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Research Article
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In vitro evaluation of resin-based liner materials and condensable composites utilizing filled and unfilled adhesives
Simone Deliperi, dds, David N. Bardwell, dmd, ms, Aikaterini Papathanasiou, dds, Ronald Perry, dmd, ms
Abstract: Purpose: To evaluate the efficacy of differing resin based liner materials in reducing microleakage. Methods: 80 freshly extracted caries-free human premolars and molars were used. MO/DO Class II standardized preparations were performed with the gingival margin placed 1 mm above the CEJ. Teeth were randomly divided into two groups; each one was divided into four subgroups (A-B-C-D for Group 1 and E-F-G-H for Group 2). Each prepared tooth was etched with 32% H3PO4 (Uni Etch); in Group 1, one coat of One Step and in Group 2, two coats and two cures of Prime & Bond NT adhesives were applied. In each group 1 mm layer of three different liners was used: A2 Heliomolar RO for A and E; A2 Heliomolar Flow for B and F; A2 Bisfil 2B for C and G. No liner was used for D and H subgroups. Teeth were then restored using 2 mm increments of Pyramid A2 Dentin and A1 Enamel and cured with a VIP curing light. Teeth were thermocycled x500 between 5°C and 55°C with a dwell of 30 seconds and then placed in a 0.5% methylene blue dye solution for 24 hours at 37°C. Samples were sectioned longitudinally and evaluated for microleakage at the gingival margin under astereomicroscope at x20 magnification. Dye penetration was scored using an Ordinal Scoring System where 0= no penetration; 1= enamel penetration; and 2= dentin penetration. Results: A Chi Square Test revealed a statistically significant difference between Group 1 and Groups 2 (P< 0.001). Group 1 yielded the most microleakage. No statistically significant difference was noted between the subgroups of each group; a statistically significant difference of B and D vs. E and H (P< 0.01> 0.001) and B vs. G and D (P<0.05> 0.01) was also noted. The dentin bonding agent in Group 2 contributed to a reduction of microleakage when compared to Group 1. (Am J Dent 2003;16:000-000).
Clinical significance:In the experimental model adopted for this study, the use of a nanofilled adhesive system may help to reduce microleakage independently to the use of resin based liner materials. The use of a microfilled flowable composite resin was associated with the higher microleakage scores in both groups.
: Dr. Simone Deliperi, Via G. Baccelli 10/b 09126 Cagliari, Italy. E-:
1
Introduction
Marginal discoloration, recurrent decay and postoperative sensitivity may be associated with the use of resin-bonded composite (RBC). This is the result of bacteria and oral fluid penetration due to an incomplete marginal adaptation at the enamel-dentin composite resin interface.
In the early 80s, to overcome this problem, research efforts were directed to improve the bond strength of adhesive systems.1 Further studies2-5 helped to demonstrate that the marginal adaptation of RBC doesn’t depend only on bond strength to enamel and dentin but it’s influenced by several other factors: the configuration factor or C-factor,3 the filling technique,6 the modulus of elasticity of RBC5-8 and the light curing system.9-11 Lately an increasing interest has developed around the use of low viscosity resin composites and filled adhesives as liner materials. This increasing popularity is related to some in vitro findings in the early 90s; Kemp-Scholte & Davidson7 introduced the concept of “elastic cavity wall” because they found that the application of multiple layers of adhesive or liners between the cavity walls and the composite resin improved the marginal adaptation and the shrinkage stresses of RBC. Van Meerbeek et al,8 stated that the use of such a liner or a filled low viscosity resin allows the formation of a gradient of elasticity from the rigid dentin substrate to the shrinking RBC relieving stresses during polymerization. Nevertheless, the application of multiple coats has been shown to reduce the bond strength of the adhesive.12 Based on these data filled adhesive and flowable composite resin has been introduced in the attempt to create a stress-absorbing layer.13-16 The use of flowable composite resins alone or in combination with filled adhesive has been shown to reduce microleakage.17-18 On the other hand Labella et al,19 demonstrated that flowable composite resins shrink more but are less rigid than traditional composite resins so their ability to seal the cavosurface margin is questionable. This seems to be confirmed by recent in vitro findings where the use of a flowable resin as a liner is not able to reduce microleakage in Class II composite resin restorations if not associated with a soft start or pulse polymerization.20 Microfilled composite resins possess a favorable combination of low shrinkage and low rigidity that may help to reduce the microleakage at the cavosurface margin.19,21 The null hypothesis tested in this project is twofold: (1) the use of a microfilled flowable composite resin is associated with increased microleakage; and (2) the use of a nanofilled adhesive system is associated with reduced microleakage scores when compared to an unfilled system.
