Shear Bond Strength of 4 Current Adhesives in Caries-Affected Dentin

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1 BALKAN JOURNAL OF STOMATOLOGY ISSN STOMATOLOGICAL SOCIETY Shear Strength of 4 Current Adhesives in Caries-Affected Dentin SUMMARY Objective: To evaluate the shear bond strength of 4 current dentin adhesives in caries-affected dentin, after thermocycling. Methods: The materials used were Prompt-L-Pop, Gluma Comfort, Clearfil SE and Nano-. 80 freshly extracted human molars were used. 40 of them had proximal caries (group A). After the removal of carious lesions, the molars were cut in order to obtain flat surfaces. Surfaces were bonded with Prompt-L-Pop, Gluma Comfort, Clearfil SE or Nano-, according to manufacturers recommendations. Composite resins were added to the surfaces by packing the material into a cylindrical-shaped plastic matrix. After storage of the specimens in distilled water for 10 days, half of the specimens with each material were submitted to thermal cycles (5 0 ; ; ; C). The other 40 teeth were healthy human molars (group B), which were submitted to the same process. All of the specimens were loaded to shear forces at a rate of 0.5 mm/min until failure. Statistical analysis was performed with 3-Way ANOVA at the level of significance p<0.05. Results: All adhesives attained higher shear bond strengths in the normal dentin (control, group B), than in the caries-affected dentin (group A). Significantly lower results were obtained using the adhesives Prompt-L-Pop and Gluma Comfort compared to Nano- and Clearfil SE. Thermal-cycled specimens attained lower shear bond strengths. Conclusion: The results suggested that the altered structure of cariesaffected dentin reduce the adhesion performance of resins. Key Words: Dentin ing Agents; Caries-Affected Dentin; Shear Strength; Thermal Cycling Maria Stergiou, Eugenia Koliniotou-Koumpia Aristotle University, School of Dentistry, Department of Operative Dentistry Thessaloniki, Greece ORIGINAL PAPER (OP) Balk J Stom, 2005; 9: Introduction Micromechanical retention is considered to be the most important mechanism in bonding resin to dentin. Such retention is achieved when the dentin is infiltrated by resin monomers, and a hybrid layer is established 1. There are 3 cardinal steps in bonding resin to dentin, which are as follows 2 : Etching: consisting of removal or modification of smear layer and exposure of collagen fibrils; Priming: altering chemical reactivity of dentinal surface, thus facilitating the penetration of resin monomers; Adhesion: characterized by the establishment of the hybrid layer. Currently, dentin adhesives are available as 3-steps, 2-steps and single-step, depending on how the 3 former steps are accomplished or simplified 2. 2-steps systems are subdivided into self-priming adhesives, which require an extra etching step 3 and self-etching primers that combine etchant and primer in 1 bottle 4. Recently introduced allin-one adhesives combine all bonding steps in 1 application. The efficiency of these simplified bonding systems is still a matter of controversy 5. Practically most published reports use normal dentin as the substrate; however, the most common substrate in clinical procedures is cariesaffected dentin. Caries-affected dentin is the hard, sometimes stained dentin beneath excavated carious lesions 6. Its structure is different from normal dentin as dentinal tubules are occluded by mineral crystals 7 ; consequently, caries-affected dentin is much less permeable than intact dentin 8.

