Mechanical and aesthetic properties of dental composite materials have been substantially improved in recent years. However, clinical failures due to composite resin restoration are still occasionally reported . According to long-term clinical studies, between 12% and 19% of unsuccessful treatments are due to the dental composite material. It is also reported that about 6% of this occurred because of the wear mechanism . Intraoral tribology wear can be defined as the net loss of volume that occurs as a result of the interaction between two surfaces. Based on intraoral tribology, four basic mechanisms of wear can be mentioned. They are two-body wear (attrition or occlusal contact area), three-body wear (abrasion or occlusal contact free), fatigue wear, and corrosive wear. These wear mechanisms can occur alone or in combination . In the literature, it is reported that two-body wear and three-body abrasive wear are the basic wear mechanisms for composite restorative materials . Two-body wear is related to the material or tooth surface loss when the surfaces are in direct contact, without the presence of another body. Three-body abrasion is due to the presence of an abrasive third body, which acts between two antagonistic surfaces. This third abrasive surface can be pieces of food between two moving surfaces during the chewing cycle. Fatigue wear can be explained by the occurrence of subsurface cracks due to repeated loading. Chemical wear originates from the corrosive effects of food on the surface layer. This acid layer can easily be carried in the mouth with the antagonist.
It is desirable to expose the composite materials to mechanical testing in the laboratory environment prior to clinical testing. Several in vitro methods have been developed to evaluate the wear properties of composite restorative materials fin the past 40 years . However, in none of these methods the complex structure of the intraoral tribology has been taken into account. No international standard existed for the many dental wear testing machines. Also, the different characteristics of the chewing simulators, such as the loading force, temperature changes, and frequency, do not allow correlation between the results obtained in these studies. However, in 2001, standard parameters were established for two-body wear and three-body testing with an ISO specification . In this specification, the capabilities of the chewing simulator and the experimental parameters are specified. The chewing simulator used in this study has the capability of three different chewing simulations of the ISO 2001 technical specification. Many studies in the literature have evaluated the two-body wear behavior of dental composite materials , . However, none of them involved an abrasive third medium and their results have not been compared with those of materials used clinically for many years. It is crucial to select an abrasive third medium for clinical wear simulation. The selected abrasive third medium will affect negatively or positively the wear behavior of composite materials. In the ISO technical specifications, poppy seed slurry is used to simulate natural foods, and particles of the synthetic material poly(methyl methacrylate) (PMMA) are used as the abrasive third medium. Thus the purpose of this study is to investigate three-body wear resistance rate of five restorative composite materials at different mastication cycles and to compare their results with those from an amalgam material.
2 Materials and methods
2.1 Experimental details
The restorative composite materials tested in this study are shown in Table 1 (information provided by the manufacturer). For investigating the dental materials, a chewing simulator capable of simulating the mouth’s environment was designed and produced by our research group. Figure 1 schematically shows the two-axis movement of the chewing simulator. The simulator can exert a load of 49 N in the vertical direction and move by 0.7 mm along the horizontal axial when the antagonist material touches the specimens (detection was performed with a magnetic sensor). When the loading along the vertical axis on the specimen is decreasing, the specimen returns to the starting point again (Figure 1C). Thus, during the chewing tests, wear takes place in the same region of the material surface. Five specimens of each material were produced (8 mm height×4 mm diameter) under conditions recommended by the manufacturer. All specimens were kept in distilled water for 1 week before the three-body wear tests. Then all the specimen surfaces were finished with 600, 1500 and 4000 grit SiC abrasive papers, and their surface of roughness (Ra) and Vicker’s hardness (HV) were determined (Table 2). Steatite balls (6 mm diameter, soapstone; Ceram Tec AG) were used in each chewing test as the antagonist material. Specimens were exposed to three-body wear tests using a chewing simulator with steatite balls as the antagonist (1.6 Hz, 49 N load, 120,000, 240,000 and 480,000 mechanical cycles, and additional thermal cycling between 5 and 55°C at 5 min/cycle for 3000 cycles) immersed in poppy seed slurry. The values of Ra and the mean volume loss of the specimens were measured after each chewing test using a three-dimensional (3D) noncontact profilometer (Bruker Contour). In addition, a random specimen was selected from each test group, and scanning electron microscopy (SEM) images were taken for analyzing the wear tracks (SEM, Zeiss Sigma 300).