This study evaluated the microleakage of Pyramid condensable composite resina when an unfilled adhesive (One Stepa) and a nanofilled adhesive (Prime & Bond NTb) were used in combination with three different resin based liner materials: Heliomolar ROc conventional microfilled composite resin, Heliomolar Flow c microfilled flowable composite resin, and Bisfil 2B a self-cured composite resin.
Materials and Methods
Eighty freshly extracted caries-free human premolar and molar teeth were kept in distilled water at 4C for 24 hours. The preparation was standardized as 4 mm long, 3 mm wide and 5 mm in depth. MO/DO Class II restorations were prepared with the gingival margin 1 mm above the CEJ using a 245 carbide burd with a high speed handpiece and copious amounts of water. Teeth were divided into two groups with each further divided into four subgroups.
Each prepared tooth was etched with Uni Etch 32% H3PO4a for 15 seconds, rinsed for 20 seconds, and then gently blown to remove excess water, being careful to maintain a moist surface. In Group 1, one coat of One Step unfilled acetone based adhesivea and in Group 2, two coats/two cures of P&B NT nanofilled acetone based adhesiveb were applied. In group G, P&B NT was used in combination with the Self Cure Activator, P&B NT-Dual Cure.b A 0.0015 inch Tofflmire metal matrix band was used to reconstruct the proximal surface and simulate clinical conditions. In each group three different resin based liner materials were used: A2Heliomolar RO microfilled composite resinc for subgroups A and E; A2 Heliomolar Flow microfilled flowable composite resinc for subgroups B and F; A2 Bisfil 2B hybrid self cured flowable composite resina for subgroups C and G. Flowable resins were applied either on the gingival or pulpal floor using 1mm even layers; these low viscosity resins were in contact with either dentin or enamel on the gingival floor and just dentin on the pulpal floor. No liner was used for control subgroups D and H (Table 1). The restorations were completely filled with the Pyramid composite resin systema which corresponds to either condensable (to substitute dentin) or microhybrid (to substitute enamel) composite resins. A2 Pyramid dentin condensable composite resina was used to build-up the dentin; vertical apico-occlusal increments were applied to the level of the occlusal fossae without enamel contact (each layer not being more than 2 mm). The cusps were then built up one unit at a time6 with A1 Pyramida enamel microhybrid composite resin. Both the liners and the Pyramid composite resin system were cured using a VIPa quartz tungsten halogen light. A conventional continuous mode of polymerization (600mw/cm2 continuous energy output for 20 seconds) was used to cure each composite resin layer. Restorations were finished using carbide burs and polished using One Glosse points and cups. All teeth were stored in distilled water at 37C for 24 hours.
The restored teeth were thermocycled x500 in a 5 - 55C water bath with a dwell time of 30 seconds in each bath. The samples were then blotted dry with a paper towel and the entire tooth was covered with a clear nail polish with the exception of the restoration and the area surrounding the cavosurface margins. All specimens were then immersed in 0.5 methylene blue dye solution for 24 hours. The dye solution was buffered to pH 7 in order to avoid demineralization of the tooth structure, thus giving false readings. Teeth were rinsed in running water, blotted dry, and embedded in an orthodontic acrylic resin block.b The teeth were sectioned longitudinally from mesial to distal with a water-cooled diamond wheel saw. Dye penetration at the gingival margin was examined using a stereomicroscope at x20 by two independent evaluators and scored utilizing an Ordinal Scoring System where 0= no penetration; 1= enamel penetration; 2= dentin penetration. A statistical analysis was performed using a Chi-Square test.