2 M. Stergiou, E. Koliniotou-Koumpia Balk J Stom, Vol 9, 2005 The purpose of this study was to test the shear bond strength of 4 dentin adhesive systems (Prompt-L-Pop, Gluma Comfort, Clearfil SE, Nano-) in normal and caries-affected dentin, after subjection to thermal cycling. Materials and Methods 80 freshly extracted human molars were divided to 2 groups. The first group involved 40 molars with proximal caries (group A), while the second group consisted of 40 caries-free molars (group B). After extraction, all teeth were stored in distilled water, to which some crystals of thymole (Merck, Darmstadt, Germany) were added to provide antibacterial protection (70 ml of distilled water and 1 small crystal of thymole. The roots of the teeth were removed just below the DEJ using a diamond saw in a low-speed handpiece with copious water spray. In group A, carious lesion was repeatedly stained using a caries detector dye solution (CDDS, Kuraray Co Ltd, Osaka, Japan) and the outer layer of carious dentin was removed using rotary low-speed round steel burs, according to the staining of CDDS until the dentin was no longer stained by the caries detector solution. In both groups (A and B), in order to obtain a dentin section approximately 2 mm thick, the teeth were cut longitudinally in the centre of the tooth parallel to the buccal-lingual axis, using an Isomet saw (Buehler Ltd, Lake Bluff, IL, USA), under water spray. The dentin samples were embedded in light-cured resin (Z-250, 3M Dental products, St Paul, MN, USA) and the resin was polymerized in the middle of a metal ring mould for 40 sec. To create the smear layer, each specimen was finished with wet 600 grit silicon carbide paper. The materials used are listed in table 1, while table 2 illustrates the different bonding procedures. 10 dentin samples from each group (total 80 dentin samples) were treated according to the manufacturers instructions for each bonding system. Each bonding agent was applied strictly following the manufacturers recommendations. After the bonding procedure, in order to limit the bonding area, a Teflon mould of a 2.5 mm diameter and 3.00 mm in height was placed over the dentin sample. The mould was filled with the composite resin corresponding to the adhesive system in 2 increments, following the layering technique. Each increment was light-cured for 40 sec with a conventional visible light source (Coltolux 4, Coltene/ Whaledent GmbH, Konstanz, Germany). The intensity of the lamp was tested for light output (600 mv/cm 2 ). This procedure resulted in cylindrical specimens of composite resin measuring 2.5 mm in diameter and 3.00 mm in height. Next, all samples were stored in distilled water, at room temperature, for 10 days. Then, 5 specimens from each group were thermo-cycled for cycles (5 0 C; 37 0 C; 55 0 C; 37 0 C) with a dwell time of 15 sec. The rest of the specimens remained stored in distilled water. Dentin Adhesive System Table 1. Materials tested Code Prompt L- Pop PLP Z250 Gluma Comfort Clearfil SE GCB CSE Restorative Composite Resin Charisma Nano- NB Smile Prompt-L-Pop Gluma Comfort Clearfil SE Nano- Clearfil AP-X Table 2. ing procedures Manufacturer 3M-ESPE St-Paul, MN USA Heraeus-Kulzer GmbH & Co. KG Gruner Weg 11D Hanau Kuraray Europe GmbH Schiess-strasse Dusseldorf Germany Jeneric-Pentron/Inc P.O. Box 724 Wallingford, CT USA Apply with scrubbing for 15 s; gently air dry. Light cure for 10 s Etching for 20 s; rinsing and gentle air dry; apply adhesive for 15 s. Light cure for 20 s Apply primer for 20 s; evaporate water with mild air; apply adhesive, gentle air stream. Light cure for 10 s Apply primer for 30 s; mild air stream; apply adhesive; gentle air stream for 10 s. Light cure for 10 s The shear bond test was conducted using an Instronlike machine (AMETEK Accu Force III 500 Mansfield and Green Division) at a cross-head speed of 0.5 mm/sec (Fig. 1). The force at failure was recorded and the shear bond strength values (MPa) of each adhesive system were compared. 3-way ANOVA and Student-Newman-Keuls Post Hoc test were used for multiple comparisons of means at significance level of p<0.05. In addition, the de-bonded surfaces were air-dried and examined at x20 and x50 magnifications with a stereomicroscope (Olympus Co, Tokyo, Japan) to determine the mode of failure. Failure modes were classified as cohesive (occurring within the dentin or the composite resin), adhesive (taking place at the top level of the adhesive layer) or mixed (containing areas of both adhesive and cohesive modes of failure).

