Effect of Bleaching on Microhardness of Esthetic Restorative Materials
This study evaluated the effect of a high-concentration carbamide peroxide–containing home bleaching system (Opalescence PF) and a hydrogen peroxide–containing over-the-counter bleaching system (Treswhite Supreme) on the microhardness of two nanocomposites (Filtek Supreme XT and Premise) and leucite-reinforced glass ceramic (Empress Esthetic), glass ceramic (Empress 2 layering), and feldspathic porcelain (Matchmaker MC). A total of 100 specimens, 20 of each kind of the restorative materials, 2 mm in thickness and 10 mm in diameter, were fabricated. Then the specimens were polished with SiC paper and 1 μm alumina polishing paste. After polishing, porcelain specimens were glazed in accordance with the manufacturer's instructions. Each type of restorative material was then randomly divided into two groups (n=10), and the specimens were treated with either Opalescence PF or Treswhite Supreme. The microhardness of the specimens before bleaching (baseline) and after bleaching was determined using a digital microhardness tester. Data were analyzed using the Mann-Whitney U-test and the Wilcoxon test. Opalescence PF significantly influenced the hardness of all the restorative materials. Statistically significant decreases with respect to before bleaching were found for Premise (p=0.005), Empress Esthetic (p=0.003), Empress 2 layering (p=0.005), and Matchmaker-MC (p=0.003), whereas a statistically significant increase was observed in Filtek Supreme XT (p=0.028). The difference in the microhardness values between before and after bleaching using Treswhite Supreme was statistically significant only for Premise (p=0.022). High-concentration carbamide peroxide–containing home bleaching may affect the microhardness of restorative materials.SUMMARY
INTRODUCTION
The esthetic appearance of a person's smile is influenced largely by the color, shape, and position of the teeth.1 Correction of discolored, malformed, crooked teeth often results in improved confidence, personality, and social life.2 Discolored teeth can be esthetically treated with bleaching, microabrasion, macroabrasion, veneering, and placement of porcelain crowns.2 Among these approaches, bleaching is the most minimally invasive method to get an optimum esthetic result in natural teeth.1,3-7 Bleaching techniques can be classified by whether they involve vital or nonvital teeth, by whether the procedure is performed in the office or has an at-home component, or both.8 In-office bleaching techniques include the use of a gel or paste composition that commonly contains 30% to 35% hydrogen peroxide, whereas 10% to 15% carbamide peroxide bleaching material in gel or paste form is generally used for at-home techniques.8 On the other hand, many over-the-counter products are available on the market that contain varying concentrations of hydrogen peroxide in strip, paint-on, or prefabricated tray-based designs.9
Despite the fact that vital bleaching is not regarded as creating macroscopically visible defects, numerous studies have exhibited microstructual changes of dental hard tissues induced by bleaching agents, particularly when peroxides are applied in high concentrations.10 Bleaching materials as well as techniques may also have a varying influence on restorative materials.9,11-14 Especially when bleaching was not performed under the supervision of a dentist, inadvertent application of the bleaching agent to restorative materials may occur more often. Chemical softening resulting from bleaching may affect the microhardness of restorative materials and the clinical durability of tooth-colored restorations.1,15 Because of their organic matrix, composite resins have been shown to be more prone to chemical alteration compared to inert metal or ceramic restorations.16
However, the impact of bleaching on the surface microhardness of composite resins is controversial in the literature. Decreases11,17 as well as increases11,18,19 in microhardness by various types of bleaching regimens have been found, but no significant alterations have been pointed out.20-22 With this background, the purpose of this study was to evaluate the effect of a high-concentration carbamide peroxide–containing home bleaching system (Opalescence PF) and a hydrogen peroxide–containing over-the-counter bleaching system (Treswhite Supreme) on the microhardness of two nanocomposites (Filtek Supreme XT and Premise) and leucite-reinforced glass ceramic (Empress Esthetic), glass ceramic (Empress 2 layering), and feldspathic porcelain (Matchmaker MC).
