|DENTAL SCIENCE - ORIGINAL ARTICLE
|Year : 2014 | Volume
| Issue : 5 | Page : 74-79
Scanning electron microscopic study of teeth restored with fiber posts and composite resin: An in vitro study
KS Sridhara1, Sunil Mankar2, CM Jayshankar3, K Vinaya4
1 Department of Conservative Dentistry, Krishnadevaraya College of Dental Sciences, Bengaluru, Karnataka, India
2 DAPMRV Dental College and Hospital, Bengaluru, Karnataka, India
3 Siddartha Dental College and Hospital, Tumkur, Karnataka, India
4 Department of Prosthodontics, M. R. Ambedkar Dental College and Hospital, Bengaluru, Karnataka, India
|Date of Submission||30-Mar-2014|
|Date of Decision||30-Mar-2014|
|Date of Acceptance||09-Apr-2014|
|Date of Web Publication||25-Jul-2014|
Dr. Sunil Mankar
DAPMRV Dental College and Hospital, Bengaluru, Karnataka
Source of Support: None, Conflict of Interest: None
| Abstract|| |
Aims and Objectives: The aim of this study is to compare and evaluate the thickness of resin dentin interface zones (RDIZ) obtained by luting carbon fiber post to intra-radicular dentin, either with All-Bond 2 bonding agent and C and B composite cement or Panavia F dentin-bonding system and Panavia F resin cement. Materials and Methods: Twenty single rooted mandibular premolars of similar sizes were prepared for the carbon fiber post after biomechanical preparation and obturation. They were divided into two groups, Group 1 and 2 of 10 samples each. Carbon fiber posts used for Group 1 samples were luted using All-Bond 2 and C and B cement. For Group 2 carbon fiber posts were luted using Panavia F dentin-bonding system and Panavia F resin cement. All the 20 samples were sectioned longitudinally and marked at three points on the length of the tooth from the dentin-core interface to the apex at 2 mm, 5 mm, and 8 mm to get coronal, middle, and apical areas, respectively. The formation and thickness (width) of the RDIZ at the marked areas was evaluated by scanning electron microscope using ×1000 magnification. The results were statistical analyzed. Results: Irrespective of the adhesive systems used all specimens showed a RDIZ formation. Microscopic examination of Group 1 showed significantly higher percentage of RDIZ (P < 0.05) than Group 2. RDIZ morphology was easily detectable at coronal and middle areas of all specimens. Conclusion: All-Bond 2 showed denser and wider RDIZ compared with the Panavia F.
Keywords: Bonding, fiber post, hybrid layer, resistance
|How to cite this article:|
Sridhara K S, Mankar S, Jayshankar C M, Vinaya K. Scanning electron microscopic study of teeth restored with fiber posts and composite resin: An in vitro study. J Pharm Bioall Sci 2014;6, Suppl S1:74-9
|How to cite this URL:|
Sridhara K S, Mankar S, Jayshankar C M, Vinaya K. Scanning electron microscopic study of teeth restored with fiber posts and composite resin: An in vitro study. J Pharm Bioall Sci [serial online] 2014 [cited 2019 Nov 15];6, Suppl S1:74-9. Available from: http://www.jpbsonline.org/text.asp?2014/6/5/74/137392
Endodontically treated teeth with defective clinical crowns very often need to be restored with a post and core as a foundation for the final restoration. Commonly used form of restoration for the endodontically treated teeth are cast posts, which shows several disadvantages such as, loss of retention, potential for the post and root fracture, and a risk of corrosion when different metals are used in the system. 
Carbon reinforced resins were considered viable alternatives to metals in many fields, when strength, lightness, and resistance to corrosion and fatigue are required. Carbon compounds in different forms have been studied in various dental and surgical applications. Where they have proved to be biocompatible and mechanically satisfactory for many purposes. 