3 Balk J Stom, Vol 9, 2005 Adhesives and Caries-Affected Dentin 3 Table 4. Mode of failure Mode of failure Normal dentin Caries-affected dentin Adhesive 22.5 % 5 % Cohesive 35 % 32.5 % Mixed 42.5 % 62.5 % Figure 1: Instron-like machine Results The results of shear bond testing are summarized in table 3, while the failure modes are shown in table 4. For all adhesive systems, 3-way ANOVA analysis showed significant differences in mean values of shear bond strengths in caries-affected and normal dentin at the significance level of p<0.05 (Fig. 2). Table 3. Shear bond strength values (MPa) PLP GCB CSE NB Thermal cycling Normal Dentin Mean Std. deviation Caries-affected Dentin Mean Std. deviation No 22,39 ± 2,72 14,83 ± 3,49 Yes 14,82 ± 1,06 8,20 ± 1,32 No 22,97 ± 2,23 17,96 ± 4,39 Yes 13,21 ± 2,65 13,40 ± 2,11 No 31,81 ± 5,12 22,79 ± 4,49 Yes 20,25 ± 5,05 16,46 ± 1,67 No 32,10 ± 3,49 25,74 ± 2,69 Yes 23,25 ± 2,23 16,73 ± 1,50 Figure 2: Shear bond strength values (MPa) in normal and caries-affected dentin Among the materials used, CSE and NB showed significantly higher shear bond strength values than GCB and PLP (p<0.05), in both normal and caries-affected dentin. There were no statistically significant differences (p>0.05), neither between PLP and GCB nor between NB and CSE (Fig. 3). Figure 3: Shear bond strength values (MPa) of each material tested

4 4 M. Stergiou, E. Koliniotou-Koumpia Balk J Stom, Vol 9, 2005 Figure 4: Comparison of shear bond strength values (MPa) between thermo-cycled and non-thermo-cycled samples For all materials, shear bond strength values significantly decreased (p<0.05) when they were submitted to thermal cycling (Fig. 4). In stereomicroscope evaluation, failures appeared mostly as mixed between the adhesive layer and the composite (52%). About 34% of specimens showed cohesive failure and 14% adhesive failure. Discussion The results of this study indicate that in vitro bonding to normal dentin, in all tested dentin bonding systems, produced shear bond strength values that were significantly higher (p<0.05) than those in caries-affected dentin. Structural lesions in the dentin owing to the presence of caries may be a possible explanation for this phenomenon. In order to achieve good resin-dentin adhesion, resin monomers must penetrate through the demineralised dentinal subsurface 9. The degree of penetration of resin monomers is thought to depend in part on the characteristics of the demineralised collagen fibril network before application of the resin adhesive. The structural or physical characteristics of the caries-affected collagen fibrils that are exposed by etching may be different from that of the normal dentin. Spaces between the collagen fibrils in normal dentin are occupied by normal calcium-deficient carbonate-rich apatite 10. In caries-affected inter-tubular dentin, the mineral occupying the inter-fibrilar spaces may be different due to cyclic demineralization-remineralization. Due to its partial demineralization, caries-affected dentin is softer than normal dentin Because of loss of minerals, carious inter-tubular dentin exhibits a higher degree of porosity than normal dentin 14. This results in easier diffusion of acidic conditioners and adhesive monomers and causes the formation of thicker hybrid layers in caries-affected dentin 11,14. However, the thickness of the hybrid layers is unrelated to bond strengths in dentin 11, Even when adhesive systems infiltrate deep into inter-tubular dentin, there will always be a porous and demineralised underlying zone that was not infiltrated 19. Dentinal tubules in caries-affected dentin are occluded with acid-resistant minerals that hinder the infiltration of adhesive resins and the formation of tags 19. When smear layers are created on caries-affected dentin, it is likely that they contain acid-resistant crystals and extrinsic proteins that have permeated into these layers during cycles of demineralization 18. These smear layers may be more resistant to the action of primers. It is possible that primers hybridize the smear layer without penetrating into underlying intact dentin. This phenomenon tends to result in lower bond strengths 23, 24. The lowest shear bond strengths (SBS) were attained with Prompt-L-Pop (22.4±2.72 in normal dentin, and 14.83±3.49 in caries-affected dentin). These findings are in agreement with previous reports Prompt-L-Pop is a strong self-etching system, which dissolves the smear