| Abstract|| |
Purpose: To determine the fatigue resistance and failure mode of endodontically treated premolars using direct composite resin restorations.
Materials and Methods: Eighty-four human premolars were divided into seven groups of 12, prepared as follows: Intact teeth used in Group 1 as control, the second group covers the endodontically treated teeth, restored with direct onlays using Z250 composite resin, the next two groups (i.e. 3,4) were similar to the second group, but subjected to 1 and 2 million fatigue load cycles, respectively. Groups 5, 6, 7 were similar to groups 2, 3, 4, however, in these groups Tetric Ceram was used as the restorative material. All specimens were loaded using a Universal Testing Machine until fracture occurred. One-way Anova andTukey's HSD tests were used to analyze the data of onlay groups.
Results: All specimens withstood the masticating simulation. The mean fracture strength for Goups 1 to 7 was: 1276.92, 1373.47, 1269/70, 486/35, 484/12, 1130/49, 1113/79 Newton, respectively. No statistically significant differences were found between the groups in fracture strength and failure mode.
Conclusions: No statistically significant differences in fracture strength were found between sound teeth and composite onlays that were subjected to 1 and 2 million fatigue load cycles.
Keywords: Fiber reinforced composites, fiber position and orientation, fracture resistance
|How to cite this article:|
Moezizadeh M, Mokhtari N. Fracture resistance of endodontically treated premolars with direct composite restorations. J Conserv Dent 2011;14:277-81
|How to cite this URL:|
Moezizadeh M, Mokhtari N. Fracture resistance of endodontically treated premolars with direct composite restorations. J Conserv Dent [serial online] 2011 [cited 2014 Apr 21];14:277-81. Available from: http://www.jcd.org.in/text.asp?2011/14/3/277/85816
| Introduction|| |
Restoration of endodontically treated teeth is one of the biggest challenges in the field of operative dentistry. Since in endodontically treated teeth, most part of the tooth structure is lost because of dental caries, access cavity preparation and biomechanical preparation during root canal therapy, they become very weak and brittle and also more prone to fracture compared to vital teeth. 
Measurement of cuspal flexure using strain gauge has shown that bonded restorations can reinforce the remaining tooth structure and increase strength of restored teeth. 
Demand for having tooth-colored restorations has increased during the past decade.  Saving more amount of healthy tooth structure has been the prime concern of patients as well as dentists, so use of such restorations for treating root canal-treated teeth has been considered widely, because not only is it more conservative but it also maintains the health of the periodontal tissue in addition to being more economical compared to a full-coverage crown. 
Among tooth-colored restorations, direct composite has been considered as one of the most favorable restorations in operative dentistry among dentists which is mainly due to its ease of work, single sitting and excellent esthetics. 
Although indirect composite and ceramic restorations are also used, compared to direct composite, their use is very time-consuming and more expensive.  But, the most important factor is long-term prognosis of tooth and the longevity of restoration.  During the masticatory process, teeth are continuously subjected to mechanical and thermal cycles and under such stresses, restorative materials undergo fatigue and ultimately will fracture. 
So, researchers have designed fatigue laboratory tests in order to simulate the oral environment with dynamic and continuous load application along with thermal stress (thermocycling).  With data collected from these tests, they can predict the long-term prognosis of the restoration.  By evaluating the data collected from different studies, it can be said that there are controversial results and conclusions about the long-term prognosis of the effect of composite restorations on the fracture resistance of a tooth, in some studies, short term prognosis and in some other longer term favorable prognosis of composite restorations of posterior teeth have been reported. 
The aim of the present study was to determine the longevity and long-term prognosis of direct composite onlays on posterior teeth and
What would be the effect of mechanical and thermal stresses to oral conditions on the fracture resistance of these restorations?
| Materials and Methods|| |
In the present study 84 human premolars which were extracted due to orthodontic treatment or periodontal disease were used.
After removing periodontal soft tissue and calculus from the root and crown surfaces using hand scaler (Gray curette se 17/18; Hufreedy; Chicago III), they were kept in 1% Chloramine solution for 12 h, rinsed with water and till the time of the test were kept in distilled water at room temperature. Three criteria were considered for using the samples in the study:
1) Normal anatomy without any anomalies; 2) Presence of no crack or caries on crown or root surfaces with use of transillumination; 3) Their dimension be the same as measurement done by Galan JR for crown (gingivo-occlusal: 7.8-8.8 mm, mesiodistal: 7-7.4 mm, buccolingual: 9-9nmm). With the use of Research Randomizer Software ( http://www.ResearchRandomizer.com ),  the teeth were randomly divided into seven groups of 12.
Except one group of 12 teeth (control group), the rest of the teeth were endodontically treated and mesio-occluso-distal (MOD) cavity preparation was done using high-speed hand piece with air-water spray and fissure bur.
