total-etch adhesive systems [5 8]. The degree of wetness that is ideal for resin dentin bonding varies widely among the different adhesive systems and depends on their solvents [9]. Adhesive systems can use acetone, ethanol, and/or water as solvents. The ability of these solvents to form a hydrogen bond with the peptides of the collagen fibrils determines their capacity to re-expand and maintain the demineralized dentin in an expanded state during resin infiltration. Water is the strongest hydrogen bonding solvent [10] and the application of solubility parameter theory to the procedures of dentin bonding [11] clarifies what happens to adhesives and dentin and their interactions during the adhesive procedure. The optimal amount of surface wetness necessary for wet bonding varies among marketed total-etch adhesive systems. While water-based systems tolerate a relatively dry dentin surface, the acetone-based systems require a wetter dentin surface to achieve high bond strength [7]. The regional differences among dentin surfaces in the same preparation cause non-uniform resin bonding because it is not uncommon to have over-wet and over-dry regions on the same surface [12]. Several studies have shown that the moist bonding technique produces high immediate bond strength, but is not stable upon aging [6,13 21]. During resin hybridization, two events may result in the creation of porosities within the hybrid layer: (1) under over-dry conditions, restricted diffusion of resin monomers occurs within a shrunken collagen matrix; (2) under overly moist conditions, residual water entrapped within the fibrils may compromise resin diffusion and subsequent polymerization of resin monomers. These events render the hybrid layer porous and more susceptible to hydrolytic degradation over time, leading to the reduction of bond strengths both in vivo [15] and in vitro [14,22]. While it is known that immediate bond strength of totaletch adhesive systems are high on a moist surface and that these values reduce over time, there is little information on the effect of residual water in dentin on the stability of bonding. Moisture is necessary for good bonding to dentin, but residual water may prevent complete monomer infiltration to the bottom of the demineralized zone, and cause phase separation in some adhesive systems that compromises ideal adhesive infiltration and polymerization [4,23 27]. These factors could be responsible for the degradation of resin dentin interfaces over short periods of time. The instability of bonds over longer time periods has been attributed to the degradation of both exposed collagen and resin monomers. Dry bonding is more likely to result in exposed collagen fibrils, but may allow better removal of solvents and residual water from the adhesive. Conversely, wet bonding may facilitate resin infiltration, but residual water may compromise the cure and durability of the resin monomers. Therefore, the objective of this study was to determine the effects of surface moisture on immediate and 3 months bond strength of four total-etch adhesive systems to dentin. The null hypothesis was that there is no influence of surface moisture on the bond strength of different solventbased adhesive systems, regardless of the time of storage. 2. Materials and methods 2.1. Tooth preparation Thirty-two extracted human third molars that were kept in saline containing 0.1% timol at 4 C for no longer than 6 months were used in this study. The study was approved by the local Ethics Committee under the protocol #145/2002. The teeth had their crowns sectioned at right angles to the long axis of the tooth, midway between the cusp and the cementum enamel junction using a low-speed machine equipped with a rotating, diamond-impregnated, copper disc under copious water supply (IMPTECH PC 10, Boksburg, Republic of SA) to expose a flat mid-coronal dentin surface. The roots were severed parallel to the first cut and the pulp tissue was removed. Using the same diamond blade, a groove was cut along the diameter of the exposed surface to demarcate two halves of the same surface. The dentin surface was polished with a 600 grit SiC paper for 30 s to create a standard smear layer Bonding procedure Four total-etch adhesive systems with different solvents were tested: Single Bond (SB 3M/ESPE, St. Paul, MN, USA) and Optibond Solo Plus (OB KERR Corporation, Orange, CA, USA), both ethanol-/water-based; One Step (OS Bisco, Schaumburg, IL, USA) and Prime & Bond NT (Caulk/Dentsply