|Year : 2015 | Volume
| Issue : 1 | Page : 8-13
Effect of coronal cement base and its thickness on the fracture resistance of endodontically treated teeth
Cihan Yildirim1, Ugur Aydin1, Abdul Semih Ozsevik2, Fatih Aksoy1, Samet Tosun2
1 Department of Endodontics, Faculty of Dentistry, Gaziantep University, Gaziantep, Turkey
2 Department of Restorative Dentistry, Faculty of Dentistry, Gaziantep University, Gaziantep, Turkey
|Date of Web Publication||27-Jan-2015|
Abdul Semih Ozsevik
Gaziantep University, Faculty of Dentistry, 27060 ?ehitkāmil, Gaziantep
Source of Support: None, Conflict of Interest: None
Objective: To compare the fracture resistance of endodontically treated teeth with mesiodistocclusal (MOD) cavities restored with only composite resin, 3 mm glass-ionomer cement (GIC) base + composite resin, and 5 mm GIC base + composite resin. Materials and Methods: Fifty extracted intact mandibular molars were randomly divided into five groups each including 10 teeth. Group 1: No cavity preparation or endodontic treatment was applied (intact teeth). Group 2-5: Root canals were prepared with step-back technique and filled lateral condensation of gutta-percha and sealer. Group 2: No coronal restoration was achieved. Group 3: Teeth were coronally restored with only composite resin. Group 4: Coronal restorations were performed with composite resin following 3 mm GIC base placement. Group 5: Composite resin placed over 5 mm GIC base. After finishing and polishing, all specimens were kept in an incubator at 37°C in 100% humidity for 24 h and fracture resistance was tested with a Universal Testing Machine. Mean force load for each sample was recorded in Newtons (N). Results were statistically analyzed with one-way analysis of variance (ANOVA) and post-hoc Tukey's tests. Results: The mean force required to fracture each sample was as follows: Group 1: 2,745.3; Group 2: 325.9; Group 3: 1,958.1; Group 4: 1,756.3; and Group 5: 1,889.1. Fracture resistance of intact teeth (Group 1) was significantly higher than all other groups. Fracture resistance of teeth in Group 2 (not coronally restored) was significantly lower than all other groups. Fracture resistance values of other three experimental groups (Groups 3, 4, and 5) were not significantly different from each other. Conclusion: Placing a GIC base and its thickness did not significantly affect the fracture resistance compared with composite resin alone.
Keywords: Composite, fracture resistance, glass-ionomer cement, root-filled teeth
|How to cite this article:|
Yildirim C, Aydin U, Ozsevik AS, Aksoy F, Tosun S. Effect of coronal cement base and its thickness on the fracture resistance of endodontically treated teeth. J Res Dent 2015;3:8-13
|How to cite this URL:|
Yildirim C, Aydin U, Ozsevik AS, Aksoy F, Tosun S. Effect of coronal cement base and its thickness on the fracture resistance of endodontically treated teeth. J Res Dent [serial online] 2015 [cited 2019 Jun 16];3:8-13. Available from: http://www.jresdent.org/text.asp?2015/3/1/8/150015
| Introduction|| |
Several studies revealed that endodontic treatment is a major cause resulting in tooth fracture ,, due to excessive tissue loss.  Such kind of fractures occurs as a result of crack formation between the tooth and the material. Progression of these cracks leads to both secondary caries and fractures.  For this reason, coronal restorations of root-filled teeth is important in terms of both microleakage and fracture resistance. 
Direct composite resins have been widely used for the coronal restoration of root-filled teeth with satisfying long-term outcomes due to their bonding properties which makes composite resins a good alternative to previously preferential cusp coverage restorations.  However, polymerization shrinkage is an unfavorable sequel of compositesand to reduce shrinkage stresses; several liners and base materials including glass-ionomer, flowable composite resins, and polyacid-modified glass-ionomer have been used. ,,
Glass-ionomer cement (GIC) is one of the most popular materials used under restorations because of its biocompatibility and chemical adhesion to dental hard tissues. , They are composed of polyketonic acids and fluroaluminosilicate glass and cure with acid-base reaction after mixing of powder and liquid.  The strength of GICs is directly proportional to the ratio of powder in the mixture. 
