Abstract
Objective:
Wear and surface hardness of resin composites are of relevance from the clinical standpoint. With the incorporation of novel filler system, more studies need to be performed to investigate newly marketed resin composites. The objective of this study was to investigate the abrasive wear and surface hardness of dental restorative with different filler types.
Methods:
Nanohybrid filled Harmonize (HM) and Tetric N-Ceram (TNC), microhybrid filled Filtek Z250 (Z250), nanofilled Filtek Z350 (Z350) were included in the study. Twelve cylindrical resin composites specimens with 10 mm in diameter and 6 mm in thickness were prepared for abrasive wear test. Eight hundred cycles under 17 kg load were conducted for final wear by CW3-1 wear machine. The specimen was cleaned with an ultrasonic unit for 3 min followed with drying procedure. After measurement of weight loss and the density of specimens, the specimens were kept for measurement of surface hardness. Surface hardness was measured using a micro-hardness tester with a Vickers diamond indenter after polishing. Three specimens of each material were observed by scanning electron microscopy (SEM) after the abrasion to evaluate the morphology of the surface. Data were analyzed using one-way analysis of variance (ANOVA), followed by Tukey HSD test (α = .05).
Results:
Z250 performed the least volume wear loss (41.1 ± 2.1 mm3), as well as the hardest value (102.7 ± 2.9 HV). There was no significant difference with the volume wear loss (p = 1.000) and surface hardness (p = 0.874) of HM and TNC. SEM images of nanohybrid filled HM and TNC represented smoother surface compared with other types of resin composites.
Conclusions:
Microhybrid Z250 showed the highest wear resistance and surface hardness, nanofilled and nanohybrid resin composites may still face the insufficient of wear and surface hardness quality.
Introduction
Resin-based composite restoratives play a valuable part for direct restorations in clinical dentistry. 1 Over the years, resin-based composites have developed continuously regarding their filler content, resin matrix, and initiator system for better esthetic behavior and physical properties.2,3 Nowadays, nanotechnology has been widely applied in dentistry, with the decreasing of the size of filler particles, from traditional microhybrid filled and microfilled particles, to the nanofilled and nanohybrid filled composite materials.3–6
Wear performance is one of the greatest physical property of direct posterior restorations, especially on the occasion of abnormal occlusive habits such as clenching or bruxing.6,7 Physiological wear is the result of a series of complex processes, including the abrasive of food, the type of antagonistic material, the parafunctional chewing behavior, etc. 8 Moreover, Occlusive wear unavoidably cause the loss of the anatomical morphology of the resin composite restorations, which would further affects marginal discrepancies and crack propagation in resin composite restorations. 9 Therefore, wear resistance property of resin-based composites are of vital importance for long-term durability of restorations. In general, resin-based composite restorations should yield wear resistance comparable to the antagonistic natural teeth, however, the wear resistance properties of resin composite restoration commercially in market is still inferior to the natural teeth in a large degree.8,9
Wear of resin composites can be determined by many factors, such as the content and the size of filler particles, the composition of resin matrix, as well as the bonding performance between the filler and the matrix.7–10 Through the years, many researchers have evaluated the effect of the size of filler particles on wear resistance property.10,11 As mentioned by Stawarczyk et al., 10 in terms of wear resistance, enhanced wear resistance of resin composite associated with the decrease of filler particles size. On the other hand, Turssi et al. have proposed that microfiller contained resin composites performed similarly or better wear resistance compared with nanofiller contained resin composites. These results may be explained by the possibility that the filler particles with nano-sized were too small that unable to offer load support during occlusion. 11
Nanohybrid filled resin composite Harmonize (HM) is widely applied in restorative dentistry since it comes to the market. 12 It was claimed that the nanoparticle filler network of HM provided better blending effect and color matching abilities to better match the shade of the surrounding teeth.12,13 In some previous studies, it has been proposed that HM’s superior esthetic properties such as color matching capability, color stability compared with traditional resin composites.12,13 However, very little information is available regarding nanohybrid filled resin composite HM.
The current study evaluated the wear properties and surface hardness of a novel commercial esthetic nanohybrid resin composite Harmonize (HM) with the comparison of nanofilled and microhybrid resin composite materials, in an effort to identify whether filler types affect wear properties. The objective of this in vitro study was to evaluate the wear performance and Vickers hardness of different filler types of dental restorative materials (nanofilled, nanohybrid filled, and microfilled). The null hypotheses were (1) the wear properties, (2) the surface hardness of nanohybrid resin composite would not differ from the other types of filler contained resin composites.
