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ORIGINAL ARTICLE
Year : 2019  |  Volume : 22  |  Issue : 3  |  Page : 328-334

Evaluation of the influence of various restoration techniques on fracture resistance of endodontically treated teeth with different cavity wall thicknesses


1 Department of Restorative Dentistry, Yeditepe University Faculty of Dentistry, Istanbul, Turkey
2 Department of Restorative Dentistry, Istanbul University Faculty of Dentistry, Istanbul, Turkey

Date of Acceptance21-Nov-2018
Date of Web Publication6-Mar-2019

Correspondence Address:
Dr. E T Basaran
Yeditepe Universitesi Dis Hekimligi Fakultesi Bagdat Cad. No: 238, 34728 Kadikoy, Istanbul
Turkey
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/njcp.njcp_346_18

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   Abstract 


Aims: The aim of this study was to compare the effect of different restoration techniques on fracture resistance of endodontically treated teeth with different wall thicknesses. Materials and Methods: Extracted and endodontically treated 210 premolars were randomly divided into three thickness groups [2 mm (A), 1.5 mm (B), and 1 mm (C)] and, each group was further divided into seven restoration subgroups (n = 10): direct composite (control) (K), composite with fiber on cavity floor (KT), composite with fiber on occlusal level (KO), fiber post and composite (FP), inlay (L), fiber on cavity floor and inlay (LT), and inlay and fiber on occlusal level (LO). Fracture test was performed, and data were compared with Kruskal–Wallis and Mann–Whitney U tests (P < 0.05). Results: There were no differences between the subgroups in A and C statistically (P > 0.05). However, in B, KO subgroup showed statistically higher values (P = 0.039). Conclusion: Wall support of 2 mm was adequate, and support of 1 mm was completely insufficient. When the wall thickness was 1.5 mm, direct restoration with fiber at the occlusal level significantly improved resistance.

Keywords: Composite restoration, compressive strength, endodontically treated teeth, reinforcement materials, tooth fracture


How to cite this article:
Basaran E T, Gokce Y. Evaluation of the influence of various restoration techniques on fracture resistance of endodontically treated teeth with different cavity wall thicknesses. Niger J Clin Pract 2019;22:328-34

How to cite this URL:
Basaran E T, Gokce Y. Evaluation of the influence of various restoration techniques on fracture resistance of endodontically treated teeth with different cavity wall thicknesses. Niger J Clin Pract [serial online] 2019 [cited 2019 Mar 25];22:328-34. Available from: http://www.njcponline.com/text.asp?2019/22/3/328/253451




   Introduction Top


Endodontically treated teeth have critically higher risk of fracture compared with vital teeth, because of their structural differences and very large tissue losses. Huang et al. investigated the resistance to compressive and tensile forces of dry and moisturized dentin specimens, and whether the brittleness of endodontically treated teeth is related to the reduction in moisture content. As a result, they reported that moist samples were more resistant to forces.[1] In a previous study, the authors stated that the hardness of an endodontically treated tooth was reduced by 5%, whereas the hardness of a tooth prepared with MOD cavity preparation decreased by 60%.[2] For these reasons, the choice of restoration to be applied after endodontic treatment has great importance.

In addition to direct composite resin restorations, another type of restoration that is required to be applied to teeth with excess tissue loss is indirect restorations fabricated with various materials. In recent years, such restorations (inlay/onlay) have often been preferred because they cover and protect existing tooth tissues.[3],[4]

Contrarily, when tissue loss is excessive, different applications are made to increase the support of composite resins and make the remaining dental tissue more resistant. For this purpose, the use of additional fiber structures with an elasticity modulus close to the dental tissue has come to the forefront. In teeth with excess tissue loss, the retaining structures to which composite resins were applied were evaluated separately in previous studies.[5],[6],[7],[8] These applications, which are used in composite restorations, are shown in studies to provide support to the remaining tooth tissues.[9],[10],[11],[12]

Until now, studies have shown that additional support structures contribute to the fracture resistance of teeth with endodontic treatment, but to the authors' knowledge, there is no existing study in which these support structures are evaluated and compared. Moreover, in previous studies, there was no research project in this context, nor have any studies investigated the use of these applications regarding the effects of different amounts of tissue loss on endodontically treated teeth.

The aim of this study was to evaluate the effect of direct and indirect restorations and additional fiber applications on the fracture resistance of endodontically treated teeth with different cavity wall thicknesses. The null hypothesis was that the reinforcement applications would not affect the restorations' fracture resistance in all wall thickness groups.


   Materials and Methods Top


Approval for this study was obtained from the Ethics Committee of Istanbul University Faculty of Dentistry (No. 2015/40). The study was conducted in full accordance with the World Medical Association Declaration of Helsinki of 1975, as revised in 2000. Each patient provided informed written consent, and the ethics committee form was signed by each patient at the beginning of the study. The clinical part of this study included the collection of teeth, which were extracted for periodontal and orthodontic reasons in patients ranging in age from 20 to 30 years to avoid structural variability.