MATERIALS AND METHODS
Opalescence PF (Ultradent, S Jordan UT, USA), a commercially available home bleaching system containing 20% carbamide peroxide, and Treswhite Supreme (Ultradent), an over-the-counter bleaching system containing 10% hydrogen peroxide, were used in the study. Esthetic restorative materials selected for the study include two nanocomposites (Filtek Supreme XT Enamel; 3M-ESPE, St Paul MN USA; Premise Enamel, Kerr Corp, Orange, CA, USA) and leucite-reinforced glass ceramic (Empress Esthetic; IvoclarVivadent, Schaan, Liechtenstein), glass ceramic (Empress 2 layering; IvoclarVivadent), and feldspathic porcelain (Matchmaker MC; Davis Schottlander & Davis Ltd, Letchworth Garden City,Hertfordshire England). Materials, contents, and their manufacturers are listed in Table 1.
Specimen Preparation
Composite specimens (A2 shade), 2 mm in thickness and 10 mm in diameter, were fabricated using polyurethane molds. The molds were slightly overfilled with the composites and then covered with a mylar strip (Hawe Neos Dental, Gentilino, Switzerland) and pressed with a glass slab to remove voids and excess material. They were then polymerized according to the manufacturer's instructions using a halogen light curing unit (Optilux 501; Kerr Corp) with a light intensity of 550 mW/cm2. Following removal of the Mylar strip, the specimens were stored in distilled water at 37°C for 24 hours. In order to prepare feldspathic and glass ceramic discs, stainless-steel plates, 2 mm in thickness, were held together by welding. Before welding, four holes, 1 cm in diameter, were drilled in one of the plates to obtain flat and parallel bottom discs. The metal mold was boxed, and silicone duplicating impression material (Flexisil Duplicating Silicone; Davis Schottlander & Davis Ltd, Letchworth Garden City, Hertfordshire, England) was mixed in accordance with the manufacturer's directions and poured into the boxed mold to duplicate the holes. Investment material (Veneer Investment; Davis Schottlander & Davis) was poured into the silicone mold by a vibrator. After the material was set, the duplicated mold, which was made of investment, was removed from the silicone impression. This procedure was repeated until 40 investment molds were produced.11 Feldspathic and glass ceramic discs were fabricated on the investment molds according to the manufacturer's instructions. After the fabrication, investment material from the discs was cleaned by air particle abrasion using 50 μm aluminum oxide, and then trimming of the discs was performed with a cylindrical diamond bur.
A rubber mold was used (Preshurnk Mold Ruber, KerrLab, Orange, CA, USA) to facilitate the fabrication of the leucite-reinforced glass ceramic discs by the lost wax technique (Pro-mod VKS Al dente Dental Produkte Meckenbeuren, Germany). After fabrication of the porcelain discs in a furnace (Ivoclar EP 600 combi, IvoclarVivadent, Schaan, Liechtenstein), cleaning of the investment material (IPS PressVEST, IvoclarVivadent, Schaan, Liechtenstein) and trimming of the discs were performed as previously mentioned.
A total of 100 specimens, 20 of each kind of the restorative material, were fabricated. The specimens were polished by the same operator using a sequence of 600-800-1200-2500-grit silicon carbide paper (Buehler, Lake Bluff, IL, USA) and 1 μm alumina polishing paste (MetaDi Diamond Paste; Buehler). The polished specimens were cleaned in distilled water in an ultrasonic cleaner for five minutes to remove any debris. After polishing, porcelain specimens were glazed in accordance with the manufacturer's instructions. Then all specimens were stored in 37°C distilled water for 24 hours.23 Each type of restorative material was then randomly divided into two groups (n=10), and the specimens were treated with either Opalescence PF or Treswhite Supreme. The bleaching agents were painted onto the top surface of each specimen according to the manufacturer's instructions at room temperature and then stored at 37°C during the bleaching period. Treswhite Supreme was applied one hour per day for 10 days, whereas Opalescence PF was applied six hours per day for seven days. After each treatment, the specimens were rinsed for a standardized time of one minute with distilled water to remove the bleaching materials. In order to simulate the clinical situation between each bleaching treatment, the specimens were also kept in distilled water at 37°C.
