|Year : 2017 | Volume
| Issue : 1 | Page : 6-11
A comparative evaluation of the internal adaptation of various lining materials to dentin under light cure composite restorations: A scanning electron microscope study
Nishin K John, KV Manoj, Bobby Joseph, Aby Kuruvilla, Noushin Faizal, Binila S Babu
Department of Conservative Dentistry and Endodontics, Al-Azhar Dental College, Thodupuzha, Kerala, India
|Date of Web Publication||28-Feb-2017|
Nishin K John
Department of Conservative Dentistry and Endodontics, Al-Azhar Dental College, Thodupuzha, Kerala
Source of Support: None, Conflict of Interest: None
Aims and Objectives: To examine adaptation of lining materials to dentin and resin under scanning electron microscope in Class V restorations. Materials and Methods: Seventy-five caries-free extracted maxillary and mandibular molars were selected, and Class V restoration cavities prepared. The cavities were filled with Dycal as liner, resin-modified glass ionomer (Vitrebond [VT]) as base and restored with composite in different combinations. The statistical software SPSS version 11.0 and Systat 8.0 were used for the analysis of the data. Results: Microgaps were seen when Dycal and VT were used as liners and bases under composite resin restorations. Conclusion: Hybridization of dentin significantly reduced microgaps, especially after adhesive was applied.
Keywords: Calcium hydroxide, composite resins, dental materials, glass ionomer, polymerization shrinkage
|How to cite this article:|
John NK, Manoj K V, Joseph B, Kuruvilla A, Faizal N, Babu BS. A comparative evaluation of the internal adaptation of various lining materials to dentin under light cure composite restorations: A scanning electron microscope study. J Int Oral Health 2017;9:6-11
|How to cite this URL:|
John NK, Manoj K V, Joseph B, Kuruvilla A, Faizal N, Babu BS. A comparative evaluation of the internal adaptation of various lining materials to dentin under light cure composite restorations: A scanning electron microscope study. J Int Oral Health [serial online] 2017 [cited 2019 Nov 14];9:6-11. Available from: http://www.jioh.org/text.asp?2017/9/1/6/201089
| Introduction|| |
For decades, restorative material for cavities in teeth was amalgam. In the recent years, its usage has been limited mainly due to esthetic concerns, other reasons being mercury toxicity, lack of bonding to tooth, and galvanic interactions with other materials. Hence, composite resin restorative materials were introduced to overcome amalgam drawbacks.
Apart from esthetics, the purpose of introducing composite resins was to secure an adhesive bond between the restorative material and the cavity walls, which will resist microleakage, thereby preventing postoperative sensitivity and recurrent caries. Success of light cure composite resins depends on perfect marginal adaptation to cavity walls and a gap-free internal bond between composite resin and dentin of tooth. Even after continuing efforts by manufacturers to improve resin-based composites, polymerization shrinkage still remains the main drawback of these polymerizable resins.
Factors that influence the magnitude of forces at bonded interfaces are polymerization shrinkage, adherence of the resin to the cavity walls, elastic modulus, and viscous flow of the resin and the configuration factor of the resin, i.e., the ratio of bonded to unbonded composite surfaces.,
Heitmann and Unterbrink observed pulpal reaction with similar to that seen with silicate cements and suggested to use lining materials based on calcium hydroxide. According to Brännström and Nyborg, microinfiltration between the restoration and cavity walls is the chief cause of pulpal irritation and suggested to use lining materials to prevent pulpal damage.
Bonding strength of composite resins must be high enough to counteract the dimensional changes that occur during resin polymerization, thereby preventing disruption of marginal sealing; otherwise marginal gap formation will result., Here, we carry out our study to evaluate the internal adaptation of various lining materials to dentin under light cure composite restorations. To the best of our knowledge after extensive literature review, ours is the first such study evaluating lining materials' adaptation to dentin under composite resins in vitro under scanning electron microscope (SEM).
We carried out this in vitro study to observe microgaps in Class V restorations at the junction between the dentin, adhesive system, lining materials (Dycal and Vitrebond [VT]), and composites.
| Materials and Methods|| |
Seventy-five caries-free extracted maxillary and mandibular molars (30 maxillary and 45 mandibular randomly selected) were included in this retrospective study, from January 2015 to May 2015.