Int., Inc. Milford, DE, USA), both acetone-based systems. The composition and batch numbers are listed in Table 1. Tooth specimens were ran- Table 1 Composition and batch number of the materials used in the study Material (manufacturer) Composition Batch number Resin composite (Z 250 (3M/ESPE, St. Paul, MN, USA)) UDMA, Bis-EMA, TEGDMA, filler 4FF/4BC Acid conditioner (Scotch Etchant (3M/ESPE, St. Paul, Phosphoric acid 35% 3GN MN, USA)) Single Bond (SB) (3M/ESPE, St. Paul, MN, USA) Bis-GMA, HEMA, dimethacrylates, polyalkenoic acid 3HN copolymer, initiators, water, and ethanol OptiBond Solo Plus (OB) (KERR Corporation, Orange, CA, USA) Bis-GMA, HEMA, GPDM, sodium-fluorsilicate, ethanol, water, initiator Prime & Bond NT (PB) (Caulk/Dentsply Int., Inc., Milford, DE, USA) PENTA, UDMA, resin5-62-1, resin-t, resin-d, bisphenol A dimethacrylate, acetone, nanoscale filler cetylamine hydrofluoride One Step (OS) (Bisco, Inc., Schaumburg, IL, USA) Bis-GMA, HEMA, BPDM, initiator, and acetone

3 478 dental materials 24 (2008) domly divided into four groups and the entire dentin surface (both halves) was etched with 35% phosphoric acid gel (3M ESPE) for 15 s and rinsed with water for 20 s. One half of the surface was blot-dried separately with absorbent paper and the groove was dried with paper points. A glass cover-slip was vertically inserted into the groove prior to the application of an air blast for 30 s at a distance of 10 cm to the other half of the surface. The glass slide cover-slip prevented the adjacent moist surface from drying by the use of the air-stream. In this way, one half of each surface was over-dried and the other half was maintained moist, as recommended for the wet bonding technique. The glass cover-slip was then removed and the adhesive systems applied simultaneously to both halves. After maintaining the surface undisturbed for 30 s for solvent evaporation, the adhesive agent was light-cured for 20 s. After bonding, the entire dentin surface received four layers of Z-250 (3M ESPE) resin composite to build up a crown approximately 4 mm high. Each 1 mm increment was light-cured for 40 s with a light-curing unit (Degulux Soft Start, Degussa, Hanau, Germany) that delivered 500 mw/cm 2. All bonding procedures were carried out by a single operator at room temperature (24 C) and 45 55% relative humidity [28] Microtensile bond test and storage The bonded teeth were then stored in water at 37 C for 24 h. Subsequently, the teeth were vertically, serially sectioned into several 0.9-mm slabs, which were then longitudinally sectioned to separate the moist from the dry halves. Each half slab was gently trimmed with an ultra-fine diamond bur under magnification to an hourglass shape to reduce the bonded interface to a cross-sectional area of approximately 0.8 mm 2 [29]. Eight teeth were used per adhesive. Approximately 4 6 slabs were produced from each tooth. Half of them were tested at 24 h and the other half stored to be tested after 3 months. Specimens were stored individually in distilled water (ph = 5) at 37 C. The storage water was changed monthly [22] and its ph was monitored before substitution by ph indicator paper strips (Merck KGaA, Darmstadt, Germany). Preservatives and antimicrobial agents were not used in the storage water in this study. The number of prematurely debonded specimens per group during specimen preparation or storage was recorded. Slabs originated from the areas immediately above the pulp chamber were measured for their remaining dentin thickness (RDT) with digital calipers and recorded (Digimess, Shinko precision gaging, Ltd., City, China). The specimens were then fixed with cyanoacrylate glue to the fixtures of a Vitrodyne V1000 testing machine (Chatillon Bros, Greensboro, NC, USA) and tested in tension at 0.6 mm/min. until fracture. After failure, the specimens were removed from the Vitrodyne and the load at failure (Kgf) was divided by the cross-sectional area of the failed interface (cm 2 ), measured with a digital caliper to the nearest 0.01 mm, and expressed in MPa. The bond failure modes were evaluated at 40 under a light microscope (Laborama, São Paulo, SP, Brazil) and classified as cohesive when exclusively within dentin (CD) or resin composite (CR), adhesive (A) when failure occurred at the dentin/resin interface, or mixed (M) when two modes of failure happened simultaneously. Table 2 Specimens that prematurely debonded during preparation or storage Groups 24 h (%) 3 months (%) PB Dry 1/21 (5) 5/21 (24) Wet 0/20 (0) 1/25 (4) OS Dry 5/25 (20) 6/19 (31) Wet 1/25 (4) 1/20 (5) OB Dry 0/22 (0) 0/19 (0) Wet 0/22 (0) 0/19 (0) SB Dry 2/23 (9) 1/20 (5) Wet 0/22 (0) 0/18 (0) Number of failed specimens/total number (relative %). Dried and moist dentin specimens obtained from the same bonded surfaces were used to permit comparisons between moisture conditions for each adhesive system. Differences in bond strengths were tested using two-way ANOVA, followed by Student-Newman-Keuls (SNK). Statistical significance was set in advance at = 0.05%. 