Microhardness Testing
For each restorative material, the microhardness of the specimens before (baseline) and after bleaching was evaluated using a digital microhardness tester (Vickers Hardness Testing Machine; Buehler). For porcelain specimens, a 1000-g load was applied through the Vickers indentor with a dwell time of 15 seconds,24 whereas a 200-g load with 30 seconds25 was applied for the composite specimens. Three indentations were made at different points on each specimen, and an average value was obtained from these three measurements.
Scanning Electron Microscopy
Representative scanning electron micrographs were taken from each group showing the microhardness similar to the mean microhardness of the corresponding group. The specimens were dried, gold-sputter coated, and observed using a scanning electron microscope (JSM 6335F; JEOL Ltd, Tokyo, Japan).
Statistical Analysis
Multiple comparisons between two bleaching systems and before- (baseline) and after-bleaching groups were performed using the Mann-Whitney U-test and the Wilcoxon test at a significance level of 0.05.
RESULTS
Mean Vickers hardness values and standard deviations of the tested composites and porcelains before and after bleaching are shown in Tables 2 and 3, respectively.
Using the over-the-counter bleaching Treswhite Supreme, a statistically significant difference with respect to before bleaching was found only for the nanocomposite Premise. The microhardness of Premise was significantly decreased (p=0.022) by Treswhite Supreme application. Treswhite Supreme did not produce any statistically significant effect on the surface microhardness of the nanocomposite Filtek Supreme XT (p=0.959), leucite-reinforced glass ceramic (Empress Esthetic; p=0.959), glass ceramic (Empress 2 layering; p=0.484), and feldspathic porcelain (Matchmaker MC; p=0.508).
The home bleaching system Opalescence PF significantly influenced the microhardness of all the restorative materials. A statistically significant decrease with respect to before bleaching was found for Premise (p=0.005), Empress Esthetic (p=0.003), Empress 2 layering (p=0.005), and Matchmaker MC (p=0.003), whereas a statistically significant increase was observed in Filtek Supreme XT (p=0.028).
Scanning electron micrographs of Filtek Supreme XT and Premise as well as of the Empress Esthetic, Empress 2 layering, and Matchmaker MC porcelain before-bleaching groups revealed smooth surfaces (Figures 1a, 2a, 3a, 4a, and 5a, respectively) with apparent different inorganic particle sizes for the composites. On the other hand, the Opalescence PF–applied Filtek Supreme XT group showed a protruding of the filler particles and an erosion of the organic matrix (Figure 1b), whereas no evident differences were observed in the Treswhite Supreme–applied Filtek Supreme XT group (Figure 1c). The Opalescence PF–applied Premise group showed many irregularities with displaced inorganic filler particles with respect to before bleaching (Figure 2b). Similar to the Treswhite Supreme–applied Filtek Supreme XT group, no evident differences were observed in Treswhite Supreme–applied Premise group.
Citation: Operative Dentistry 36, 2; 10.2341/10-078-L
Citation: Operative Dentistry 36, 2; 10.2341/10-078-L
Citation: Operative Dentistry 36, 2; 10.2341/10-078-L
Citation: Operative Dentistry 36, 2; 10.2341/10-078-L
DISCUSSION
The physical properties of restorative materials, such as microhardness, flexural strength, flexural modulus, and fracture toughness, influence the quality and durability of restorations.26 A reduction in microhardness due to organic matrix erosion may enhance the roughness of restorations and may decrease their wear resistance.27 Similar to composites, the microhardness of porcelains characterizes their resistance to deformation, densification, and fracture.28 Therefore, this study aimed to investigate the effect of two bleaching systems—a commercially available tray-based home bleaching system containing 20% carbamide peroxide (Opalescence PF) and an over-the-counter bleaching system containing 10% hydrogen peroxide (Treswhite Supreme)—on the microhardness of nanocomposites and porcelains. To represent clinically relevant bleaching systems that followed the manufacturer's instructions, the high-concentration home bleaching system Opalescence PF was applied to the specimens for six hours per day and a total of seven days,29 whereas the over-the-counter bleaching system Treswhite Supreme was applied for one hour per day and a total of 10 days.30 Between each bleaching treatment, the specimens were stored in 37°C distilled water so that the specimens were not continuously exposed to bleaching products to simulate cumulative effects over time.