Inclusion criteria were molar teeth, teeth free of cracks and defects, and noncarious teeth
Exclusion criteria were carious teeth, nonvital teeth, and teeth which undergone regressive alterations such as attrition, abrasion, and erosion.
The teeth were cleaned of gross debris and stains with ultrasonic scaler. The teeth were examined under ×10 magnification for finding whether they had any breaks, flaws, or cavities. Then, they were kept in 0.5% chloramine agent for infection control purpose until they were used. Class V cavity form was prepared either on facial or lingual surface depending on whichever surface is broad with high-speed Airotor and No. 245 carbide bur under copious water irrigation. The preparations were circular with cavity dimensions of 6 mm diameter; 3 mm of total depth, in which depth of the cavity from the outer surface was 2 mm for composite (3M ESPE) and the remaining 0.5 mm for base and rest of the 0.5 mm depth was for the liner, respectively. This was done to simulate the variations that arise in natural situation. Cervical margin of the cavities was prepared about 1 mm beneath the crown-root interface. Cavosurface bevels were not made. The burs were changed for every five preparations. Cavity measurements were gauged by means of a periodontal probe. The cavities were filled with Dycal (calcium hydroxide-DENTSPLY) as liner, VT (resin modified glass ionomer-3M ESPE) as base and restored with composite in different combinations as well as in different orders [Table 1]. The bonding agent used was self-etching adhesive (Adper Prompt-3M ESPE). All the materials were used according to the manufacturer's instructions.
The liner was prepared according to the manufacturer's instructions, by mixing identical quantities of base and catalyst. Then, the obtained cement was placed into the floor of the preparation and was left to set for a time of 4 min.
The base was prepared by mixing the powder and liquid according to the manufacturer's instructions and then was placed in the prepared cavity and cured with light.
Equal drops of both liquid A and liquid B bottles of Adper were added to the mixing well and mixed with an applicator tip until uniform color appeared. The mixed adhesive was brushed onto the cavity surface for 15 s, with a gentle stream of air the adhesive was dried to thin film. Second coat was applied to the surface and again with a gentle stream of air the adhesive was dried to thin film. It was light cured for 10 s.
The prepared teeth were randomly assigned to five groups [Table 2].
For all the groups, horizontal increments of <2 mm were successively placed and cured with for 40 s incrementally using soft-start light emitting diode (Turbo LED, Anthos, Germany) by increasing the intensity of light gradually beginning with 80–540 mW/cm 2. The specimens were finished and polished immediately after restoration of the cavity. Later, filled specimens were finished using abrasive discs (SofLex pop-on, 3M ESPE) and kept in distilled water. After they were thermocycled, the teeth were cut to remove roots and then mesiodistally into two halves with water-cooled diamond disc. Subsequently, each half was sectioned along the longitudinal axis through the center of the restoration.
Then, the specimens were polished using silicon carbide papers. Later, they were ultrasonicated with absolute alcohol for approximately 10 min to remove the debris (for only a few minutes so that Dycal will not get dislodged), and the restored surfaces were etched with 10% orthophosphoric acid gel for 5 s and cleaned for 10 s to remove smear layer. Then, they were then dehydrated and dried. Subsequently, the specimens were mounted on aluminum stubs, sputter coated with gold-palladium (5 nm thickness), and examined under SEM (JSM 840 A JEOL LTD, Japan) by direct method at an acceleration voltage of 7–10 kV.
To compare the means of all the groups, we used two-way analysis of variance test. The statistical software SPSS version 11.0 and Systat 8.0: SPSS Inc. Chicago, Illinois, USA were used for the analysis of the data and Microsoft Word and Excel have been used to generate graphs and tables. To find out, in which group there exists a significant difference, we carried out multiple comparisons using Bonferroni test and the results are given below.
