|Year : 2016 | Volume
| Issue : 3 | Page : 93-96
Effect of thermal cycling and microhardness on roughness of composite restorative materials
Efe Cetin Yilmaz, Recep Sadeler
Department of Mechanical Engineering, Faculty of Engineering, Ataturk University, Erzurum, Turkey
|Date of Web Publication||10-Aug-2016|
Dr. Efe Cetin Yilmaz
Department of Mechanical Engineering, Faculty of Engineering, Ataturk University, Erzurum
Source of Support: None, Conflict of Interest: None
Objective: This study is aimed to investigate the effect of thermal cycling and microhardness on surface roughness of four different composite restorative materials. Materials and Methods: In this study Nonofilled(Ivoclar Heliomolar, 3M ESPE Filtek Supreme) and microhybrid(3M ESPE Filtek Z250, Kuraray Clearfil AP-X) composites were used. The surface roughness (Ra) was initially measured in a profilometer using a cut off 0,25mm, after 6000 and 12000 thermal cycles. In addition to microhardness of composites Vicker hardness (HV) were determined. Data were subjected to Anova and Tukey's test.(α=0,05). Results: One-way Anova indicated significant differences in Vicker hardness(HV) between four composite resins. Significant lowest HV was found for Heliomolar (HV=22); mean values were considerably lower than three composite resins. In addition to overall 6000 thermal cycles increased the surface of roughness values for all materials and there was a trend in all groups to decrease the roughness after 12000 thermal cycles. Conclusion: The material composition including type of organic matrix could be more relevant to roughness maintenance over time than the general behavior of composites based on particle fillers. Moreover, this study revealed that correlations between microhardness (HV) and surface of roughness were poor.
Keywords: Composite resin, roughness, thermal cycle
|How to cite this article:|
Yilmaz EC, Sadeler R. Effect of thermal cycling and microhardness on roughness of composite restorative materials. J Res Dent 2016;4:93-6
|How to cite this URL:|
Yilmaz EC, Sadeler R. Effect of thermal cycling and microhardness on roughness of composite restorative materials. J Res Dent [serial online] 2016 [cited 2017 Dec 16];4:93-6. Available from: http://www.jresdent.org/text.asp?2016/4/3/93/188233
| Introduction|| |
Since the beginning of the 1960s, composite resin has been available as an esthetic material for restorative procedures. The use as a posterior restorative material has been substantially increased over the last few years.  This material consists of a resin matrix and filler particles that are chemically linked by silane coupling agents. Several composite materials are available for direct dental restorations, comprising microhybrid, microfilled, and nanofilled composites.  Surface roughness of restorative materials has been recognized as a parameter of high clinical relevance for plaque accumulation, staining susceptibility, and wear.  If the restoration has a surface roughness of 0.2 mm (Ra) or more, dental plaque accumulation may increase the risk for both caries and periodontal inflammation.  The mechanical properties of composite resin can be affected by hydrolytic degradation.  In in vitro studies, the long-term water storage and thermal cycling are considered relevant conditions to test the durability of resin bonds.  Furthermore, roughness of some resin-based materials can be changed by the toothbrushing and thermocycling process, and could affect the durability of the composite restorations.  Microhardness is a composite property, which is correlated with resistance to wear, also in case of thermal fatigue. Investigations of microhardness allow evaluating mechanical properties of the composite materials. As it has been demonstrated, there is a strong correlation between composite microhardness and elasticity modulus values, photopolymerization depth, and the strongest with a polymerization shrinkage degree. In addition, a correlation with the degree of composite filler conversion has been demonstrated.  Microhardness studies can be also used to evaluate a local gradient of photopolymerization, which is a specific homogeneity of composite in the area of impact of the light spectrum, influence of polymerization time, and the wavelength of the light.  Undertaking in vitro thermal fatigue simulation studies of the mechanical tooth-composite filling system, the loads conditions reflecting physiological conditions in the human oral cavity should be ensured. The following parameters should be controlled: Temperature of the operating liquid (artificial saliva or normal saline), retention time of the operating liquid in the container with samples, or the studied sample in the container with operating liquid, as well as a number of load cycles. In the previous studies, different assumptions have been made with regard to the experimental parameter.  The lower operating liquid temperature applied in the experiments was between 2°C and 24°C, whereas heated liquid temperature was in a range of 45°C and 60°C. 
The aim of this in vitro study was to investigate the effect of thermal cycling procedure and microhardness on the surface of roughness of nanofilled (3M ESPE Filtek Supreme, Ivoclar Heliomolar) and microhybrid (3M ESPE Filtek Z250, Kuraray Clearfil AP-X) composites.
| Materials and Methods|| |
The materials, including nonofilled (3M ESPE Filtek Supreme, Ivoclar Heliomolar) and microhybrid (3M ESPE Filtek Z250, Kuraray Clearfil AP-X) composite resins, used in this study are shown in [Table 1]. From the selected materials (n = 8), disk shape samples with 6 mm diameter and 2 mm thickness were prepared. Photopolymerization process was conducted with the use of halogen lamp. The exposure time of the samples was according to manufacturer's recommendation. In this study, a thermal shock simulator was designed and used to investigate its effect over the dental materials. The device design and programming were realized by our working group. The thermal shock simulator consists of the microprocessor control system, infrared temperature sensor, and step moving mechanism. The device was connected to the computer via DB25 parallel port, and thus real-time data were saved to the computer. The device enables the creation of thermal shocks in the samples placed in the measuring container located in the simulator. Thermal cycle device consists of the cooler pumping and heater pumping that heated (55°C) and cooled operating liquid (5°C) from two independent temperature conditioning systems. Time of the subsequent procedure performance within each thermal shocks cycle was programmed and repeatable which was controlled by computer reported. Retention time of the cooled and heated liquid was 5 min time pumping in and out of operating liquid was 10 s. The surface roughness (Ra) was initially measured with a profilometer using a cutoff 0.25 mm after 6000 and 12,000 thermal cycles. In addition to microhardness of composites, Vicker hardness (HV) was also determined. Data were subjected to one-way ANOVA and Tukey's test (α =0.05). Selected samples from each experimental group were mounted on metal stubs, gold sputter coated and surface morphology was evaluated under a scanning electron microscopy (Inspect S50).