The bur was changed after every fifth preparation. Buccolingual dimension of cavity at isthmus area was kept two-thirds of distance between the cusp tips and after going around the cusps it was converted to mesio-occluso-distal cavity. Width of the proximal box was kept two-thirds of the buccolingual width of the tooth. Gingival floor of proximal boxes was kept 1 mm above cement-enamel junction. After preparing MOD cavity, buccal and palatal cusps were reduced to 1.5 mm following cusp slope in order to cover them with composite later. Before preparing the cavity, an impression was made of each tooth using additional silicon rubber base impression material (ColteneSpeedex putty, Type I, very high, No 4970) and that impression was used as an anatomical guide during restoration. The groups were as follow:
Group 1: Sound teeth as control group.
Group 2: Restoring the teeth with Filtek Z250 composite resin and evaluating their fracture resistance after being subjected to static load as baseline (without cyclic loading).
Group 3: Restoring the teeth with Filtek Z250 composite resin and evaluating their fracture resistance after being subjected to fatigue with application of 1 million thermo-mechanical cycles equal to four years of chewing.
Group 4: Restoring the teeth with Filtek Z250 and evaluating their fracture resistance after being subjected to fatigue with application of 2 millions thermo- mechanical cycles equal to eight years of chewing.
For Groups 5, 6 and 7 Tetric Ceram composite was used but force application was similar to Groups 2, 3 and 4.
Names, compositions and manufacturers of materials used in this study are given in [Table 1].
Before restoring the teeth with direct onlay, 2 mm of guttapercha was removed from the canals in order to pack the composite inside the canals (corono-radicular technique). Then teeth were etched using 37% phosphoric acid for 15 sec and rinsed for 15 sec with air-water spray and gently air-dried for 3 sec without completely drying the cavity (wet bonding). In Groups 2, 3 and 4, in order to use resin composite (Filtek Z250), single-bond adhesive was applied and then cured for 20 sec, using an LED light cure unit (LED Demetron/ Kerr, Danburay, CT, USA with) with 600 mw/cm 2 . In the next step, Z250 resin composite was placed incrementally using buccolingual layering technique with 2-mm thickness of each layer and was then light-cured for 20 sec. With the use of the silicon mold which was made before cavity preparation, the last layer of composite (occlusal surface) was placed so that the anatomy and thickness of onlay was controlled. After finishing the restoration, curing of all aspects of tooth was done for 40 sec.
In Groups 5, 6 and 7 Tetric Ceram composite was used. After completion of restoration, samples were kept in distilled water at room temperature (maximum time of storage was 48 h), till the test was done. All teeth were mounted in PVC molds of self-cured acrylic resin with 25 mm diameter and 20 mm height, and were kept 1 mm below CEJ. Groups 3, 4, 6 and 7 were transferred to an artificial mouth machine. In Groups 3 and 6, the number of cycles applied was 1 million cycles and in Groups 4 and 7, 2 million cycles were applied (3 Hz frequency, 50 Newton load).
After finishing the load cycles in the artificial mouth machine, all teeth were subjected to stress with a speed of 1mm/min in a universal testing machine (BDO-FBOZ TN/Zwick / Roell) which was kept perpendicular to the occlusal surface of the teeth till they fractured. Data collected from the results were analyzed using SPSS software. One-way ANOVA in conjunction with Tukey HSD test were used for comparison of fracture strength of the six experimental groups with onlay composite. Two-way ANOVA test was used for evaluating the effect of type of material and load cycle.
Non-parametric Kruskal Wallis and Mann-Whitney tests were used for comparison of fracture strength of groups with onlay composite and control group.
| Results|| |
Statistical criteria with regard to the study groups are given in [Table 2].
The first part of the statistical analysis belongs to the six experimental groups (with onlay composite), without considering the control group.
The ANOVA test showed that there is a statistically significant difference between the fracture resistance of groups (P<0.0.5). Tukey HSD test showed that there were no statistically significant differences in the fracture strength of the important groups of the present study (based on the aim of the study) (P>0.05). The average amount of fracture load in the seven experimental groups has been shown in [Figure 1].
[Figure 2] shows the average amount of fracture resistance of the experimental groups at three different time intervals (0, 4 and 8 years) for two different types of composites used in study. The graph shows that the average fracture strength of groups with Z250 composite at similar times was higher than the Tetric Ceram groups (but the difference was not statistically significant). Regarding the comparison of the fracture strength of two composites at 0, 4 and 8 years, two-way ANOVA demonstrated that the effect of type of material and number of cycles (time) was not statistically significant (P=0.934)
|Figure 2: Average of fracture resistance of study groups at three different time intervals (0, 4, 8 years) using two composites (Z250 and Tetric Ceram)|
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Results showed that type of material had a significant effect on strength with P value of less than 0.01; but number of cycles (time effect) did not have a statistically significant effect on fracture strength (P=0.056).