GIC placement under composite restoration may act as a stress absorption layer and improve the quality of composite restoration by reducing polymerization shrinkage. , Furthermore, the amount of GIC may be important because the more cement used, the less composite bulk resulting in lesser polymerization shrinkage.  Although the physical properties of GICs such as thermal conductivity and marginal sealing are ideal, they are weaker materials compared to other restorative materials such as composite resins. Because of their low fracture resistance, they should be preferred at low stress sites.  When weaker structural and strong bonding properties of GICs are considered together, using them in combination with an overlaying composite resins may be beneficial especially in posterior teeth  because GICs are not manufactured to be enough against high occlusal loads.  However, Mongkolnam and Tyas  and Banditmahakun et al.,  stated that using cement base under restoration might weaken the restored teeth due to its weaker structure compared to the main restorative materials. Thus, benefits of using composite resins alone or in combination with an underlying GIC base are a controversial issue.  Especially for endodontically treated teeth, which are more prone to cuspal and vertical fractures, this subject should be clarified.
For these reasons, using a base material and its thickness under composite restorations should be questioned in terms of resistance. In general, protective effect over pulp against irritation, bond strength, and caries prevention properties of GICs in vital teeth were studied. , But studies regarding their reinforcing effects on endodontically treated teeth against fractures are limited. ,,
The present study aimed to evaluate the fracture resistance of endodontically treated teeth with directly applied composite restorations and composite resins restored over different thicknesses of GICs.
| Materials and methods|| |
Fifty intact, human mandibular molars of similar dimensions, extracted for periodontal reasons and free of caries, restorations, abrasions, and fractures were included in the present study. Any calculus and soft tissue remnants were removed with scalers. The teeth were kept in physiological saline at + 4°C until they used. All samples were randomly divided into five groups each including 10 teeth. The teeth in each group were prepared as follows:
Group 1: No endodontic or restorative intervention was applied to the teeth in this group.
Groups 2-5: Access cavities were prepared with high-speed diamond burs (Medin, A.S.Vlachovicka 619592 Nove Mesto na Morave Czech Republic) under water-cooling allowing an easy inlet to the root canals. A size 15K-file (Sybron Endo, Scafati, Italy) was inserted through the canal until the file tip was visible at the apex. Working length was determined to be 1 mm beyond this point. Root canal preparation was conducted until apical enlargement reached size 40. Further preparation was achieved by step-back technique, 1mm withdrawn after each file until size 70. Coronal enlargement was performed with Gates-Glidden burs (Thomas, Bourges, France). After each file, canals were rinsed with 2 ml 1% of NaOCl. At the end of preparation canals were dried with absorbent paper points (Dentplus, Choonchong, Korea). Root canal obturation was performed with lateral condensation of gutta-percha (Dentsply, DeTrey, Konstanz, Germany) and sealer (Dentsply, DeTrey, Konstanz, Germany). Excessive gutta-percha was removed with a hot instrument at the level of canal orifices (Gutta Cut, WDV GmbH, Germany). Cavities of all samples were modified to mesiodistocclusal (MOD) configuration until obtaining a thickness of 2 mm at the buccal occlusal wall, 2.5 mm at the buccal cementoenamel junction, 1.5 mm at the lingual occlusal surface, and 1.5 mm at the lingual cementoenamel junction. All cavity surfaces were cleansed with an alcohol-absorbed cotton pellet to remove sealer remnants and debris. Roots of all samples were embedded in self-curing polymethylmethacrilate (Meliodent, Bayer Dental, Leverkusen, Germany)-filled cylinder molds with dimensions of 2 cm diameter and 2 cm height to the level of the cementoenamel junction prior to coronal restorations and fracture test. Coronal restorations were completed as follows:
Group 2: No further restoration was applied [Figure 1].