A sample size of 10 within each subgroup for the fracture resistance test was determined using power analysis with α = 0.05 and 80% power to detect a large effect size. A total of 210 human premolar teeth, extracted for periodontal and orthodontic reasons, with similar dimensions were selected and stored in 0.5% chloramine-T solution at 4°C until use. The materials and their compositions used in the study are shown in [Table 1].
Table 1: Product names and compositions of the materials used in this study

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Endodontic procedures

Endodontic access cavities were prepared using a diamond bur with water-cooling high-speed hand instrument, and then the pulp tissue was removed. The working length was set 1 mm shorter than apical foramen with a size 15 K-file (Dentsply Maillefer, Ballaigues, Switzerland). The canals were prepared to a size 30 K-file at working length with a step-back technique. One milliliter of 1% sodium hypochlorite (NaOCl) and 1 mL of 15% ethylenediaminetetraacetic acid solution (Ogna Laboratori Farmaceutici, Muggiò, Italy) were used for irrigation during canal instrumentation. The canals were dried and then filled with gutta-percha (Diadent International, Inc. Vancouver, BC, Canada) and sealer (AH Plus; Dentsply DeTrey, Konstanz, Germany) using the lateral condensation technique.

Cavity preparation and restoration procedures

After root canal treatments, specimens were divided into three groups according to the remaining wall thicknesses, which were 2 mm (A), 1.5 mm (B), and 1 mm (C), respectively, by measuring with a digital caliber [Figure 1]. MOD cavities were prepared using diamond bur for both direct and indirect restorations. Floor margins of MOD cavities were ended 1 mm coronal to the cement–enamel junction (CEJ). The teeth were placed in self-curing acrylic resin at the 2 mm apical side of the CEJ. These teeth were then divided into seven subgroups according to the type of restoration to be applied (n = 10).
Figure 1: Adjusting cavity walls with caliper (a: 2 mm, b: 1.5 mm, c: 1 mm)

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Group K: Phosphoric acid 35% (Scotchbond Universal Etchant; 3M ESPE, St. Paul, MN, USA) and universal adhesive system (Single Bond Universal; 3M ESPE) were applied to the cavities according to the etch and rinse concept and the manufacturers' instructions. Then, the adhesive system was cured for 10 s with a halogen curing light (Optilux 501; Kerr Corp., USA). The cavity floor was covered with a 1-mm layer of flowable composite (Filtek Ultimate Flowable; 3M ESPE) and cured. Then cavities were restored with composite resin (Filtek Z250; 3M ESPE) using layering technique. Each layer was light-cured for 20 s.

Group KT: After etching and bonding steps, the cavity surfaces were sealed with flowable composite. A piece of fiber network (EverStick NET, Stick Tech Ltd, Finland) (10 mm long, 4 mm wide) was prepared and located inside the flowable composite from the occlusal level of the buccal wall to the lingual wall before curing. After 40 s light-curing, the cavities were restored with direct composite as mentioned in group K [Figure 2].
Figure 2: Representation of group KT (C: composite, F: fiber, GP: guta-percha)

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Group KO: In this group, grooves with a width of 2 mm and depth of 1 mm were prepared on the occlusal level of the cusps.[8] The cavities were restored as mentioned before in Group K until the grooves opened at the occlusal level. Flowable composite resin was applied to the grooves and on the composite but was not cured. A 2-mm-wide fiber network was placed into the uncured flowable composite resin. This combination was cured for 40 s, and then the exposed fiber and empty spaces were covered with composite resin [Figure 3].
Figure 3: Representation of group KO. (C: composite, F: fiber, G: groove, GP: guta-percha)

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Group FP: The post space was prepared with drills from the post manufacturer (Bisco, Schaumburg, IL, USA) at the two-thirds of the canal length. The root canal walls were cleaned with 1% NaOCl and saline solution and then dried. For fiber post cementation, the adhesive system was applied to the fiber post (DT Light Post No. 2, Bisco) and root canal wall but not cured. Then, dual-cure resin cement (RelyX Ultimate; 3M ESPE) was applied on the fiber post surface. Fiber posts were cemented in the prepared post spaces. After initial preparation, polymerization was performed with light for 40 s. After that, composite resin was placed as described above.

Group L: Impressions of all teeth were taken in silicon impression material (Elite HD; Zhermack, Rovigo, Italy) and restorations were fabricated with indirect composite resin (Tescera ATL; Bisco). Indirect composite restoration cementation was performed as in group FP with dual-cure resin cement.

Group LT: In this group, polyethylene fiber was applied to the cavity floor as described in group KT. After fiber insertion, specimens were impressed, and indirect composite inlays were prepared and cemented as in Group L.