All posts, to a greater or lesser extent, gain their final retention by cementation into the prepared root canal. Studies have provided conflicting results in relation to the adequacy of different types of cement to retain dental posts. ,, The ability of different cements to retain dental posts is related to: (1) The mechanical properties of the cement, (2) the bonding efficiency of the cement to the two surfaces being joined, (3) the durability of cement and (4) the configuration of the post and the prepared canal, which influence the stress distribution within the cement layer.
Recently, the use of adhesive systems for post cementation has increased in popularity, which avoids friction between post and root canal walls.  The aim of the present in vitro study is to compare the scanning electron microscopic (SEM) evaluations of resin dentin interface zones (RDIZ) obtained after luting carbon fiber post either with All-Bond 2 with C and B composite cement or Panavia F dentin-bonding system and Panavia F resin cement.
| Materials and Methods|| |
Twenty single rooted mandibular premolars of similar sizes were selected, which were removed for periodontal and orthodontic reasons free of caries, resorptive defects, calculus, and fractures. The teeth were cleaned of debris, calculus, and any periodontal ligament remnants and stored in normal saline at room temperature.
The crowns were horizontally sectioned using sectioning disks and the lengths of the teeth were standardized to 14 mm. The endodontic treatment followed the step back technique. The canals were instrumented with K - files and H - file under irrigation with 2.5% sodium hypochlorite and final irrigation with normal saline. The canals were then obturated using lateral condensation method with Gutta-percha cones combined with AH26 sealer. The root diameter was enlarged with low-speed drill provided by the manufacturer to a depth of 9 mm from the cementoenamel junction.
The 20 specimens were randomly divided into two groups of 10 teeth each and treated as follows:
- Group 1: Carbon fiber post cemented with All Bond 2 and C and B cement
- Group 2: Carbon fiber post cemented with Panavia F dentin-bonding system and Panavia F resin cement.
All samples in Group 1 were etched with 37% of phosphoric acid for 30 s, washed with a water syringe and gently air-dried. Three drops of All-Bond 2 primer A and B were mixed and 4-5 coats of primer adhesive were applied into the canal wall with a small brush provided by the manufacturer. Excess primer adhesive was gently removed with a paper point before applying a layer of Dentin prebond resin onto the canal walls with a small brush provided by the manufacturer. A layer of All-Bond-2 primer B was applied on #1 carbon fiber post before C and B cement was applied on the post surface and the post was inserted into the canal and the cement was allowed to set.
Samples in Group 2 were treated with Panavia F bonding systems by mixing three drops of liquid A and B of ED primer, 4-5 coats of primer adhesive was applied on the root canal walls using a small brush provided by the manufacturer. Excess primer was gently removed with a paper point. Panavia F resin cement was applied on the #1 carbon fiber post surface after priming the samples for 60 s. Posts were then inserted into the canal, a small amount of the oxyguard gel, provided by the manufacturer was applied around the canal access for 3 min, to allow the anaerobic setting reaction of the Panavia F.
Core was built for all 20 samples in Group 1 and 2 with Filtek P60 light cure polymerizing composite resin and was stored in saline solution for 3 weeks. The teeth were then longitudinally sectioned along the long axis of the tooth using a diamond disc at a slow speed under water.
Three locating notches for standardized examination of the RDIZ were made, by a scalpel perpendicular to the long axis of the sectioned tooth at 2 mm, 5 mm, and 8 mm from the dentin-core interface toward the apex to form the coronal, middle, and apical areas.
The sectioned surface of the sample was etched with 10% phosphoric acid for 15 s, washed for 30 s with tap water and then deproteinized for 30 s with 2% sodium hypochlorite. Each section was mounted on aluminum stubs and sputter coated with gold as shown in [Figure 1]. The formation and thickness (width) of the RDIZ or hybrid layer of the dentin-bonding systems were evaluated at the previously marked areas by scanning electron microscope using ×1000 magnification as shown in [Figure 2] and [Figure 3]. The results obtained were subjected to statistical analysis.
|Figure 2: Scanning electron microscopic image of resin dentin interface zones of sample bonded with All Bond 2 and C and B cement: (a) Coronal area (b) Middle area (c) Apical area|
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|Figure 3: Scanning electron microscopic image of resin dentin interface zones of sample bonded with Panavia F dentin-bonding system and Panavia F resin cement: (a) Coronal area, (b) Middle area, (c) Apical area|
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There is a significant difference in the mean shear strength of the two groups, that is, μ1 ≠ μ2 .
