|Year : 2011 | Volume
| Issue : 1 | Page : 22-27
SEM evaluation of the effect of fiber placement or flowable resin lining on microleakage in Class II adhesive restorations: An in vitro study
Vivek Sharma, Sukesh Kumar, Sumita Giri Nishad, Anil Tomer, Mayur Sharma
Department of Conservative Dentistry & Endodontics, DJ College of Dental Sciences & Research, Niwari Road, Modinagar, Uttar Pradesh, India
|Date of Web Publication||4-Mar-2011|
Department of Conservative Dentistry & Endodontics, DJ College of Dental Sciences & Research, Niwari Road, Modinagar, Uttar Pradesh
Source of Support: None, Conflict of Interest: None
| Abstract|| |
Aims : To evaluate the effect of two fibers (polyethylene or glass) and a flowable resin liner on microleakage in Class II adhesive restorations.
Materials and Methods : Class II cavities were prepared on mesial and distal surfaces of 40 extracted sound human molars. The cavity margins were below the CEJ on one side and above the CEJ on the other. The teeth were randomly divided into four groups according to the restoration technique: group 1: restored with a resin composite in bulk after SE Bond application; group 2: flowable resin liner was used before composite restoration; group 3: polyethylene fibre and in group 4: glass fiber was placed into the bed of flowable resin before composite restoration. Samples were finished, stored in distilled water and then thermocycled. Apices were sealed and tooth surfaces were coated with nail polish within 1 mm of the margins and placed in basic fuschin dye for 24 h at 37ºC. Teeth were rinsed and sectioned longitudinally through the restorations. Microleakage was evaluated and scored.
Statistical Analysis Used : Kruskal-Wallis, Mann-Whitney U-test.
Results : Flowable resin, everStick NET, and Ribbond THM used in combination with flowable resin significantly reduced leakage at occlusal enamel margins (P<0.05).
Conclusions : Use of flowable composite alone or in combination with polyethylene or glass fibers reduces occlusal leakage in Class II adhesive cavities with enamel margins.
Keywords: Cavity lining, fiber-reinforced composite, flowable composite resins, microleakage
|How to cite this article:|
Sharma V, Kumar S, Nishad SG, Tomer A, Sharma M. SEM evaluation of the effect of fiber placement or flowable resin lining on microleakage in Class II adhesive restorations: An in vitro study. J Interdiscip Dentistry 2011;1:22-7
|How to cite this URL:|
Sharma V, Kumar S, Nishad SG, Tomer A, Sharma M. SEM evaluation of the effect of fiber placement or flowable resin lining on microleakage in Class II adhesive restorations: An in vitro study. J Interdiscip Dentistry [serial online] 2011 [cited 2019 Jun 18];1:22-7. Available from: http://www.jidonline.com/text.asp?2011/1/1/22/77195
The use of resin-based composite materials for the restoration of posterior teeth is increasing not only because of their advantages for esthetic properties, but also because of their increasing adhesion to dental tissues. Although resin-based composite materials have made significant improvements in their properties, composite restorations may still be unsuccessful clinically due to wear, inadequate polymerization and microleakage, which may also result in postoperative sensitivity and recurrent caries and/or possible loss of the restoration.
Microleakage is one of the most frequently encountered problems for posterior composite restorations, in particular, at the gingival margins of Class II cavities.  Studies have reported efforts to develop methods to decrease this problem with Class II composite restorations.  These include methods for light polymerization proposed to reduce the amount of composite volumetric shrinkage, reducing C-factor, and following strategic incremental placement techniques in order to reduce residual stresses at the tooth/restoration interface. 
The use of flowable resin composites as liners in difficult areas of access has been suggested. This assumes that these less viscous materials flow easily onto all prepared surfaces, resulting in less leakage and postoperative sensitivity. Flowable resin composite liners may also act as a flexible intermediate layer that helps to relieve stresses during polymerization shrinkage of the restorative resin. 
The elastic modulus describes the relative stiffness of materials within their elastic range. A higher modulus of elasticity and lower flexural modulus of the polyethylene fibers was reported to have a modifying effect on the interfacial stresses developed along the etched enamel-resin boundary.  It has been found that, by embedding a leno-woven ultrahigh modulus (LWUHM) polyethylene fiber into the bed of a flowable resin before composite restoration, higher microtensile bond strength could be achieved in prepared cavities with a high C-factor. The C-factor affects dentin adhesion, but by using an appropriate layering technique, bond strengths to deep cavity floors can be increased. 