Methods
A novel nanohybrid resin composite Harmonize (HM), a microhybrid filled resin composite Filtek Z250 (Z250), a nanofilled resin composite Filtek Z350 (Z350), and a conventional nanohybrid composite Tetric N-Ceram (TNC) were chosen for this in vitro study. TNC is one of the most popular conventional resin composites applied in clinical dentistry. Detailed information was presented in Table 1.
Characterization of the resin composites tested.
Bis-EMA: bisphenol A polyethylene glycol diether dimethacrylate; Bis-GMA: bisphenol glycidyl methacrylate, TEGDMA: tetraethyleneglycol dimethacrylate; UDMA: urethane dimethacrytate.
Specimen preparation
A 10 mm in diameter and 6 mm in thickness cavity was made using silicone impression (Correcsil, Yamahachi Dental Mfg., Co., Aichi, Japan). Twelve specimens were prepared by filling the cavity with tested resin composites (HM, Z250, Z350, TNC) (n = 3) using layering technique. Irradiation (BluePhase, Ivoclar Vivadent, Shaan, Liechtenstein) was applied with an LED curing light with an output of 850–950 mW/cm2 for 20 s for each layer of the resin composites. After irradiating the tested resin composites, the samples were measured using the digital Vernier Caliper (Altraco Inc., Sausalito, California, USA) with calibration to 0.01 mm to confirm the accuracy of the sample size. All the specimens were ultrasonically cleaned for 10 min, then stored in an incubator (WGLL-65BE, Taisite, Tianjing, China) with the condition of distilled water at 37°C for 24 h prior to testing.
Abrasive wear testing
The abrasive wear of the resin composite restorations was measured with a CW3-1 wear machine (Peking University, Beijing, China) developed by Xu et al 14 in the 1990s. The device contained an antagonist with a Shore hardness of 67 and a thickness of 6.5 mm and diameter of 15 cm, a slurry of fluorite powder mixed with distilled water as the abrasive. After 100 cycles of preliminary wear under a 5 kg load to remove the smear layer created of the surface of specimen, the weight of the specimen were recorded using a balance (XS105, Mettler Toledo, Columbus, OH, USA) with 0.01 mg accuracy, the density was measured by means of a density meter (DT100, Beijing Optical Instrument Factory, Beijing, China). Eight hundred cycles under 17 kg load were conducted for final wear. Detailed of abrasive as described in the previous study. 15 The specimen was cleaned with an ultrasonic unit for 3 min followed with a natural-draft drier.
Determination of Vickers hardness
After abrasive wear test, the Vickers hardness of the specimen was measured with a with an HMV-2T microhardness tester (Shimadzu, Tokyo, Japan). The specimen were employed the load of 0.98 N and 10 s time at a temperature of 20°C. The Vickers hardness values were calculated for three spots for each specimen. Indentations with greater than 0.5 mm distance between adjacent was maintained with the purpose of avoiding the influence of the residual stress.
Surface observation
The surface performance of specimens after abrasive wear were further confirmed under scanning electron microscope (SEM, EVO 18, Zeiss, Wetzlar, Germany) at an accelerating voltage of 20 kV. The specimens were cleaned ultrasonically for 2 min after the abrasive wear procedure, then preserved in an incubator (WGLL-65BE, Taisite, Tianjing, China) 37°C for 24 h prior to testing. Twelve specimens were surface coated with Pt-Pd by an ion sputtering device (E-1030, Hitachi, Tokyo, Japan) for 120 s, then observed using SEM with the magnifications of 1000× and 2000×. Relative quantitative analysis of the images was done using Image J (Version 1.47 V; National Institutes of Health, Bethesda, MD).
Statistical analysis
The normality and homogeneity of wear loss and Vickers hardness were performed using the Shapiro-Wilk test and Levene’s test. One-way ANOVA was performed to compare the volume loss and surface hardness values among the resin composite restorations, followed by Tukey HSD test with a significance level of α = 0.05. The statistical analysis was performed with SPSS 21.0 for Windows (SPSS Inc., Chicago, IL, USA).
Results
Abrasive wear
Volumetric wear loss and hardness data were represented in Table 2. Tukey HSD test revealed that volume loss was significantly affected by the resin composite restorations (F = 30.22, p = 0.000). Z250 performed the least volume wear loss (41.1 ± 2.1 mm3), with significant difference compared with other materials (p = 0.000 for HM and TNC, p = 0.001 for Z350), followed by Z350 (62.5 ± 6.3 mm3), TNC (70.1 ± 3.9 mm3), and HM (70.2 ± 3.4 mm3). There was no significant difference between the group of HM, Z350, and TNC (p = 0.218, 0.218, 1.000 respectively).
Mean volumetric wear loss (±SD) and hardness (±SD) of tested resin composites.