layer and plugs 28 ; although this etching aggressiveness is unrelated to the bond strengths attained. This might possibly be explained by the formation of dry spots. Due to its low viscosity, Prompt-L-Pop spreads thinly and therefore is not properly polymerized It is also reported that all-in-one systems do not yet possess all the necessary requirements in order to provide optimal adhesion to the tooth structure 25. It has been suggested that either their formula or mode of application need to be modified 29. Furthermore, it seems that in Prompt-L-Pop the time allowed for chemical reactions to take place is extremely short 25. Clearfil SE contains an unsaturated methacrylated phosphate ester, 10-methacryloxydecyl dihydrogen phosphate (MDP) as the acidic resin monomer 30. Due to the presence of MDP, Clearfil SE has a milder formula that preserves the smear layer and plugs and incorporates them into a hybridized complex 31. With the use of mild self-etching primers, collagen fibrils are not completely deprived of hydroxyapatite. This residual hydroxyapatite may serve as a receptor for additional intermolecular interaction with specific carboxyl or the functional monomers of the primer 2. Clearfil SE produced SBS values that range between 31.81±5.12, in normal dentin and 22.79±4.49 in caries-affected dentin. Sengün et al 32 refer SBS values that range between 29.91± 8.95 in normal dentin and between 24.49±5.38 in cariesaffected dentin. The SBS values for Nano- were 32.1±3.48 in normal dentin and 25.7±2.69 in caries-affected dentin. The interesting feature about this material is that the manufac-

5 Balk J Stom, Vol 9, 2005 Adhesives and Caries-Affected Dentin 5 turer claims that it is a nano-technology dentin adhesive. The bonding agent contains nano-filler sized particles that, as it is claimed, increase the bond strength of the material due to their capacity to penetrate the spaces between the collagen microfibrils, provide nano-retention. The nanofiller may be able to infiltrate the tubules and the demineralised inter-tubular dentin 33. However, Tay et al 34 found that nano-fillers from the adhesive layer were congested around tubular orifices but not located within the interfibrillar spaces of the hybrid layer. Moreover, manufacturer claims that Nano- consists of polyhedral-shaped nano-particles containing surface functional groups to improve the properties of the resin. These structures are characterized by their 3-dimensional cage-like molecular shape that serves to increase the modulus and hardness of the material 35. However, no study to date has confirmed these claims. Gluma Comfort produced SBS values that range between 22.97±2.23 in normal dentin and between 17.96±4.3 in caries-affected dentin. Gallo et al 36 reported SBS values in normal dentin for Gluma Comfort that range between 26.6±2 when the dentin was moist and 20±4 when it was dry. It seems that in this material, the degree of moisture is critical for SBS values 37. It is also possible that within this particular system, low SBS values are related to technique sensitivity during etching and rinsing. SBS values for all adhesive systems were significantly lower when specimens were submitted to thermal cycling. Thermal cycling allows bonded specimens to be subjected to extreme temperatures, which simulate intraoral conditions. In a way it resembled an in vitro process of the aging of materials. The effects of thermal cycling on bond strength depended on the adhesive system used 38,39. During thermal cycling specimens were subjected to exposure to water. The decrease in bond strengths is thought to have resulted from the effect of hydrolysis at the interface of the bonding resin and hybrid layer 40. Conclusions All the adhesives evaluated in the present study showed higher strengths in normal dentin, compared with caries-affected dentin. These differences on shear bond strength values were statistically significant in all the adhesive systems tested. Variations in the intrinsic structure of caries-affected dentin may have contributed to these results. Moreover, the chemical composition of the bonding system could affect bonding mechanism performance. Thermal cycling significantly reduced the bond strength of all the adhesive systems tested. References 1. Nakabayashi N, Kojima K, Mashuara E. The promotion of adhesion by the infiltration of monomers into tooth substrate. 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