Level of significance
In order to determine whether there was a difference between All-Bond-2 and Panavia F2 hybrid layer measurements student t-test was used.
The P value is compared with the level of significance, which is also termed as the confidence interval. The hypothesis is accepted if the P value is significant at < 0.05 level of significance and it is concluded that there is a significant difference in the mean measurement of the groups. Otherwise, the hypothesis is rejected.
| Results|| |
Results showed that the RDIZ (hybrid layer) at the coronal, middle, and apical areas formed by All-Bond 2 with C and B resin cement were denser when compared with the Panavia F bonding system and Panavia F resin cement [Table 1] and [Graph 1].
|Table 1: Comparisons of mean values of the RDIZ between All-Bond-2 and Panavia F |
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Among the observations made for the three areas within the group for Group 1 that is, carbon fiber post cemented with All Bond 2 and C and B resin cement the values at the coronal area came high followed by middle area. The apical area showed the least values [Table 2].
The observations made for the three areas within the group for Group 2 that is, carbon fiber post cemented with carbon fiber post cemented with Panavia F dentin-bonding system and Panavia F resin cement showed the same pattern as that of Group 1 [Table 3].
| Discussion|| |
The longevity of endodontically involved teeth has been greatly enhanced by continuing developments made in endodontic therapy and restorative procedures. It has been reported that a large number of endodontically treated teeth were restored to the original function with the use of intra-radicular devices. 
Several hypotheses have been formulated to explain the weakening action of the posts cemented into the root canal: It was thought that the increase of the cement layer thickness was able to increase fracture rates, and therefore the use of tapered and cast posts was suggested, but it was shown that cement thickness did not increase the risk of root fracture. 
The cementation technique must also be taken into consideration because the mechanical stress induced in the root structure by cementation procedures of the so-called active posts can cause root fractures both during the cementation and during functional loading. The use of an adhesive technique might reduce the stress created by post cementation. 
The post core systems include components of different rigidity. Since, the more rigid component was able to resist forces without distortion; stress was transferred to the less rigid substrate and causes its failure. The difference between the elastic modulus of dentin and post material was a source of stress for the root structures. Therefore, the use of composite, glass ionomers, and amalgam pins has been advocated to avoid the insertion of metal posts into the root canal. It was found that endodontically treated premolars restored with acid etch resin technique were more resistant to root fractures than teeth restored with glass ionomer cement. 
In vitro resistance to fracture of endodontically treated molars, the cuspal coverage was more important than the preservation of tooth structure. Unfortunately, if no post was cemented into the root canal, the link that can be obtained between the radicular and coronal parts of the restorative materials may not be strong enough to resist the chewing forces resulting in horizontal crown or root fractures. 
To achieve optimal results, the materials used for the posts should have physical properties similar to that of dentin, be bonded to the tooth structure, and be biocompatible in the oral environment.  It should also act as a shock absorber by transmitting only limited stress to the residual tooth structure. 
Carbon fiber posts consist of pyrolytic carbon fibers arranged longitudinally in an epoxy resin matrix with the carbon component constituting 64% of the structure. They provide advantages such as biocompatibility, corrosion resistance, strength, and modulus of elasticity similar to dentin. 
The presence of a good seal at the resin dentin interface of composite cores used for crown build ups of endodontically treated teeth was a fundamental step for prevention of bacterial penetration, recurrent caries, decementation, and root fracture. 
Newer adhesive resin luting agents were advocated for the luting of posts because they were reported to bond the post to the tooth structure in comparison to the traditional cements, which produce only frictional resistance. 