Group 3: After applying bonding agent (Clearfil S 3 Bond, Kuraray, Osaka, Japan) according to the manufacturer's instructions, it was gently dried and light-cured for 10 s. Cavities were restored with composite resin (Universal Restoratif 200, 3M ESPE, Dental Products, St Paul, MN55144-1000 ABD) from canal orifices to the occlusal surface with incremental technique by placing 2 mm resin in each turn [Figure 2]. To provide standardization, the light source was applied just over the cusp tips and after every 10 samples; the power of the light source was controlled with a dental radiometer (Demetron, Kerr) to avoid the light source device declining in intensity to less than 1,000 mW/cm 2 .
|Figure 2: Cavities were restored with only composite resin. GP = Gutta Percha, DBS = Dentin bonding system, CR = Composite resin|
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Group 4: Composite restoration was achieved after placing a 3 mm thickness (excepting pulp chamber) of conventional GIC (Spofa Dental-Kerr, Markova, Czech Republic) [Figure 3].
|Figure 3: Composite restoration was achieved after placing a 3 mm thickness of conventional GIC. GIC = Glass-ionomer cement, GP =Gutta Percha, DBS = Dentin Bonding System, CR = Composite Resin|
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Group 5: Composite restoration was achieved after placing a 5 mm thickness (excepting pulp chamber) of conventional GIC (Spofa Dental-Kerr, Markova, Czech Republic) [Figure 4].
|Figure 4: Composite restoration was achieved after placing a 5 mm thickness of conventional GIC. GP = Gutta Percha, DBS = Dentin Bonding System, CR = Composite Resin, GIC= Glass Ionomer Cement|
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After finishing and polishing, all specimens were kept in an incubator at 37°C in 100% humidity for 24 h. Then, all specimens were placed in a Universal Testing Machine (AGS-X, Shimadzu, Kyoto, Japan). A round-shaped tip made of steel with a diameter of 4 mm was mounted to the testing machine in contact with three points including restoration surface, buccal and lingual walls of the teeth. Force parallel to the long axis of each tooth at a crosshead speed of 0.5 mm/min was applied to the samples. Force necessary to fracture each tooth was recorded in Newtons (N). Statistical analysis was performed with one-way analysis of variance (ANOVA) and post-hoc Tukey'stests.
| Results|| |
[Table 1] shows the mean fracture resistance (N) and the standard deviation (SD) for each group.
|Table 1: Mean fracture resistances and standard deviations (SDs) of five experimental groups|
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The fracture resistance of Group 1 (intact teeth) was significantly higher than that of all other groups (P < 0.05). There were no significant difference between Groups 3, 4, and 5 (P > 0.05).
Fracture resistances of unrestored teeth (Group 2) were significantly lower than that of all other groups (P < 0.05).
| Discussion|| |
Although it was believed that moisture loss of endodontically treated teeth is an important factor causing teeth fractures,  studies revealed that there were no significant difference between vital teeth and root-filled teeth in terms of water content. ,, Root-filled teeth entertain the risk of fracture mainly due to treatment procedures resulting in hard tissue loss. 
Advances in bonding technology resulted in more frequent use of composite resins due to decreased microleakage and postoperative sensitivity. , Despite composites require more technical sensitivity than amalgam restorations, they have been widely used for the restoration of posterior teeth. However, polymerization shrinkage is an important drawback of these materials.  The incremental layering technique results in better shrinkage results compared to the bulk technique. , For this reason, incremental layering technique was achieved in the present study.
Using a liner or base cement under composite restoration is another application reducing polymerization shrinkage,  especially in deeper cavities closer to pulp 0.5 mm or less.  But this subject is not clear for root-filled teeth. Stress distribution through tooth and restoration interface may differ when different types of cement bases were used.  For these reasons, the present study aimed to evaluate if placement of base cement and its thickness is important for endodontically treated teeth. According to the results, using a GIC base and its thickness did not affect fracture resistance significantly in MOD cavities of mandibular teeth. However, these results should be corroborated with other studies including different types of teeth and/or cavity designs.