Hydrogen peroxide has a high capacity for oxidation and reduction by generating free radicals.31 It also demonstrates an extensive diffusion ability.32 Peroxides have been claimed to induce oxidative cleavage of polymer chains, and thereby unreacted double bonds are expected to be the most vulnerable parts of the polymers.9 Therefore, in the present study, a significant reduction in microhardness after bleaching of the tested nanocomposites was expected since microhybrid and nanocomposites contain a high concentration of resin matrix to be oxidized by hydrogen peroxide.17 However, a statistically significant decrease with respect to before bleaching (baseline) was found only for the nanocomposite Premise with both of the bleaching systems. Free radicals induced by peroxides may impact the resin–filler interface and cause a filler–matrix debonding.31 This fact may lead to the displacement of inorganic filler particles in the Opalescence PF–applied Premise group, as shown in the scanning electron micrograph (Figure 2b). In addition to this, the microhardness of the composites is highly influenced by the amount and type of the inorganic fillers.33 Hence, a reduction in the surface microhardness may be due to the inorganic filler loss on the surface of Premise (Figure 2b). The organic matrix of Filtek Supreme XT consists of UDMA, Bis-EMA,13 and minor amounts of TEGDMA. UDMA and Bis-EMA13 resins are of higher molecular weight and therefore have fewer double bonds per unit of weight. The higher molecular weight of the resin is affirmed to result in less shrinkage, reduced aging, and a slightly softer resin matrix.34 The dissolution effects of the bleaching agent on the materials depend on the penetration depth of the bleaching agent into the restorative materials.35 If the restorative material is tightly crosslinked by high-molecular-weight polymer molecules, the bleaching agent may need more time to diffuse through the link. Hence, the bleaching effect was affirmed as material and concentration dependent.15 This may explain the differences between the microhardness values of the Opalescence PF– and Treswhite Supreme–applied Supreme XT groups (Table 2). The recommended application protocol of the bleaching agent Opalescence is much more than Treswhite Supreme, which may lead the bleaching agent to have sufficient time to diffuse into the high-molecular-weight organic matrix of Filtek Supreme XT. Scanning electron micrographs also showed no evident differences between before and after bleaching for the Treshwhite-applied Supreme XT group (Figure 1a and 1c, respectively), whereas the protruding of inorganic filler particles and erosion of the organic matrix was evident in the Opalescence PF group (Figure 1b).
Commercially available over-the-counter bleaching systems containing hydrogen peroxide in strip and paint on gel form did not have any effect on either the surface morphology or the surface roughness of porcelains,36,37 whereas increased surface roughness38,39 and decreased surface microhardness11,40 were evaluated with carbamide peroxide–containing tray-based systems. Consistent with the previously mentioned literature, in the present study, the tray-based carbamide peroxide–containing bleaching system Opalescence PF significantly decreased the microhardness of leucite-reinforced glass ceramic, glass ceramic, and feldspathic porcelain with respect to before bleaching, whereas the hydrogen peroxide–containing over-the-counter bleaching agent Treswhite Supreme did not significantly affect their microhardness. However, no evident effects were detected on the surface morphology of any of the porcelains and bleaching regimens tested (Figures 3b,c, 4b,c, and 5b,c), which was also in accordance with the findings of Schemehorn and others41 for the feldspathic porcelain. A reduction of surface SiO2 content in feldspathic porcelain after bleaching was shown by energy-dispersive X-ray microanalysis. As SiO2 forms the matrix, this reduction was assumed to affect the surface hardness.12 All the different porcelains used in this study consist of SiO2, and the reduction in microhardness might be due to the possible loss of surface SiO2 content.