Historically, dentin-bonding agents were categorized into generations based on chemistry and the manner in which they treat the smear layer a new classification of adhesives was introduced based on the number of working steps and treatment of smear layer in 2000, ,, accordingly, four types of adhesives have been listed:
- Type I adhesives (formerly 4 th generation): Removes smear layer, comprises of three steps-etching, priming, bonding
- Type II adhesives (formerly 5 th generation): Also removes smear layer, is a two-step system as priming and bonding is combined in one step
- Type III adhesives: Two step self-etching systems
- Type IV adhesives: Self-etching one step all in one products
- The Type III and IV adhesives are regarded as 6 th generation systems. With the development of Type I and II (4 th and 5 th generations) bonding systems, two strategies for bonding have evolved.
Van Meerbeek et al. using SEM confirmed that the application of recent adhesive systems induced structural changes in the dentin surface morphology, creating a retentive interface called the Inter Diffusion Zone between the deep, untouched dentin layers and the composite filling material. 
The penetration of resin into the decalcified dentin and its subsequent polymerization results in a hybrid or resin reinforced layer that serves as a transitional zone between the intact dentin substrate and the cured resin. 
Direnzo et al. suggested that adhesives bind chemically to the dentin substrate,  later many investigators in their study have shown that no covalent or primary chemical bonding between the dentin adhesives and dentin substrate. ,
Through a series of microscopic investigations, it was well-accepted that bonding mechanism of adhesive systems to root dentin substrate is essentially of a micro-mechanical nature, based on infiltration of the demineralized surface, and on the formation of RDIZ, resin tags, and adhesive lateral branches. ,,
In this study, the uniformity and thickness of RDIZ was measured using Scanning Electron Microscope at three areas coronal, middle, and apical from the dentin-core junction.
The RDIZ formed by the All-Bond-2 was more uniform and wider in all the three areas tested than the Panavia F, probably due to the result of an increase in the surface area made available for bonding by the effect of etching the dentin. , Furthermore, three step bonding agent All-Bond-2 forms a wider RDIZ by removing the smear layer from the dentinal surface, than the self-etching bonding agent Panavia F which only modifies the smear later without completely removing it from the dentinal surface. ,,,
The morphology of the RDIZ was well observed in the coronal and middle areas than at the apical areas in both groups, it could be due to:
- Increased primer adhesive application pressure by the brush was maximum at the coronal and middle thirds than at the apical thirds resulting in lesser penetration of the primer adhesive into and around the tubules 
- Increased density of tubules in coronal and middle areas 
- The gaps noted between the RDIZ and resin cement in a few samples were probably caused by vacuum pressure of the sputtering device and SEM chamber. 
The self-etching bonding agent must diffuse by demineralizing through the smear layer and into sound dentin to produce high bond strength. Because smear layer have a significant buffer capacity and tend to buffer the acidity of acidic monomer used as the self-etching agent. This property in addition to tight packing smear layer particles to each other seems to limit the depth of penetration of monomer.
The findings of this in vitro study do not represent a hybridization standard for the adhesive systems used and cannot be directly correlated to assert clinical effectiveness, since other factors are also involved including the dentinal substrate and the relationship between dentinal substrate and adhesive system. Hence, long-term clinical trials are necessary to substantiate these results and establish the superiority of one over the other.
| Conclusion|| |
Deriving from the results it could be concluded that,
• Resin dentin interface zone is well observed in the coronal and middle thirds than apical thirds of the tested samples in both groups
• The RDIZ formed at all the three tested areas, coronal, middle and apical, with All-Bond-2 is wider (with a mean value of coronal 6.765, middle 5.298 and apical 2.759) than Panavia F (mean value of coronal 4.441, middle 3.559, and apical 1.099) and is statistically significant.
Hence, "three-step" bonding system is more effective and can create wider micro-mechanical interlocking between the adhesive material and the etched dentin than the "one-step" system and therefore can be used as a bonding agent for the fiber post.
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[Figure 1], [Figure 2], [Figure 3]
[Table 1], [Table 2], [Table 3]