Recently, glass fibers have also demonstrated their ability to withstand tensile stress and stop crack propagation in composite material.  When a glass fibers layer is applied, the internal stress patterns of the restorative material may change.  There is a major stress at the dentin/composite interface, and modifications that would reduce or eliminate the interfacial stress concentrations can reduce gap formation and microleakage. 
This in vitro study investigated the effects of two fibers (polyethylene and glass) and a flowable resin liner on microleakage in class II adhesive restorations.
| Materials and Methods|| |
Forty extracted human molars were debrided and stored in physiological saline until use. They were free of visible caries, previous restorations, and visible hard or soft tissue structural defects.
Class II cavities were prepared on the mesial and distal surfaces of each tooth. The cervical margins of the mesial cavity were located 1 mm occlusal to the cementoenamel junction (CEJ), while the cervical margins of the distal cavity were located 1 mm apical to the CEJ. No bevels were prepared. A minimum of 2 mm of tooth tissue remained occlusally between the two cavities. The bucco-lingual width of the cavity was 3 ± 0.5 mm on the occlusal and gingival side. The gingival wall was approximately 2.5 mm deep to the axial wall. Internal angles were rounded. The preparations were accomplished with diamond burs (BR-41 and SF- 21; Mani; Tochigi, Japan) under profuse water-cooling.
The teeth were then randomly divided into four groups according to the restorative technique and prepared as follows:
Group 1: Cavity surfaces were treated with Clearfil SE Primer (Kuraray) for 20 s. and gently air dried for 10 s. Clearfil SE Bond was applied, thinned with brush and light cured for 20 s, using a halogen curing unit (Dentsply Maillefer USA) with an intensity of 620 mW/cm 2 . The cavities were then bulk filled using a resin composite (AP-X, Kuraray).
Group 2: Cavities were lined with a flowable resin (Protect Liner F, Kuraray) after adhesive treatment (SE Primer and SE Bond) at a thickness of 0.5-1 mm. Contamination of margins with flowable resin was avoided. After curing the resin for 20 s., the cavities were bulk filled with the same composite resin used in Group I.
Group 3: 3 × 3 mm polyethylene fiber (Ribbond THM, Ribbond) was cut and saturated with adhesive resin (Clearfil SE Bond) for 2 min. The excess of the resin was removed from the fibers surfaces using a hand instrument in the fibers' direction. After adhesive treatment, flowable resin was applied inside the cavities as described in Group 2. Preimpregnated fiber was then placed into the bed of uncured flowable resin. Contamination of the margins with flowable resin was avoided, and light curing was performed for 20 s. The cavities were then bulk filled with composite resin and cured for 40 s.
Group 4: 3 × 3 mm, preimpregnated glass fiber (everStick NET) was cut and placed into the bed of uncured flowable resin as described in Group 3. This combination was then light cured for 20 s before composite resin restoration.
All the composite restorations were finished and polished. (Enhance polishing system, Dentsply, Asia.) After storage for 1 week at room temperature in water, all specimens were subjected to 300 thermocycles between 5 and 55 o C with dwell time of 1 min per bath and 5 s transit time between baths. Following thermocycling, open apices of the specimens were sealed with resin composite, and all tooth surfaces except a 1-mm-wide zone around the margins of each restoration were sealed with one coat of nail polish. After completing the sealing, the teeth were placed in 0.5% basic fuchsin dye for 24 h at 37°C. Teeth were rinsed under running water to remove excess dye, and nail polish was removed using alcohol. The teeth were sectioned longitudinally through the centre of the restorations using a low-speed water-cooled diamond saw (Isomet, Buehler; Lake Bluff, IL, USA). Microleakage was determined by observing the distance dye penetrated into the specimens. Each section was viewed under a stereomicroscope (JSM-81-0 A JEOL Cambridge equipments Ltd.) at 20× and blindly scored by an observer on a 0-4 scale [Table 1]. One-half from each group was randomly selected for SEM analysis.