3. Results The number of specimens that debonded prematurely during preparation and/or storage is shown in Table 2. Overall analysis showed 11.7% of premature failure for dry-bonded specimens and only 1.7% for wet-bonded specimens. When bonded in the dry condition, the percent of premature failure was 12.5% for acetone-based adhesives and 4.5% for water-/ethanol-based adhesives. After storage, premature debonding was 27.5% and 2.5% for acetone-based and water- /ethanol-based adhesives, respectively. The mean RDT for all specimens was 1.77 mm, indicating that the bonded interfaces were located mid-dentin [29,30]. Regression analysis between RDT and BS revealed no influence of dentin depth on bond strength (p > 0.05, not shown). The mean cross-sectional area by group is shown in Table 3. No differences among the treatment groups were detected (p > 0.05). The initial ph value (5.0) of the water increased after 30 days to values that ranged from 7.0 to 8.0. This pattern was constant after 30, 60, and 90 days for all groups tested. Table3 Averagecross-sectional area (cm 2 )of specimens tested in all experimental conditions (N) Groups 24 h 3 months PB Dry (0.001) [20] (0.000) [16] Wet (0.001) [20] (0.001) [21] OS Dry (0.001) [20] (0.001) [13] Wet (0.001) [24] (0.001) [19] OB Dry (0.001) [22] (0.001) [19] Wet (0.001) [22] (0.001) [19] SB Dry (0.001) [21] (0.001) [19] Wet (0.001) [22] (0.001) [18] No significant differences were found for any comparison (p > 0.05).

4 dental materials 24 (2008) Table 4 Mean(S.D.) (MPa) of bond strength as a function of bonding moisture condition and storage time (N) Dry-bonded Wet-bonded 24 h 3 months 24 h 3 months PB (7.27) A,a [20] 9.64 (9.35) A,a [16] (16.40) B,a [20] (21.30) B,a [21] OS (5.28) A,a [20] 4.00 (4.70) B,a [13] (13.26) C,b [24] (12.26) D,a [19] SB (9.70) A,b [20] (8.72) B,a [19] (7.72) C,b [22] (12.60) C,b [18] OB (11.68) A,b [22] (15.55) A,b [19] (16.27) B,b [22] (15.07) B,b [19] Same superscript letters indicate no statistically significant difference (p > 0.05). Capital letters compare data in the same row and lower cases in the same column. The overall bond strengths for the experimental groups are expressed in Table 4. Two-way ANOVA showed statistically significant effects for the interactions only for SB (p < 0.05). There was a significant effect of the storage time on bond strength for SB in the dry condition (p < 0.05). Regardless of the bonding technique, significant reduction in bond strength was observed after 3 months storage for OS (p < 0.05). Wet bonding resulted in more stable bonds over 3 months water storage for most of the materials tested. Most of the bond failures that occurred in specimens bonded using the dry technique were adhesive in nature for acetone-based systems (PB and OS), and mixed for water- /ethanol-based systems (SB and OB) (Table 5). However, the majority of failures of specimens bonded using the wet bonding technique were mixed for all adhesives systems. 4. Discussion These results confirm the findings of other studies that totaletch adhesive systems require a moist dentin surface for bonding as bond strength values were higher in moist dentin when compared to dry dentin after storage for 24 h [5 8,18]. When wet bonding, water maintained the collagen fibrils in the expanded condition and the adhesive systems were able to infiltrate and produce high bond strengths. The protocol for dry bonding varies among different studies. Some authors only dried dentin for 5 s [31], while others applied air for 30 s [7,32], but all such results demonstrate lower bond strengths of varying degrees on dry bonding. When demineralized dentin is air-dried, the water within the collagen matrix is removed and collagen fibrils are brought into close contact. Shrinkage of demineralized dentin decreases the permeability of intertubular dentin, which results in the inhibition of monomer infiltration into conditioned dentin. Wherever resin is unable to infiltrate, water will