Group LO: Indirect composite restorations were manufactured as in Group L. After the cementation of the restorations, on the occlusal level of the restorations, a groove 2-mm-wide and 1-mm deep between the buccal and lingual cusps was prepared. The etchant and adhesive system were applied to the groove and cured. The flowable composite resin was applied, and then the fiber network was placed in it and cured for 40 s.

Testing procedure

All the specimens were polished with fine Sof-Lex discs (3M ESPE) and then stored in an incubator (H11420BD; Termal Laboratuvar Aletleri San., Turkey) at 37°C for 24 h before testing. The specimens were then placed into a universal testing machine (AGS-J; Shimadzu, Japan). Stainless steel rounded bar with 5-mm diameter was placed to the upper part of the test machine. The rounded bar was kept parallel to the long axis of the teeth and touched buccal and lingual cusps of the teeth. Compressive force was loaded at a crosshead speed of 1 mm/min vertically, and the force necessary to fracture each specimen was recorded as kilonewton [Figure 4].
Figure 4: Diagram of the fracture resistance test for the specimen

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Fractured specimens were analyzed under a stereomicroscope (SZ6; Olympus, Munster, Germany). The types of failure were determined and compared; a distinction was made between catastrophic fractures (nonreparable, below the CEJ) and noncatastrophic fractures (reparable, above the CEJ).[13]

Statistical analysis

Kolmogorov–Smirnov test was used to assess whether the data obtained in the study were appropriate for normal distribution. As a result, it was observed that the parameters were not in normal distribution. For this reason, Kruskal–Wallis test was used to compare the groups. A post hoc Mann–Whitney U comparison test was used to compare the subgroups. The statistical significance level was established at P < 0.05. Statistical analyses were performed with MedCalc Statistical Software version 12.7.7 (MedCalc Software bvba, Ostend, Belgium).


   Results Top


The mean fracture resistance values, standard deviations, and the modes of failure observed in the groups are listed in [Table 2]. Kruskal–Wallis test was used to compare all the different restoration techniques, keeping the wall thickness constant. When all the subgroups with different restoration techniques were compared with each other, there were no statistically significant differences in the groups with wall thicknesses of 2 and 1 mm (P > 0.05). On the other hand, when comparing different restoration subgroups found in the group with 1.5-mm wall thickness, the difference was statistically significant (P = 0.039).
Table 2: The mean values and catastrophic fracture rates of the groups

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Furthermore, Mann–Whitney U tests showed that subgroup KO with 1.5-mm wall thickness had a significantly higher fracture resistance than did subgroups K, KT, FP, and LT, all with the same wall thickness (P < 0.05).

When the effect of different wall thicknesses on the fracture resistance of the restoration types was evaluated, there was no statistically significant difference in subgroups K, KT, FP, LT, and LO (P > 0.05). In contrast to these subgroups, direct composite restoration with fiber insertion at the occlusal level (P = 0.023) and indirect composite restoration subgroups showed statistically significant differences when wall thicknesses were compared (P = 0.044). In the KO subgroup, in pair comparisons, no significant differences were observed between A and B wall thicknesses or between A and C wall thicknesses (P > 0.05). On the other hand, in the KO subgroup, when the wall thicknesses of groups B and C were compared, group B showed statistically significant higher fracture resistance values (P = 0.007). In the indirect restoration subgroup in the pair comparisons, there was no significant difference between the wall thicknesses of A and B (P = 0.529), whereas the wall thickness of group C showed statistically significant lower values than groups A (P = 0.029) and B (P = 0.043).

In general, when the experimental groups were examined in terms of fracture types, a catastrophic fracture type was observed in all groups with a rate of 50% and higher. When all the groups were examined, subgroups that inserted a fiber network at the occlusal level showed a lower rate of catastrophic fracture than the other restoration groups did.


   Discussion Top


The null hypothesis was rejected as reinforcement applications affected fracture resistance in 1.5-mm wall thickness group significantly. In this study, MOD cavity preparation was done to decrease fracture resistance and to better evaluate the contributory effects of restoration techniques. Various wall thicknesses have been formed to evaluate the effect of the remaining dental tissues on fracture strength. In studies investigating adequate wall thicknesses, Macpherson et al. reported that the critical thickness for restoring MOD cavities was 2.25 mm.[14] On the other hand, a research also showed that there is no difference in the 1- and 3-mm wall thicknesses.[15] In indirect adhesive restorations, the optimum wall thickness was reported as 2 mm, but the critical thickness was expressed as 1.5 mm.[16] With this information, the wall thickness groups in this study were categorized as 2, 1.5, and 1 mm.