Results of evaluation of both occlusal and cervical leakage were subjected to nonparametric statistical tests. Possible differences between the various experimental groups were explored with the Kruskal (Wallis and Mann) Whitney U-tests.
| Results|| |
Materials and fibers used and Experimental groups used in the study are given in [Table 2] and [Table 3] respectively.
Statistical analysis among the groups showed no statistically significant differences in microleakage at the gingival margins when cavity lines were below or above the CEJ (P>0.05).
Use of an intermediate material (flowable resin, Ribbond THM or everStick NET in combination with flowable resin) did not change the leakage scores at the gingival margins. When the values were evaluated at occlusal margins in cavities with gingival margins below the CEJ, no statistically significant difference was observed among the groups (P>0.05). However, statistically significant differences in microleakage were observed among the groups at the occlusal margins in cavities where gingival margins were above the CEJ.
Statistical analysis revealed that placement of an intermediate material (flowable resin, Ribbond THM or everStick NET) before composite restoration significantly reduced occlusal leakage (P<0.05).
When Clearfil SE Bond was applied without an intermediate material, all the samples showed microleakage involving the gingival margins below the CEJ (score 1) [[Figure 1]a].
When the cavities were treated with flowable resin, 70% of the samples showed gapfree occlusal margins, but 40% to 60% of the samples showed cervical leakage (scores 1 to 2). [[Figure 1]b]. Ribbond THM and everStick NET groups showed 50% to 60 % gap-free gingival margins when the cavity lines were below the CEJ. 90%Ninety percent of the samples showed no dye penetration at occlusal margins in the samples restored with Ribbond THM in combination with flowable resin [[Figure 1]c]. With everStick NET, percent leakage was the same (80% gap-free occlusal margins) when the cavity lines were above or below the CEJ [[Figure 1]d].
|Figure 1: (a) Specimen showing leakage pattern with sample restored with direct composite resin (Clearfil APX) and Clearfil SE Bond.); (b) Specimen showing leakage pattern with sample restored with SE Bond and flowable resin (Protect Liner F); (c) Specimen showing leakage pattern with sample restored with bulk composite resin, flowable resin in combination of Ribbond THM; (d) Specimen showing leakage pattern with sample restored with bulk composite resin, flowable resin in combination of EverStick NET|
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SEM images indicated that interfacial integrity of all the materials was good and no gaps were observed in the center of the cavities opposite to the cavity margins [[Figure 2]a-d].
|Figure 2: (a) SEM picture of adhesive interface of a specimen restored with SE Bond and hybrid composite in bulk; (b) SEM picture of the composite - dentin interface of a specimen restored with flowable resin; (c) SEM picture of the composite - dentin interface of a specimen restored with flowable composite adhesive resin in combination with Ribbond THM; (d) SEM picture of the composite - dentin interface of a specimen restored with flowable compositeadhesive resin in combination with EverStick NET|
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Mean values and standard deviations of experimental groups is seen in [Table 4] and Kruskal-Wallis test for statistical significance between test groups in [Table 5].
|Table 5: Kruskal-Wallis test for statistical significance between test groups |
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| Discussion|| |
Marginal microleakage is one of the major disadvantages of resin composite restorations. Failure of the material to adapt to dentin structure causes microleakage, generally at the gingival margin. Once a layer of resin composite is inserted into the cavity and is light cured, a competition between polymerization shrinkage of the composite and adhesion to the substrate begins. Stresses produced by polymerization shrinkage are critical to adhesion between the resin composite and the tooth structure. 
This shrinkage stress depends on factors, such as cavity size and shape, substrate type and location of the margins, restorative material, and the technique of placement and polymerization. If bond strength is weaker than shrinkage stresses between the resin and adhesive system, the tooth-restoration interface may break, forming a gap that will allow for marginal microleakage. 
To reduce marginal leakage, flowable composites have been recommended.  Low elasticity modulus of flowable composites used as a liner under high filled resins in posterior restorations provides flexibility for the bonded restoration and lining with them might lead to a more equal distribution of stresses over the adhesive interface and reacts as a stress breaker. ,
However, some studies suggest that flowable composites are not suited for high stress situations because of their lower elastic moduli, lower filler content, and inferior mechanical properties. Their contraction stress was shown to reach similar levels to those developed by high-viscosity composites.  On the other hand, a low-modulus lining material can reduce stresses which occur along the adhesive interface. 