On the other hand, the pH value is an important factor for the rate of reaction in the bleaching process. Ionization of buffered hydrogen peroxide in the pH range of 9.5 to 10.8 produces more perhydroxyl HO2 free radicals. The result is a 50% greater bleaching effect in the same period as other pH levels.42 The bleaching agents Opalescence PF and Treswhite Supreme used in this study have an average pH value of 6.5. However, carbamide peroxide–containing Opalescence PF decomposes first into hydrogen peroxide and urea, which further continues to decompose into CO2 and ammonia. Ammonia is a strong base that then offers an elevated pH environment, one that is more favorable for producing stronger free radicals.42,43 As previously mentioned, the difference between the two types of bleaching systems on microhardness may be due to the different application period, the diffusion ability of the materials, and, moreover, the elevated pH environment.
CONCLUSIONS
On the basis of the results of this study, the following can be concluded:
1) The effect of bleaching on the microhardness of restorative materials is material dependent.
2) The application times used for bleaching, representing hydrogen peroxide–containing over-the-counter bleaching, affected the microhardness of only the nanocomposite Premise.
3) The application times used for bleaching, representing carbamide peroxide–containing home bleaching, resulted in a decrease in the microhardness for all the porcelains tested.
4) Before the application of bleaching systems that affect the surface properties of restorative materials, the application of a protective barrier should be considered.
5) Further research is necessary to evaluate whether the decrease in microhardness after high-concentration home bleaching will have any impact on the clinical durability of the nanocomposite, leucite-reinforced glass ceramic, glass ceramic, and feldspathic porcelain restorations.
Citation: Operative Dentistry 36, 2; 10.2341/10-078-L
(a): Scanning electron micrograph of Filtek Supreme XT before bleaching. (b): The Opalescence PF–applied Filtek Supreme XT showed protruding of the inorganic particles and erosion of the organic matrix. (c): The Treswhite Supreme–applied Supreme XT group showed no evident differences compared to its before bleaching (baseline).
(a): Scanning electron micrographs of Premise before bleaching. (b): In the Opalescence PF–applied Premise group, the displacement of inorganic particles was evident. (c): The Treswhite Supreme–applied Premise group showed no evident differences compared to before bleaching.
(a) Scanning electron micrographs of Empress Esthetic before bleaching. (b): The Opalescence PF–applied Empress Esthetic group. (c): The Treswhite Supreme–applied Empress Esthetic group.
(a): Scanning electron micrographs of Empress 2 before bleaching. (b): The Opalescence-applied Empress 2 group. (c): The Treswhite Supreme–applied Empress 2 group.
(a): Scanning electron micrographs of Matchmaker MC before bleaching. (b): The Opalescence PF–applied Matchmaker MC group. (c): The Treswhite Supreme–applied Matchmaker MC group.
Contributor Notes
Özlem Malkondu, DDS, PhD, assistant professor, Department of Prosthodontics, Faculty of Dentistry, Yeditepe University, İstanbul, Turkey
Haktan Yurdagüven, DDS, PhD, assistant professor, Department of Operative Dentistry, Faculty of Dentistry, Yeditepe University, İstanbul, Turkey
Esra Can Say, DDS, PhD, associate professor, Department of Operative Dentistry, Faculty of Dentistry, Yeditepe University, İstanbul, Turkey
Ender Kazazoğlu, DDS, PhD, professor, Department of Prosthodontics Dentistry, Faculty of Dentistry, Yeditepe University, İstanbul, Turkey
Mübin Soyman, professor, Department of Operative Dentistry, Faculty of Dentistry, Yeditepe University, İstanbul, Turkey