| Results|| |
The surface roughness
[Table 2] shows the surface roughness of composite resins before and after 6000 and 12,000 thermal cycles. After 6000 thermal cycles, there was an increase on the surface roughness for all materials, except for Filtek Z250 (P < 0.05). After 12,000 thermal cycles, there was a trend of decreasing values of surface roughness. Filtek Z250 showed the lowest values of surface roughness in all measurement when compared to all other composite resins (P < 0.05), regardless of thermal cycling procedure. In [Figure 1], all composite restorative materials are exposed on the surface of Filtek Z250 after 6000 thermal cycling procedure compared to other composite resins, which seem to have a smooth surface [Figure 1]b. There is no difference among the composite materials before and after 12,000 thermal cycles, Heliomolar and Filtek Z250. Clearfil AP-X showed the highest values of surface roughness in all measurement when compared to all other composite resins (P < 0.05), regardless thermal cycling procedures.
|Figure 1: Composite restorative materials after 6000 thermal cycling procedure: (a) Supreme Plus, (b) Filtek Z250, (c) Heliomolar and (d) Clearfil AP-X (5 KV × 24,000 4 μm)|
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|Table 2: Mean surface roughness (Ra μm) of the restorative composite materials before and after thermal cycling |
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One-way ANOVA indicated significant differences in HV between four composite resin materials [Table 3], P < 0.05]. Significantly lowest HV found for Heliomolar (HV 22, 45); values were significantly lower than all other composite resin materials. Clearfil AP-X (HV 75, 16) showed the highest values of HV when compared to all other composite resins.
|Table 3: Vickers hardness of the restorative materials that have been used in this study |
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| Discussion|| |
The finding of this in vitro study indicates that thermal cycling has a critical effect on the surface roughness of composite resins, regardless of the filler composition. After 6000 thermal cycles, there was an increase on the surface roughness for all materials, except for Filtek Z250 (P < 0.05). A previous study also showed that thermal cycling significantly affected the surface texture of composite with dislodgment of filler particles.  However, a correlation between thermal cycling and surface of roughness of dental composites is difficult due to the varied cycles number, different minimum and maximum temperatures, dwell time, and ranges between baths used in the studies. Thus, 5°C and 55°C temperature and 30 s dwell time values were commonly used in literature. Thermocycled samples have been subjected to temperature fluctuations, generating thermal stresses, and leading to microcracks in the matrix or failure at the filler/matrix interface.  Moreover, exposure to water may cause hydrolytic degradation of filler's silane coating or swelling of the matrix.  Differences in filler exposure after thermal cycling are thus most possible because of matrix degradation, leading to exposure of underlying filler particles and an increased roughness, as observed after 6000 thermal cycles for Filtek Supreme, Heliomolar and Clearfil AP-X materials tested. Composites containing hydrophilic components, such as TEGDMA or TEGMA as a matrix component, may be more susceptible to matrix degradation. , Because they allow water to penetrate more easily due to its hydrophobicity. Thus, Clearfil AP-X and Filtek Supreme materials have TEGDMA or TEGMA components that were more affected than other composite materials after 6000 thermal cycle test [Table 1]. In addition, Filtek Z250 did not affect thermal cycle tests due to the absence of TEGDMA or TEGMA components [Table 2]. The size, hardness, and amount of filler dictate the surface roughness of composite, which enhances the mechanical properties of the resin-based composites. , However, it is revealed by this study that the correlation between microhardness and surface of roughness was quite poor. A study showed that the material composition including the type of organic matrix could be more relevant to roughness after thermal cycle tests. A previous study reported higher roughness values for Clearfil AP-X, caused by the largest filler size and irregularity of particles compared with other restorative composites.  Moreover, this material has a TEGDMA hydrophilic component, which is susceptible to hydrolytic degradation. , Even though some composites, especially like Clearfil AP-X showed higher roughness values after thermal cycle tests, all of them showed roughness under the limit proposed by the literature (0.2 μm). The surface roughness parameter (Ra) represents the arithmetic average value of the departure of profile from centerline.  The increase on roughness after thermocycling procedures might cause several problems such as surface stain, dental plaque accumulation, and wear of occluding teeth. Furthermore, organic matrixes of composites would have some absorbed water content , causing hygroscopic expansion in resinous matrix and filler phase, thereby enhancing the weakening of matrix-filler interface. , Data from this study demonstrated some of the changes caused by thermal cycle tests. Surface of roughness of restorative materials can be effect of other degradation mechanisms such as vertical volume loss, erosion, and mechanical abrasion. Thus further work is needed to examine correlation between surface of roughness and two-body wear resistance.
| Conclusion|| |
Within the limitations of this in vitro study, the following can be concluded:
- In this study revealed increased the surface of roughness values for all materials after 6000 thermal cycles, and there was a trend in all groups to decrease the roughness after 12000 thermal cycles (except for Filtek Z250)
- Restorative composite materials used in this study that correlations between microhardness (HV) and surface of roughness were poor
- Clearfil AP-X and Filtek Supreme materials were more affected than the other composite materials after 6000 thermal cycle test due to the existence of TEGDMA or TEGMA components.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Table 1], [Table 2], [Table 3]