Results
Tables 2 and 3 present the microleakage scores for Groups 1 and 2. The number of samples with gap formation at the restoration enamel-dentin interface is summarized in Table 4 for Group 1 and 2.
Statistical analysis was employed using the generalized Chi-square for degrees of freedom, which revealed a statistically significant difference between Group 1 and 2 (P< 0.001), indicating Group 1 to have the most microleakage. Statistical analysis also revealed a significant difference between Groups B and D versus Groups E and H (P< 0.01> 0.001); Group B versus Group G and Group D versus Group G (P< 0.05> 0.01). No statistically significant difference was found between the subgroups of the same group (Table 5).
Discussion
The results of this study showed that the nanofilled adhesive Prime & Bond NT used with a two coat/two cure technique had a very important role in reducing microleakage when using a condensable composite resin alone or in combination with different resin liner materials (Tables 2-6).
Three different hypotheses were advanced to explain these results: (1) The two coat/two cure technique used with P&B NT may help to create a gradient of elasticity from the dentin to the composite resin relieving stress during polymerization; Swift et al,22 reported excellent clinical performance of an unfilled acetone based adhesive (P&B 2.1) applied in class 5 lesions using a similar technique. (2) The filler content of P&B NT may reinforce the hybrid layer and help to create this gradient of elasticity; Tay et al,23 and Inoue et al,24 demonstrated that the collagen fibrils network mostly filters out the nanofillers, holding them at the hybrid layer surface thus acting as an intermediate shock absorber. (3) Either the filler content or the two coat/two cure technique may be responsible for the reduced microleakage. A recent investigation reported no statistically significant difference with regard to microleakage when Prime & Bond NT was used either with the two coat/one cure technique or a two coat/two cure technique25.
The use of one bottle adhesive systems is considered a very technique sensitive clinical procedure.26-28 Even though One Step was abundantly applied in multiple layers as per manufacturer’s instructions, the high solvent to monomer ratio and the lack of nanofillers may have been responsible for the formation of a too thin hybrid layer. The use of One Step with the two coat/two cure technique may have helped to reduce microleakage because a gradient of elasticity would have been created while avoiding oxygen inhibition of extremely thin resin layers. van Dijken29reported high failure rate for One Step in Class 5 non carious lesions even using the two coats and two cure technique. This may suggest that the introduction of nanofillers in the One Step composition may help to create a uniform resin film that stabilizes the hybrid layer. According to Rueggeberg & Margeson,30 the top 15 m of the adhesive resin cannot polymerize because of oxygen inhibition. Van Meerbeek et al,31 recommended “brush thinning rather than air thinning” to prevent the film thickness from being reduced to an extent that the air inhibited layer corresponds to the whole resin layer, reducing the bond effectiveness. Residual solvent molecules may not be removed from the dentin surface using this technique and may interfere with resin polymerization.
To overcome this problem, Unterbrink & Liebenberg13proposed the use of flowable composites as filled adhesives. They combined the use of a single component adhesive as a dentin primer and a thin layer of flowable composite as a filled adhesive, which were cured together to avoid oxygen inhibition of very thin layers. The importance of precuring direct composite resin restorations was previously reported.32
Many authors have reported higher bond strength for adhesive systems with incorporated filler particles than for unfilled products.33-37 Filler particles may also reduce the shrinkage of the adhesive, thus helping to prevent microleakage.16,24,34 On the other hand, Labella et al,19 reported increased modulus of elasticity and rigidity for adhesive systems with increased filler content. This means that the filler size and content may play an important role in the clinical performance of adhesive systems.