Glass-ionomers are frequently used base materials.  Generally, type-III/lining GICs characterized with low viscosity and rapid onset are used as a base under restorations.  They represent good marginal sealing and biocompatibility.  Pagavino et al.,  stated that using composite with an underlying glass-ionomer revealed better retention. Placing GIC under resin-based composite restorations offers the advantages of chemical bonding, biocompatibility, fluoride release, reduced polymerization shrinkage, and microleakage. ,,, Furthermore, GICs bond deeper dentine, which has more water content due to dentinal fluid flow better than composite resins. 
On the other hand, Banomyong et al.,  and Peliz et al.,  found that using a GIC under resin restorations increases the risk of gap formation, which adversely affect the strength of teeth. In another study of Banomyong et al.,  using GIC lining under composite restorations did not significantly increase the fracture resistance of the teeth. Ritter  also stated that despite the favorable properties of GICs, their weak bonding limited their use as a dentine replacement material. In contrary to those studies mentioned above and the present study, Liu et al.,  found that using a 1mm GIC under composite resin decreased stress distribution significantly. In the present study, using a GIC base under composite restorations did not significantly affect fracture resistance of root-filled teeth either positively or negatively compared to the restorations with composite resin alone. These results are in accordance with the study of Taha et al.  They stated that loosing axial walls during access cavity preparation severely leads to decrease in fracture resistance.
Results of these different studies and present study highlight that using GIC base under composite restoration may be considered when ever need for its fluoride-releasing effect resulting in reduced risk of secondary caries  outweigh the risk of fracture occurrence. This is generally true for vital teeth that are more resistant to fractures than root-filled teeth. Furthermore, cervical abrasion and erosion sites should primarily be restored with GICs because of their advantageous properties mentioned above.  However, using composite resins directly for the coronal restoration of root-filled teeth seems to be more convenient.
Loss of stiffness in dental hard tissues resulting in fracture is mainly due to the disruption of marginal ridge integrity, particularly MOD cavity preparation.  Reeh et al.,  found that preparing a MOD cavity leads to loss of 63% stiffness. For this reason, MOD cavity design with thin walls was preferred to maximize the fracture risk and better evaluate effects of composite resin and GICs on fracture resistance.
The type of endodontically treated teeth is another factor that should be considered in terms of fracture occurrence. Posterior teeth are exposed to greater occlusal loads compared to anterior teeth and entertains greater risk of fracture.  Especially, root-filled mandibular molars are more prone to fractures. , For this reason, mandibular molars were selected in the present study.
In vitro testing machines are generally used for evaluating fracture resistance of root-filled teeth. Reeh et al.,  stated that, such kind of testing methods are nonphysiologic and cannot completely simulate intraoral occlusal forces. Further in vivo studies may be beneficial to better evaluate the strengthening effect of GICs and composite resins.
Modifications in restoring root-filled teeth were described by different authors to increase the fracture strength. Belli et al., , used a Leno Weave Ultra High Modulus (LWUHM) polyethylene fiber ribbon embedded in a thin layer of flowable composite under composite resins and also in an occlusal groove prepared on the finished composite restoration. They found that fiber ribbon insertion increased the fracture resistance of endodontically treated teeth. However, Sengun et al.,  and Akman et al.,  did not found a positive effect of such applications on the fracture resistance of endodontically treated teeth. Even if it is considered that this method reinforces root-filled teeth, they are time-consuming and expensive options. Thus, developing more practical techniques are necessary to improve the strength of endodontically treated teeth.
| Conclusion|| |
The present study found that using GIC bases and their thickness under composite restorations of root-filled teeth does not have any influence on the fracture resistance. Composite resins can be applied alone whenever the risk of secondary caries is not high. Further in vivo and in vitro studies are needed to better clarify this issue.
| Acknowledgement|| |
The authors deny any conflict of interest and financial support.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4]