fill naked collagen fibrils leading to low quality, porous hybrid layers [33]. Prime & Bond NT produced lower 24 h BS than the other adhesive systems. This could be attributed to the high solvent concentration in this system. Although acetone-based adhesives are sensitive to moisture on the surface [32,34], the lower bond strength for PB in this study can be explained by other factors. It was speculated that the higher concentration of volatile elements in this system (62%, as determined by weight loss) could result in a thin adhesive layer after solvent evaporation. Its higher acetone content resulted in lower values of bond strength [35] and thinner adhesive layers [36]. Additionally, thin comonomer films are more susceptible to oxygen inhibition of polymerization [37,38]. The solvent content of adhesives influences bond strength to dry dentin [7,39,40] Adhesive systems that include water as one of the solvents in the mixture have been claimed to be able to promote re-expansion of collapsed fibrils [41]. The bond strength to dry dentin was higher for water-/ethanol-based than acetone-based systems. This observation confirms the results of other studies and suggests that the water present in the adhesive may be capable of simultaneously expanding collagen fibrils during the infiltration of solvated comonomers, thus affording for better resin infiltration [3,7,31,42]. The stability of adhesive dentin interfaces is often evaluated after prolonged storage time in vitro. In this study, 3 months of storage was enough to produce significant reductions in bond strength in some experimental groups, confirming the results of other short-term durability studies [6,14,16,17,21]. The degradation of interfacial components by polymer and collagen hydrolysis, as a result of water diffusion has been demonstrated in several studies in vivo and in vitro [6,13 20,21]. The storage of specimens with reduced bonded cross-sectional surface areas and the monthly change of storage solution apparently facilitate the process of degradation [22,43,44]. The ph value changed after every 30-day period and this occurred in all changes and all groups, suggesting neutralization of the CO 2 dissolved in water (i.e. very dilute H 2 CO 3 )by the release of calcium and phosphate ions from dentin [16,22]. Table 5 Failure mode of tested specimens according to conditions Dry Wet Acetone (%) Water/ethanol (%) Acetone (%) Water/ethanol (%) Adhesive Mixed Cohesive in dentin Cohesive in resin

5 480 dental materials 24 (2008) This could promote slight demineralization of the dentin surface and may have exposed collagen, resulting in an additional factor for the reduction of bond strengths in some groups. The presence of incompletely filled interfibrillar spaces due to restricted infiltration of the adhesive, and incomplete polymerization due to residual water may also have contributed to the deterioration of the bonds over time. Reductions in 3-month bond strengths in dentin specimens bonded under dry conditions were seen for specimens bonded with OS and SB systems. Acetone-based adhesive (OS) may be unable to promote re-expansion of demineralized dentin matrix, thus creating a zone of unprotected fibrils that is susceptible to hydrolytic degradation. The other acetone-based adhesive studied (PB) did not show significant reduction of BS over 3 months in specimens bonded using the dry technique. This may be due to its already lower 24 h BS values and the higher standard deviation when compared to the other adhesive systems evaluated. The SB system showed a reduction of 24 h BS when using the dry bonding technique, but this did not occur when bonding with the OB system. As both adhesive systems are water-/ethanol-based, similar behavior was expected. It has been previously demonstrated that BS of adhesives to dentin is related to the ability of the solvent to re-expand and maintain the interfibrillar spaces within the demineralized dentin matrix [3,7,21]. Although the adhesive system SB uses water/ethanol as solvents, the water content of this material may not be enough to permit optimal re-expansion when applied to a dry dentin surface [45]. Additionally, this adhesive system contains a high concentration of polyalkenoic acid copolymers with high molecular weight that may impede the inward diffusion of dimethacrylate [25]. These molecules are preferentially retained at the superficial region of the hybrid layer, while components with lower weight (i.e. HEMA) infiltrate to the full