| Results|| |
Under SEM, Group 1 showed excellent adaptation between dentin and composite [Figure 1], Group 2 showed gap between dentin and Dycal and minimal gap between Dycal and composite [Figure 2], Group 3 showed gap between dentin and Dycal as well as between Dycal and VT, whereas excellent adaptation was seen between VT and composite resin [Figure 3], Group 4 showed gap between dentin and VT and excellent adaptation between VT and composite [Figure 4], and Group 5 showed minimal gap between dentin and VT due to the presence of adhesive in between and excellent adaptation was seen between VT and composite resin [Figure 5].
|Figure 1: Scanning electron microscopy diagram of Group 1 showing excellent adaptation between dentin and composite|
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|Figure 2: Scanning electron microscopy of Group 2 showing gap between dentin and Dycal and minimal gap between Dycal and composite|
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|Figure 3: Group 3 showed gap between dentin and Dycal and Dycal and Vitrebond also, whereas excellent adaptation between Vitrebond and composite was seen|
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|Figure 4: Group 4 showed a gap between dentin and Vitrebond and excellent adaptation between Vitrebond and composite|
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|Figure 5: Group 5 showed a minimal gap between dentin and Vitrebond due to the presence of adhesive in between and excellent adaptation between Vitrebond and composite resin was seen|
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Gap distances between dentin and materials in all the groups were noted [Table 3] and [Figure 6]. We observed that there was a significant difference (P < 0.001) between:
- Group 2 – Distance 1 and Group 2 – Distance 2
- Group 2 – Distance 1 and Group 3 – Distance 1 and Group 3 – Distance 2
- Group 2 – Distance 1 and Group 4 – Distance 1 and Group 4 – Distance 2
- Group 2 – Distance 1 and Group 5 – Distance 1 and Group 5 – Distance 2
- Group 2 – Distance 2 and Group 3 – Distance 1
- Group 3 – Distance 1 and Group 3 – Distance 2
- Group 3 – Distance 1 and Group 4 – Distance 2
- Group 3 – Distance 1 and Group 5 – Distance 2.
Whereas we observed that the distances between the following groups were not significantly significant (P > 0.05).
- Group 4 – Distance 1 and Group 4 – Distance 2
- Group 4 – Distance 1 and Group 5 – Distance 1
- Group 4 – Distance 2 and Group 5 – Distance 2.
| Discussion|| |
In this study, the samples selected were molars for standardization. Class V cavity design was selected as it limits the flow of the composite resin materials because only one surface of material will be unbonded. In our study, contraction stresses caused adhesive failure at the dentin-filling interface in many cases and microretentive surface of the adjacent acid-etched enamel neutralized these shrinkage forces and resulted in a tight marginal seal.
|Figure 6: Graph showing gap distances between dentin and materials in all the groups|
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Gaps between various interfaces are influenced by several factors such as initial adaptation of restorative material to the walls of the cavity, bond strength and ability of the bond to resist strain, and incorporation of nanofiller particles. The advantage of nanofiller particles is that they improve thermomechanical properties and also reduce the degradation of restorative material due to their larger surface area. Success can be achieved by proper cavity design and following correct procedures as indicated by manufacturer.
Polymerization shrinkage of light-cured composite resins was found to be 2%–6% of volume. Yoshikawa et al. used low-intensity light initially followed by high-intensity light and found that adaptation of resin to cavity walls was better and less polymerization contraction stress. Therefore, soft-start light-emitting diode was used in this study. Lopes et al. found that incremental placement of resin provided a better seal than the bulk condensation technique. Hence, incremental buildup of composite was employed in our study.
As polymerization shrinkage was proportional to the volume of the resin cured, sandwich or laminate restorations with resin-modified glass ionomer cement were used to restrict the volume and resulted in gap-free interfacial adaptation.
Irie et al. found no significant difference in the gap formation when the microfilled composite was polished immediately, after 30 min, 3 h, 12 h, 24 h, and 1 week. In our study, polishing was done immediately after restoration of the cavity.
In Group 1, all the samples showed excellent adaptation between the resin and dentin interface which is considered very favorable. This might be due to the shear bond strength overcoming the polymerization contraction forces of resin composite.,
In Group 2, where Dycal was used as a liner, marked gaps were seen between Dycal and dentin, whereas there were no gaps seen between Dycal and composite resin. This might be due to the lack of bonding of Dycal with dentin. The pulling forces of polymerization shrinkage stress might have caused these gaps. Another reason for the gaps could have been the sectioning process or ultrasonication process which was done to remove debris. In few of the samples, it was observed that Dycal was totally missing, this dislodgment of Dycal which might be due to the ultrasonication as was found in other studies.