Grouped values with the different letters indicated significant differences among the tested resin composites (p < 0.05, uppercase for volumetric wear loss, lowercase for hardness separately).
Surface hardness
Tukey HSD test revealed that surface hardness was significantly affected by the resin composite restorations (F = 206.210, p = 0.000). Z250 showed the hardest value (102.7 ± 2.9 HV), followed by Z350 (98.3 ± 3.8 HV). There was no significant difference of surface hardness between Z250 and Z350 (p = 0.248). There was no significant difference (p = 0.874) regarding the surface hardness of HM (63.9 ± 1.5 HV) and TNC (62.3 ± 1.7 HV). However, Z250 and Z350 showed significantly higher hardness values compared with HM and TNC (p = 0.000).
Surface observation
Representative SEM photographs of the surface of each tested resin composite material were presented in Figures 1 and 2. After abrasive procedure, shallow grooves were observed for resin composite HM, Z250, and TNC (Figure 1(a), (b), and (d)). Small pits resulting from particle detachment were detected for HM, Z250, and Z350 (Figure 2(a)–(c)). For HM resin composite, protruding nanocluster particles were observed on the surface of the resin composite, the individual nanofillers could be seen between the scratches (Figure 2(a)). SEM images of nanohybrid filled TNC represented smoother surface compared with other types of resin composites (Figure 1(d)).
SEM observation of the surface of resin composites after abrasion at 1000× magnification: (a) HM group, (b) Z250 group, (c) Z350 group, and (d) TNC group (Arrow: shallow grooves; Asterisks: pits structure of the surface).
SEM observation of the surface of resin composites after abrasion at 2000× magnification: (a) HM group, (b) Z250 group, (c) Z350 group, and (d) TNC group (Hand: protruded nanocluster; Asterisks: pits structure of the surface).
Discussion
Natural teeth and restorations are suffered with normal occlusal and physiological masticatory cycle, consequently cause the anatomical wear loss. 16 Abrasive test evaluated the abrasive performance with an attempt to reflect a real-life situation in clinical cases. Hardness values of resin composite restorations have been assumed to provide as an evidence of their wear resistance properties.17,18 For resin composite restorations, the filler particles play of vital importance for both hardness and wear performance. 19 Therefore, the wear performance and surface hardness of resin composites are of relevance from the clinical standpoint, especially in posterior restorations. The present study evaluated the wear loss and surface hardness of resin composites (HM, Z250, Z350, and TNC) with different types of filler. Esthetic nanohybrid resin composite HM resulted in less ideal wear performance and hardness characteristics, which may negatively influence the longevity of restorations in clinical situations.
Resin-based composites are made of the composition of soft, organic resin matrix, and hard, inorganic filler particles. 20 Therefore, resin matrix, the filler particles and the filler/matrix interface are factors which determine the wear of composites. Large proportion of filler particles will drastically affect the mechanical properties of the resin composite materials such as wear and hardness. Previous studies has been reported that reduce the size of filler particle as well as increase the filler loading can improve the wear resistance of resin composite.18–21 Koottathape et al. 21 assessed the wear performance of resin-based restorations with different filler type, and the results showed that the abrasive wear properties were higher for microfiller parties of resin composite, lower for other type of resin composites. Amaya-Pajares et al. 22 demonstrated the opposite, that small, regular particles of resin composite are inclined to have more wear resistance compared with resin composite of large or irregular filler particles. This results might serve as an explanation that the relatively hard nanofiller particles can be situated tightly to provide protection to the more vulnerable, softer resin matrix, consequently increasing the wear resistance.22–24
However, in the present study, the results showed the opposite. Microhybrid filled resin composite Z250 showed the least wear loss (41.1 ± 2.1 mm3), which indicated the appreciated wear behavior, followed by nano-filled resin composite Z350 (62.5 ± 6.3 mm3). However, nanohybrid filled resin composites TNC and HM showed high wear loss ( 70.2 ± 3.4 mm3 for HM; 70.1 ± 3.9 mm3 for TNC), which indicated the unsatisfied wear performance. Therefore, the first hypotheses that the wear abrasive would not vary among different type of filler contained resin composite was rejected.
Z250 belonged to microhybrid filled resin composite which contains micro-sized filler particles of varying size ranged from 0.01 to 3.5 μm. However, it may logically be kept in mind that microfilled resin composites also contain discrete nanosized filler particles of 40–50 nm diameter. 25 Therefore it cannot be ignored to find that nano-sized and microfilled resin composites actually contain filler particles of approximately the same size. Z250’s profound wear resistance behavior may be attributed to the good sealing function between smaller size of filler particles and larger filler particles, increased surface contact area may have been presented on the surface, different size of discrete filler particles penetrated into the resin matrix, causing a tight bond within the filler particles and the supporting resin matrix. Besides, the large filler particles may maintain greater friction during occlusal wear testing. Consequently, microhybrid filled composite Z250 resulted in appreciated wear resistance.