The use of microfilled composite resins didn’t help to reduce microleakeage in this study. Group 1 was associated with very high microleakage scores in all subgroups. In Group 2 there was no statistically significant difference between Heliomolar RO, Heliomolar Flow with Bisfil 2B, and Pyramid alone. Heliomolar Flow was associated with higher microleakage scores compared to the other subgroups. These results confirm previous findings that reported a questionable ability of flowable composite resin to seal the cavosurface margin.19,38-41 Flowable composite resin with reduced filler volume and increased polymerization shrinkage may not realize its supposed ability to act as an elastic stress absorbing layer when placed either on enamel or dentin. The use of Bisfil 2B self-cured composite resin was performed according to the directed-shrinkage technique introduced by Bertolotti42 and Fusayama43 in the early 90’s. Fusayama43 demonstrated with photoelastic analysis that there was increased polymerization stress for light-cured composite resins compared to self-cured ones. He theorized that the warmer cavity walls could help to favorably direct the vectors of polymerization to the cervical margin (directed-shrinkage technique), thus improving the seal of this area. Further studies44-46 demonstrated that the less stressful polymerization of the self-cured composite resin was related to a slower conversion and to the porosity and oxygen incorporation during manipulation. Contradictory results exist in the international literature regarding the use of self-cured composite resins in Class 2 restorations.47-49 However, excellent results in vivo were reported with the directed-shrinkage technique and the use of a dual-cure adhesive system.50 Similar results were obtained in this study when the self-cured composite resin Bisfil 2B was used in conjunction with Prime & Bond NT Dual Cure. The use of Prime & Bond NT with Pyramid Dentin without any resin liners resulted in the lowest dye penetration. Packable composite resins have enjoyed considerable popularity as posterior restorative materials in the last few years.51-52 Research has revealed that the physical properties of packable composite resins are similar to well known and generally successful conventional hybrid resins. Some of them performed even better than hybrids, although others were inferior.53-56 Pyramid Dentin packable composite resin demonstrated good performance with regard to microleakage in a recent in vitro study.20
The Prime & Bond NT two coat/two cure technique may have played a very important role in achieving this result. A thick, filled resin layer above the hybrid layer may contribute to creating an elastic stress absorbing zone and a gradient of elasticity between dentin and packable composite resin. Completely different results were reported for the same packable composite resin in Group 1. This supports the hypothesis that the correct application of the adhesive system and complete dentin hybridization might be the most important factors in achieving high performing composite resin restorations.
The results of this in vitro study revealed that: (1) Prime & Bond NT nanofilled adhesive systemused with the two coat/two cure technique can help to reduce microleakage independent of the use of resin based liner materials when compared to an unfilled adhesive system; (2) the combination of Prime & Bond NT and Pyramid resulted in lower microleakage scores; (3) the use of Bisfil 2B with a dual cure adhesive system was able to provide better results than with a light-cured system; and (4) the use of microfilled composite resins didn’t reduce microleakage and Heliomolar Flow wasassociated with the higher microleakage scores in both groups.
a.Bisco Inc., Schaumburg, IL, USA.
b.Dentsply-Caulk, Milford, DE, USA.
c.Ivoclar-Vivadent, Amherst, NY, USA.
d.Brasseler, Savannah, GA, USA.
e.Shofu, San Marcos, CA, USA.
Acknowledgements: To Dentsply/Caulk, Bisco Inc. and IvoclarVivadent for providing materials.
Dr. Deliperi is Postgraduate Esthetic Dentistry,Visiting Instructor and Research Associate (Tufts University School of Dental Medicine -Boston, MA) and Clinical Instructor Department of Conservative Dentistry,University of Cagliari (Italy), Dr. Bardwell is Associate Clinical Professor and Director Post Graduate Esthetic Dentistry, and Dr. Perry is Associate Clinical Professor and Director of the Gavel Center for Restorative Research, Department of Prosthodontic and Operative Dentistry, Tufts University School of Dental Medicine, Boston, MA, USA. Dr. Papathanasiou is in private practice in Athens, Greece