In Group 3, marked gaps were seen between dentin and Dycal and also between Dycal and VT. This might be attributed to the lack of adhesion of Dycal with dentin and VT. In this group also, few of the samples showed dislodgment of Dycal which might be due to the ultrasonication as was seen in other studies.,
VT and composite showed excellent adaptation. VT contains hydroxyethyl methacrylate (HEMA) and polyacrylic acid, its adaptation to dentin and composite is due to the mild etching effect of acid on dentin, and bonding was due to HEMA, respectively. These factors helped to form an interface which was free of microgaps.
The presence of microgaps between Dycal and dentin as found in Groups 2 and 3 might be due to the separation of the calcium hydroxide from the dentinal surface which was caused by polymerization shrinkage of the resin.
In Group 4, small microgaps were found at the interface between dentin and VT, whereas the interface of VT and composite showed good adaptation. However, the difference between the microgaps present between dentin and VT and the microgaps at the interface of VT and composite was found to be statistically insignificant. Chemical bond between dentin and VT usually has a bond strength in the range of 12–15 MPa, which is insufficient to defy the contraction stresses of composites so as to prevent dislodgment.
The dentin-VT and VT-composite junctions of Group 5 showed very few microgaps. Our finding showed that before applying resin-modified glass ionomer, hybridization of dentin, augments bonding. Otherwise, microleakage ensues leading to complications such as sensitivity and marginal staining. Poggio et al. found that resin-modified glass ionomer cement (RMGIC) had considerably superior shear bond strength to dentin than glass ionomer. According to them, the better bonding might be due to action of HEMA in RMGIC.,
Hannig and Friedrichs found that the interfacial adaptation between dentin and composite resins was significantly better in the cavities prepared in the laboratory than the cavities prepared in the patients' mouth. According to them, presence of living odontoblastic processes and fluid in the dentinal tubules of vital teeth might be responsible for this difference. In our study, extracted teeth were used, and the interfacial adaptation of composite with dentin was very good in Group 1. During the SEM evaluation of the specimens, it was noticed that there was a concentration of bonding agent at the corners of the cavity. This finding may be due to pushing of bonding agent to the corners of cavity by the compressed air that is used to dry the agent, and this might be seen as a thick layer of adhesive at the corner of the cavity.
In our study, thermocycling was done to simulate the oral conditions. Wattanawongpitak et al. evaluated the adaptation of composite to the cavity wall and stated that thermocycling up to 500 cycles of thermal stress gives an excellent adaptation, but above 500 cycles, there was an increase in marginal leakage.
Two methods are generally used for examining natural teeth by SEM, direct and indirect methods. In our study, direct method was used for examination of the samples. We observed that liners usually will not have sufficient bond strength to prevent microgaps created due to shrinkage of composites due to polymerization. These microgaps were lessened after resin hybridization. On the other hand, many studies showed pulp damage after application of adhesive systems.
In Group I, IV, and V, there was good adaptation with their respective materials and dentin as well. Hence, it is recommended to use these liners and bases under composite restorations, provided the marginal seal is intact. There are a number of reports which say that in deep cavities, the use of bonding agent is controversial and causes more intense pulpal response than unetched deep dentin. However, the intensity of the pulpo-dentin complex response depends on the remaining dentin thickness. Thus, the clinician should be able to judge and decide the correct and suitable agents for every circumstance based on the knowledge about adhesive system, biocompatibility of the materials available.
Further studies are necessary to suggest that Dycal can be used without any inhibitions under composite resin restorations, may be in vivo studies with long-term follow-up of the outcome are required.
| Conclusion|| |
Within the limitations of our study, the following conclusions were drawn.
- Hybridization of dentin significantly reduced microgaps
- Very few microgaps observed after application of adhesive
- Axial microgaps were seen when Dycal and VT were used as liners and bases under composite resin restorations.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6]
[Table 1], [Table 2], [Table 3]