Because of the diversity of composite resins, further modifications would seem to be necessary to improve the properties of resin-based materials used for final restoration of endodontically treated teeth. Within these modifications, the use of fiber post, fiber network, and indirect composite resin material is preferred. In this study, direct posterior restoration with fiber post application groups was included, whereas indirect composite restoration with fiber post application was not included. Studies comparing indirect restorations and indirect restorations applied with fiber post compressive strengths showed that fiber post applied before indirect restoration did not influence increasing restoration resistance.[17],[18]

In this study, no statistically significant difference was observed between the restoration subgroups in the 2-mm wall thickness group. The results of the studies evaluating the fracture resistance of different restoration types applied to endodontically treated teeth are similar to the findings of this study when taken separately.[3],[5],[7],[17],[19],[20],[21],[22],[23],[24] Researchers have reported that when the wall thickness is 2 mm or more, there is no need to use additional materials to increase tooth resistance. Harolur et al. reported that in the teeth with a wall thickness of 2 mm, the existing dental tissue was sufficient to support coronal structure.[25]

When indirect restoration subgroups were compared, the group (C) with 1-mm wall thickness had statistically significant lower results compared with the other wall thickness groups (A and B). According to previous studies, suggestions can be found indicating that when the inlay cavity preparation wall limits are closer to 1.5 mm at the functional cusp, or when less than 2 mm of wall thickness remains, the onlay preparation should be included, with a 2-mm reduction.[26] In this study, it was found that indirect composite restoration could not provide adequate integrity to the existing dental tissues when the wall thickness was insufficient.

Statistically, fiber post insertion did not increase the values of fracture resistance in all wall thickness groups. There is a consensus in the literature that posts do not strengthen endodontically treated teeth.[5],[9],[10],[23] However, they are still considered necessary for the retention of the restoration, especially in the case of severely damaged teeth.[27],[28] Numerous researchers have claimed that high fracture resistance is observed when the modulus of elasticity of post and dentin is compatible with each other. It appears that by building a mono-block of dentin-post-core with bonding agents, the force distribution in the root is improved. The post can stabilize the restorative composite resin and consequently can lessen the stress at the composite–adhesive interface.[27] In contrast to this, some other studies have reported that endodontically treated premolars without fiber posts had a parallel fracture resistance compared with those with a post. This may be because a greater part of tooth structure is removed during post placement.[9] The reason for controversy among the above findings in addition to the type of restorative materials can be attributed to the different conditions of these studies and the method of fracture force application.

In this study, in the 1.5-mm wall thickness group, comparison of the subgroups with different restoration techniques revealed statistically significant differences. Because of binary comparisons made in this group, the KO subgroup with fiber network at the occlusal level showed statistically significant higher values than subgroups of K, KT, FP, and LT. Belli et al. searched the use of fiber network embedded in flowable composite on the cavity floor and on the occlusal level. Researchers concluded that fiber network placement technique improves the resistance of endodontically treated teeth.[8] In another study, Navimipour et al. concluded that placing fiber network over and under the restoration significantly improves the resistance.[6] The researchers considered that the existence of the fiber would create a change in stress paths along the fibers and distribute the load to intact parts of the teeth and away from the bonded surfaces. Another explanation of the researchers was that the bonding ability of fiber in combination with composite resin might have increased the fracture resistance of the tooth by keeping the cusps bonded. In this study, in occlusal placement of fiber network inlay group, we preferred preparing grooves after cementation process because of the future adaptation challenge.

In this study, when the fracture types were examined, the residual wall thickness influenced the fracture mode: the 1-mm wall thickness group showed more catastrophic fracture than groups with other wall thicknesses. When restoration techniques were evaluated in terms of fracture type, in all wall thickness groups, direct restoration with fiber insertion at the occlusal level prevented the formation of catastrophic fracture compared with other restoration subgroups. This result could also be explained by the stress-modifying effect of the polyethylene fiber network, as mentioned before.

The results of this in vitro study suggested the use of different restorative techniques with insertion of fiber materials into endodontically treated teeth with MOD cavities; however, in the future, in vivo researches are required to distinguish the clinical benefits of strengthening the restorations and the tooth structure together. In addition, restoration groups that are successful in this study should be evaluated using an in vivo study.


   Conclusion Top


  1. If the wall thickness is 1 mm, additional reinforcement structures (fiber post and network) that are placed close to the occlusal surface and perpendicular to the occlusal forces become even more important. In this wall thickness, fiber post application was more effective than in other wall thicknesses in terms of providing resistance.
  2. In the presence of 1.5-mm wall thickness, the restoration type becomes more important. In this wall thickness, placement of the fiber network at the occlusal level on the direct composite restoration can provide increased fracture resistance.


Financial support and sponsorship

Science Research Projects Coordination Unit of Istanbul University (project no. 54890).

Conflict of interest

There are no conflicts of interest.



 
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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4]
 
 
    Tables

  [Table 1], [Table 2]



 

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