The second part of the statistical analysis is related to the comparison of the experimental groups with the control group in which non-parametric Kruskal-Wallis test was used. Considering P<0.05 as significant value, seven groups showed statistically significant differences, but for comparison of different groups with each other Mann-Whitney test was used which showed no statistically significant differences between the control group and experimental groups (P=0.05).
| Discussion|| |
The aim of the present study was to determine the longevity and long-term prognosis of direct composite onlays on posterior teeth, and if such restorations are subjected to mechanical and thermal stresses, what would be the effect of such stresses on the fracture resistance of the restored teeth.
The results of present study showed that the fracture strength of the Z250 composite at time intervals of 0, 4 and 8 years was higher than Tetric Ceram onlays, but fracture resistance of both the materials reduced with time, although the difference was not statistically significant.
Cyclic loads, moisture and thermal changes can lead to slow spread of one or many cracks in material and as a result, gradual loss of strength, decrease in fracture strength and failure of restoration occurs. 
The results of many studies are in agreement with the result of the present study. ,,
The results of the present study have demonstrated that both the composite materials had favorable fatigue strength and in spite of decrease in fracture strength with time, it was not very significant. This could be explained by the hybrid nature of the Z250 and Tetric Ceram composites. In hybrid composites, due to increased filler content and higher elastic modulus, slight deformation occurs under functional loads, so strain and crack formation following functional loads will not occur so easily. ,
One of the reasons that Z250 had higher fracture resistance compared to Tetric Ceram is its higher modulus of elasticity. When the elastic properties of composites approximate those of the tooth structure, lesser amount of tensile stresses will form at the tooth restoration interface and the marginal degradation which occurs due to the mechanical change in shape of the restoration during mastication will get minimized, stresses created from occlusal loads will get distributed more evenly along the tooth restoration interface, and the whole tooth restoration system will act as a single unit and will improve fracture strength.  Basically, the mechanism by which fracture toughness is increased is due to the presence of filler particles, crack pinning and crack deflection. In hybrid composites greater size and more number of fillers (compared to microfilled composites) have improved crack pinning and crack deflection mechanisms, therefore the fracture toughness increases. 
Overall, the higher mechanical properties of Z250 and more fracture resistance compared to Tetric Ceram could be due to the differences in its composition.  It seems that in addition to the greater size of filler particles in Z250, the presence of zirconia filler could also increase the strength and improve mechanical properties. Presence of aromatic rings in stiff monomers like BisGMA and BisEMA in Z250 decrease cyclization and increase crosslink in polymer, therefore better mechanical properties will form and increase its strength compared to Tetric Ceram, but for TEGDMA monomers and specially UDMA, due to higher flexibility of molecule, cyclization of intramolecule is more probable, so it can be said that the stiffness of BisGMA and BisEMA is an important factor for the longevity and efficiency of Filtek Z250. 
The BisEMA molecule has a similar molecular structure as BisGMA but the only difference is that it does not have hydroxyl groups (-OH) which are present in BisGMA, therefore due to presence of hydrogen bonds between these monomers, they had limited movement in polymer. So BisEMA, in spite of having higher stiffness due to the presence of aromatic groups, has good flexibility also  which is a reason for the favorable toughness of the Z250 composite in addition to its higher strength and modulus of elasticity. 
In addition to the properties of the filler and resin matrix, filler silanization also plays an important role in the fatigue properties of composites. 
So it can be concluded that the higher fatigue strength of the Filtek Z250 composite is due to the excellent properties of its polymer matrix (presence of hard Bis GMA and Bis EMA monomers and flexible UDMA monomer which caused increase in elastic modulus, adequate toughness, decreased cyclization and increase in cross-link), higher percentage of zirconia-silica filler particles, better combination of polymer matrix with filler particles and their uniform. microscopic structure. 
| Conclusions|| |
From the results of present study the following conclusions can be drawn:
- Teeth restored with direct composite onlays (Filtek Z250 and Tetric Cream) could increase fracture resistance. Although the strength of onlays made with Z250 was more than that of the natural sound unprepared tooth, the difference was not statistically significant.
- Direct onlays made with hybrid composites Z250 and Tetric Ceram in endodontically treated premolars, after being subjected to 1 million and 2 million thermo-mechanical cycles equal to four and eight years of chewing showed favorable fracture strength.
- Fracture resistance of hybrid composite onlays decreased 9% after four years and 11% after eight years.
- The type of material used for restoration, irrespective of the number of cycles has an effect on the fracture strength of the tooth, in such a way that Filtek Z250 showed higher fracture resistance compared to Tetric Ceram.
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Department of Operative Dentistry, School of Dentistry, Shaheed Beheshti University of Medical Sciences, Tehran
[Figure 1], [Figure 2]
[Table 1], [Table 2]