Oppositely, one can speculated that nanohybrid resin composites (HM and TNC) may confront with the loss of relatively small nano-scale filler particles. The shallow grooves of HM and TNC (Figure 1(a) and (d)) may be associated with their cluster size of fillers. In the term of wear, the greater loss of volume for the nano-sized filler particles (HM, TNC, and Z350) may be explained by the possibility that nano-sized filler particles were too small which were difficult to offer any preferential load support. Consequentially the nano-sized filler particles would easily be flushed away during wear procedure.26,27
Wear resistance was found to be linearly dependent on filler volume. The wear resistance decreased with lower volume of the filler particles, this may explained by the protection of high loading of filler to resin matrix to reduce its wear process.20,28,29 Resin matrix is softer compared with the filler particles, the wear of the resin matrix is in the first place, followed with filler particles. 28 Consequently, a comparatively high volume of particles was reserved on the surface of the restorations. The larger contact area between the fillers and the counterpart resulting in improved wear resistance.20,29 Evidence for this is the fact that high filler volume resin composite Z250 (82 wt%) showed appreciated wear resistance after regularly abrasive wear. For nanohybrid filled HM resin composite, the high filler content (81 wt%) may largely result from the extremely small sizes of these fillers, nano scale of fillers tightly crushed between each other and fit into spaces between other particles, sequentially, the overall filler content increased effectively. However, the protruding nanocluster and individual nanofillers of HM (Figure 2(a)) indicated the relatively unstable bonding of its fillers, therefore the wear behavior was still not satisfied with high filler volume content of HM resin composite.
From the surface observation, nanofilled Z350 demonstrated a more uniform and finer surface, while microhybrid composites Z250 demonstrated a coarser surface with larger filler dislodgement. This can be explained that the larger fillers of Z250 exposed in the process of abrasive wear. In some previous studies, it was speculated that there may have crack and fractures at the filler/matrix interface on the surface degradation. However, surface micro-cracks of Z250 with larger filler content were hardly found in the present study (Figure 2(b)). Nevertheless, an understanding of long-term performance of microhybrid filled resin Z250 is yet to be thoroughly evolved.
Vickers hardness signified a moderate resistance to indentation under functional stresses, many studies has been proposed that Vickers hardness is closely related to wear resistance and long-term stability in the oral environment. 30 It is reported that there were significantly higher surface hardness values of nano scale composites compared with the hybrid filled composites, 29 which is in consistent with our results. Nanofilled resin composite Z350 (98.3 ± 3.8 HV) indicated better hardness value compared with nanohybrid filler resin composite TNC (62.3 ± 1.7 HV) and HM (63.9 ± 1.5 HV). The discrepancies may be attributed to densely packed filler particles of nano scale composite resin composites. However, there was no significant difference of surface hardness of nanofilled Z350 (98.3 ± 3.8 HV) and microhybrid filled Z250 (102.7 ± 2.9 HV), the similarly surface hardness may due to the composition of the filler particles is almost the same, only different in filler particles size. Therefore, the second hypotheses that the surface hardness would not vary among different types of filler contained resin composite was partially rejected.
Over the years, many studies have been reported in the literature regarding the correlation between the hardness and wear resistance. Say et al. 31 has been proposed that there were significantly negative correlation between the Vickers hardness and its wear loss. Similarly results were also be found in this study, higher hardness value of Z250 as well as Z350 presented lower volumetric loss. Nanohybrid HM and TNC presented the opposite. Nevertheless, the data is not sufficient for the analysis of linear relationship. Further studies would be necessary to increase the amount of samples for the linear relationship analysis to investigate the relationship between hardness value and its wear behaviors.
Conclusions
Considering the limitations of this study, it can be concluded that resin composites with different filler types resulted in significantly different wear performance and hardness characteristics. Higher hardness rates and abrasive resistance properties of resin composites may indicate the longevity of restoration in clinical situations. Microhybrid resin composite Z250 exhibited the best wear resistance. Microhybrid resin composite Z250 and nanofilled Z350 showed comparable Vickers hardness of the surface. Resin composite Harmonize, which contained nanohybrid fillers, claimed to provide superior esthetic properties, however, it may still face insufficient wear and surface hardness quality in the long-term clinical situations.
Footnotes
Declaration of conflicting interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by grant No. 2021-YJJ-ZZL-061 from the Foundation of Beijing Tongren Hospital, Capital Medical University.