The results of the present study is in accordance to previous studies which reported that flowable resin did not produce gap-free resin margins in Class II adhesive cavities or in bulk-filled restorations.  However, when the leakage patterns were evaluated at occlusal margins in the cavities with enamel margins, use of flowable resin as a liner before composite restoration reduced leakage (P<0.05). This result indicates that the lower elastic modulus of the material seems to compensate negative effects of polymerization shrinkage stresses in Class II adhesive cavities with enamel margins.
The results of a linear polymerization shrinkage study confirmed that higher the ratio of bonded to unbonded surface, the greater the shrinkage in the vertical dimension. In the present study, the ratio of bonded to unbonded surfaces was approximately 2 in cavities with gingival enamel margins. C-factor was lower in the cavities without enamel margins. The bulk technique was used for the restorations and the leakage scores were similar at gingival margins (P>0.05). However, the teeth treated with Clearfil SE Bond showed more leakage at occlusal margins when compared to the other three groups (P<0.05). One possible explanation for this result is that all the intermediate materials (flowable resin, Ribbond and everStick NET in combination with flowable resin) acted as an increment and reduced the thickness of the composite resin and reduced leakage occlusally in cavities with gingival enamel margins.
In the present study, the cavities with enamel margins were more box shaped than the cavities with dentin margins. Although having enamel cavity margins provide an adequate interfacial integrity;  cavities with enamel margins showed more leakage occlusally than the cavities without enamel margins in Clearfil SE Bond treated group (P<0.05). The three-dimensional shape of the cavity with enamel margins limited stress-relieving flowability, and the leakage increased with the increase in C-factor. This result is in accordance to the findings of previous studies which report that there is a correlation between the contraction stress and microleakage.  On the other hand, no significant difference was found in microleakage among the groups at gingival enamel margins (group 2) (P>0.05). Gap formation at occlusal surfaces should have reduced the stresses inside the cavities and reduced the leakage at gingival margins.
Recent studies have demonstrated that thicker layers of relatively low-modulus adhesive can significantly reduce the contraction stress of composite restorations and reduce the overall degree of microleakage in marginal areas. Clearfil SE Bond could not prevent leakage in the present study despite its thick film layer. The thickness of the material was not great enough to withstand the contraction stresses. 
Another study demonstrates that when fiber inserts are placed in Class II composite restorations, they increase the quality of the marginal zone in two ways. First, the fibers replace the part of the composite increment at this location, which results in a decrease in the overall volumetric polymerization contraction of the composite. Second, the fibers assist the initial increment of the composite in resisting pull-away from the margins toward the light source. The fibers may also have a strengthening effect on the composite margin, which may increase resistance to dimensional change or deformation that occurs during thermal and mechanical loading and, thus, improves marginal adaptation. 
everStick NET is a woven, continuous bidirectional glass fiber sheet impregnated with bis-GMA and PMMA. The thickness of a single fiber is 6 μ.  Ribbond THM is composed of LWUHMW polyethylene fiber with a 0.18 mm thickness. Clearfil SE Bond, which contains MDP and HEMA, was used to impregnate Ribbond THM before use. The results indicated that there was no significant difference in microleakage among the groups when the cavities were lined with everStick NET or Ribbond THM. During the restorations, both fibers were embedded into the bed of flowable resin. Because of the similarity between the structures of both fiber materials, they should have acted in the same way inside the flowable resin layer, and similar leakage scores were obtained.
In the present study, dye penetration is observed in all the samples however, the occlusal margins of cavities which were treated with Ribbond THM showed the least leakage scores [Table 6].
|Table 6: Distribution of microleakeage scores at gingival and occlusal margins |
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SEM observations indicated that the interfacial integrity was good, and both materials showed an acceptable adhesion at the cavity centers opposite to the cavity margins. Improving the marginal seal is still necessary and gap formation at the margins is a persistent problem with composite restorations.
Within the limits of this in vitro study, the authors accept the null hypothesis that flowable resin in combination with a polyethylene or glass fiber can reduce leakage at least at occlusal margins in Class II adhesive cavities with enamel.
| Conclusion|| |
Use of flowable composite alone or in combination with polyethylene or glass fibers reduces occlusal leakage in Class II adhesive cavities with enamel margins.
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[Figure 1], [Figure 2]
[Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6]