2.2 Statistical analysis
Data were analyzed using a statistical software (SPSS Statics 20.0 for Windows 64 bit; SPSS Inc., Chicago, IL, USA; license by Ataturk University). Means and standard deviations of Ra, HV, and the volume loss were calculated and analyzed using one-way ANOVA. The Games–Howell test was used for post hoc analysis because Levene’s test showed significant differences in the variance of the groups. Regression analysis was performed to investigate the relation between HV and volume loss. The level of significance was set to α=0.05.
The mean measured HV values ranged between 32 and 94. Significantly, the lowest HV value was found for FIL (about 32.1 HV), which was much lower than for the other four composite materials and the amalgam. Table 3 shows the Ra values of all materials before and after the two-body wear tests. From the table, it can be seen that the Ra value of all the specimens has increased after the three-body wear tests. The mean volume loss of the specimens after different chewing cycles is summarized Table 4. After 480,000 mechanical loading cycles, all composite restorative materials showed a mean volume loss greater than that of the amalgam, but the values for Z250 and FIL were not significantly different from that of the amalgam. Figure 2 shows the horizontal and mean volume loss registered by the 3D profilometer of the FIL composite. Additionally, Figure 3 shows the horizontal axis and vertical axis volume loss registered by two-dimensional (2D) analyzer. Regression analysis showed no significant correlation between HV and the volume loss (p=0.985). Specimens of each material after 480,000 loading cycles were examined by SEM. The wear tracks on the specimen surface caused by the three-body wear mechanism are shown in Figure 4. Although the composites KAL, Z250, and FIL showed a homogenous surface distribution, DUR and AP-X showed inhomogeneous surface distribution after 480,000 chewing cycles. Table 3 shows that DUR and AP-X had larger surface roughness than the other composites irrespective of the number of chewing cycles. The large particle structure of AP-X is shown in Figure 4A. Because of this structure, it is possible to produce smaller wear tracks on the material surface. FIL showed only small losses with respect to the filler particles, but there were microcracks around the loading area (Figure 4D).
In this study, an amalgam and composite materials with five different filler types were tested for different chewing cycles using a chewing simulator. The tested dental composites are available in the market and widely used in dental treatment. The improvements in the size of the particles contained in the dental composite material resulted in a chemical composition containing two different particles. These materials are called micro- or nano-hybrid resin composites depending on the size and content of micro- or nano-particles . In addition, such resin composites are called universal. Commercially, it is often difficult to distinguish between micro- and nano-hybrids because both their microstructure and mechanical properties tend to be similar . It has been reported in the literature that many test devices are available for two-body wear and three-body wear tests , . They are accepted simulator models for in vitro wear testing. When the two types of simulators are compared, in the case of the two-body wear type, wear occurs as a result of the direct contact between the test specimens and antagonist specimens, while in the three-body type wear occurs with the abrasive slurry (e.g. poppy seed or PMMA as the third body) between the test specimens and the antagonistic specimens.
In this study, the three-body wear experiment was preferred because in practice there is always a third component between the composite and antagonist material during the chewing process. During the chewing process, the food particles are compressed on both the tooth and the antagonist material under the chewing force. From this point, further wear of the composite surface is primarily due to the sliding and disintegration of the food particles over the composite surface. The contact surface of the composite material and the antagonist material increases because of the increased wear force, which will gradually decrease because of the grinding of food particles. In this case, it will reduce the amount of wear force in progressive chewing cycles. In this study, there was no linear correlation between increasing chewing cycles and the mean volume loss occurring in the composite material (Table 4). Wear depends on many parameters such as the restorative material type, oral environment, amount of applied vertical force, horizontal wear distance, and antagonist structure, all of which can play an important role. With regard to this aspect, the use of steatite balls as an antagonist material has been reported by many researchers , . This material as antagonist has been reported to exhibit abrasive wear properties similar to those of composite materials and enamel . The force produced by the chewing simulator should simulate the chewing force produced in the oral environment. Studies in the literature have shown that tooth and dental material are normally loaded between 20 and 120 N in the oral environment . A force of 49 N applied in this study is thus an average value. In addition, the contact time (400–600 ms), loading frequency and number, change of temperature, and dwell time are the important factors affecting the wear of composite materials. The number of mechanical cycles in chewing simulators varies between 50,000 and 1,200,000 . In an in vivo study, it has been reported that the average number of chewing cycles varies between 300 and 700 . The number of mastication cycles used in this study (120,000, 240,000 and 480,000) correspond approximately to 6 months, 1 year, and 2 years in vivo, respectively. In this study, we used poppy seed slurry as the third body environment because it is representative of the daily human food. On the other hand, some researchers have selected PMMA beads as the third body environment , , even though it does not represent daily food. The characteristics of the third medium will be different and will affect three-body abrasive wear resistance of the composite material. In this study, the size of poppy seed used was 0.9–1.0 mm. The chewing simulator was programmed to perform a 2-mm vertical movement and a 0.7-mm lateral movement during the experiment, and the frequency of the loading cycle was 1.6 Hz. In the literature, experimental data on three-body wear tests are very limited. Therefore, in this study, the purpose was to investigate three-body wear resistance and hardness of five different kinds of modern composite restorative materials. In addition, the results of this study were compared with the wear characteristics of an amalgam used for long periods in clinical treatment. All specimens used in this study were stored at 37°C in distilled water for 7 days before the mastication tests. Water absorption affects the mechanical properties, such as wear resistance, hardness, and tensile strength, of dental composite materials. Previous studies have found that the three-body wear resistance and hardness of the specimens stored for 7 days in distilled water significantly higher than those exposed for 24 h to distilled water only , . According to Chadwick et al. , no significant differences in wear was found after 1 week and 1 year of storage of composite resins in water . Therefore, it can be assumed that the composite material is completely saturated within 1 week of distilled water storage. In this study, the amalgam and some composite materials showed similar three-body abrasive wear characteristics. For example, Z250 and FIL had good three-body wear resistance similar to that of the amalgam (Table 4). Amalgams have been in use for over 150 years of clinical trials because of their good wear characteristics . The results obtained in this study show that the loss volume increased in all materials with increasing number of chewing cycles. In addition, with increased number of chewing cycles, the wear areas and wear tracks increased on the material surfaces. In this study, the DUR composite suffered the highest volume loss due to the three-body abrasive wear test, which was 5 times higher than that of the amalgam. However, the wear area and wear track that resulted were different between the materials used in this study. This can be explained as due to the elastic–plastic deformation behavior of the matrix of the dental composite material. DUR material exhibited weak three-body abrasive wear characteristics, which is in accordance the results of other studies , . In previous studies, it has been seen that catastrophic failure occurred in the micro-filled composite material with increased vertical loadings . This was explained as due to the fact the micro-filled dental composite material has a lower filler volume. In previous studies, it had also been found that increasing the filler content of the material improves the wear characteristics , , . Although the filler weight of AP-X is almost equal to that of KAL, AP-X has displayed better wear resistance. The results of our study indicate that the composite resin with higher filler volume did not significantly affect the mean volume loss of three-body abrasive wear tests. This is most likely due to the fact that the reported weight percent of filler of the five materials used were similar and ranged only from 60 to 70%. FIL is characterized by a very smooth and uniformly worn surface (Figure 2B). This is because FIL has a unique nano-polymer structure. Moreover, small voids are seen on the entire surface of this material along the edges. This is because mechanical impacts of vertical loading cause plastic deformation in the composite material. This can be explained as due to the sliding motion of the particles that are snapped from the worn material surface. When comparing the mean volume loss due to wear of Z250 and FIL, the two composite materials that have very similar monomer composition with nearly the same filler volume did not have a significant effect on three-body abrasive wear rate even though they had different filler types and hardness. In this study, it was seen that the surface roughness of all tested materials increased with increasing chewing cycles (Table 3). Dental composites having tri(ethylene glycol)methacrylate (TEGMA) or tri(ethylene glycol)dimethacrylate (TEGDMA) as the matrix component may be more susceptible to matrix degradation . Because the composite material has this structure, it allows water to more easily penetrate into the matrix structure . In this study, DUR, AP-X, and FIL composites, which contain the TEGMA structure in the control group, showed higher Ra values than Z250 and KAL composites. This can be explained by the fact that all the tested specimens were kept in water for 7 days before the chewing tests.
Within the limitations of the present study, it can be concluded that the majority of composite restorative materials (Filtek Z250, Filtek Supreme, and Clearfil AP-X) had good three-body wear resistance similar to that of the amalgam. Additionally, among the composite materials used, the correlations between hardness, filler volume, and three-body abrasive wear resistance are poor. SEM analysis after 480,000 mastication cycles showed microcracks and pits on the surface of the material (particularly Figure 2B and E). These microcracks can be the continuation of cracks that occur in the subsurface of the material. This may be an indication of fatigue wear.
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About the article
Efe Cetin Yılmaz
Efe Cetin Yılmaz received the MSc and PhD degrees in mechanical engineering (Thermal Fatigue and Wear of Composite Biomaterials) from the Ataturk University, Erzurum, Turkey. His research interests include chewing simulation control system design programming, mechanical behavior of composite materials, and computer-aided mouth motion modeling. He has authored several research articles which have been cited many times.
Published Online: 2018-01-12
Published in Print: 2018-07-26