depth of the demineralized dentin. Thus, the differences in durability between the two water- /ethanol-based adhesives (SB and OB) may be attributed to differential diffusion of monomers within the hybrid layer affecting the strength and durability of the dentin infiltrated zone. The reduction of BS could be related to the unprotected interfibrillar zone, which indicates the importance of protection promoted by optimal infiltration of the adhesive systems. Recently, a similar study performed by Reis et al. [21] reported no significant reduction of BS produced on a dry dentin surface with OS and SB adhesive systems after 6 months. Conversely, the results of the current study showed significant reductions of bond strengths after 3 months. It was speculated that differences in methodology caused this variability. While the remaining pulp tissue was removed before bonding, there is no indication that this procedure was done by Reis et al. [21]. Moisture retained in the remaining pulp tissue could be responsible for some rewetting of dentin before applying the adhesive system; thus the dry dentin condition may have been more severe in the current study and the effects on demineralized, unprotected dentin more evident in a short period than in the Reis et al. study [21]. Of all of the adhesives studied, only OS showed significant reductions in bond strength. Presumably, the reduction in bond strength can be explained by inadequate hybridization, resulting in zones susceptible to hydrolytic degradation [17,26]. Acetone-based adhesive systems are moisturesensitive and the amount of surface water necessary for optimal hybridization is greater than water-/ethanol-based adhesive systems. Thus, some residual water could have been left on the surface after evaporation of acetone and compromised polymerization of the resins and created favorable conditions for polymer degradation [47,48]. The surface moisture adopted in this study for both acetone-based adhesives was similar; however, OS contains a lower concentration of volatile elements when compared to PB (unpublished observations). The authors speculated that this could have compromised the removal of the residual water present on the dentin surface. Additionally, acetone evaporates faster than ethanol/water, and this material (OS), being considerably hydrophobic, would have difficulty replacing residual water within the interfibrillar spaces [46]. The average bond strength was calculated, based on surviving specimens. Specimens that failed prematurely during preparation were disregarded in this analysis, but the frequency of prematurely debonded specimens during preparation was recorded. Although specimen preparation into an hourglass shape may induce microcracks at the interface [36,49], all groups received the same treatment and thus were not unique to the specimens that failed prematurely. Undoubtedly, a high incidence of debonded specimens during preparation in the same group suggests a greater fragility of that bonded interface. The current results showed a higher incidence of adhesive failures for the dry bonding technique and mixed failures for the wet bonding technique. These results suggest that well-hybridized dentin has better interfacial mechanical properties to resist stresses induced during specimen preparation for the microtensile test. The results of this study indicated that differences in the composition of total-etch simplified adhesive systems may create different quality hybrid layers that show differences in the durability of bonds to dentin. The ratio of hydrophilic/hydrophobic monomers [51], the type of solvent used in the adhesive system [3,50], any phase separation phenomena [9,24 27], and the plasticizing effects of water on polymers [48,52] are extensively documented factors that undoubtedly affect the bonding performance of total-etch adhesive systems to dentin. 5. Conclusions The results from this study suggest that the composition of one-bottle, total-etch adhesive systems and bond technique has a substantial effect on the stability of the interfacial structure of the dentin/adhesive bond. The null hypotheses were rejected because the results showed that the surface moisture affected the bond strength and that also influenced the stability of the bonds over time. Acknowledgments This investigation was supported, in part, by CAPES; CNPq /03-4 and /04-0, FAPESP 03/ , 03/ , and NIDCR R01 DE to DHP. The authors gratefully

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