journal of the mechanical behavior of biomedical materials 38 (2014) 105–113
Available online at www.sciencedirect.com
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Research Paper
Thermal cycling effects on adhesion of resin–bovine enamel junction among different composite resins Wen-Cheng Chena,n, Chia-Ling Koa,b, Hui-Yu Wua,b, Pei-Ling Laib, Chi-Jen Shihc,1 a Advanced Medical Devices and Composites Laboratory, Department of Fiber and Composite Materials, College of Engineering, Feng Chia University, Taichung, 40724 Taiwan, ROC b Dental Medical Devices and Materials Research Center, College of Dental Medicine, Kaohsiung Medical University, Kaohsiung 807, Taiwan, ROC c Department of Fragrance and Cosmetics Science, Kaohsiung Medical University, Kaohsiung 807, Taiwan, ROC
art i cle i nfo
ab st rac t
Article history:
Thermal cycling is used to mimic the changes in oral cavity temperature experienced by
Received 29 April 2014
composite resins when used clinically. The purpose of this study is to assess the thermal
Received in revised form
cycling effects of in-house produced composite resin on bonding strength. The dicalcium
26 June 2014
phosphate anhydrous filler surfaces are modified using nanocrystals and silanization
Accepted 1 July 2014
(w/NP/Si). The resin is compared with commercially available composite resins Filtek Z250,
Available online 9 July 2014
Z350, and glass ionomer restorative material GIC Fuji-II LC (control). Different composite
Keywords:
resins were filled into the dental enamel of bovine teeth. The bond force and resin–enamel
Composite resin
junction graphical structures of the samples were determined after thermal cycling
Surface modification
between 5 and 55 1C in deionized water for 600 cycles. After thermal cycling, the w/NP/Si
Calcium phosphates
30 wt%, 50 wt% and Filtek Z250, Z350 groups showed higher shear forces than glass
Shear force
ionomer GIC, and w/NP/Si 50 wt% had the highest shear force. Through SEM observations,
Thermal cycling
more of the fillings with w/NP/Si 30 wt% and w/NP/Si 50 wt% groups flowed into the enamel tubule, forming closed tubules with the composite resins. The push-out force is proportional to the resin flow depth and uniformity. The push-out tubule pore and resin shear pattern is the most uniform and consistent in the w/NP/Si 50 wt% group. Accordingly, this developed composite resin maintains great mechanical properties after thermal cycling. Thus, it has the potential to be used in a clinical setting when restoring noncarious cervical lesions. & 2014 Elsevier Ltd. All rights reserved.
n Correspondence to: Advanced Medical Devices and Composites Laboratory, Department of Fiber and Composite Materials, Feng Chia University, 100, Wenhwa Rd., Seatwen, Taichung, 40724, Taiwan (ROC). Tel.: þ886 4 24517250x3413; fax: þ886 4 24514625. E-mail addresses: [email protected], [email protected] (W.-C. Chen). 1 Equal contribution to correspondence.
http://dx.doi.org/10.1016/j.jmbbm.2014.07.003 1751-6161/& 2014 Elsevier Ltd. All rights reserved.
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1.
journal of the mechanical behavior of biomedical materials 38 (2014) 105 –113
Introduction
Composite resins have replaced silver amalgams in being the dominant material used in dental fillings because it can be adjusted to fit natural teeth color (Chen and Wu, 2014). Dentists are also able to closely match the material according to the esthetics and functional needs of patients. Furthermore, composite resin has better coloring and gloss without releasing ions or heavy metals from alloys (Lavigueur and Zhu, 2012). Contemporary dentists have many therapy treatment options for clinically defective restorations (Kuper et al., 2012). Among the different types of defects, clinicians must carefully evaluate enamel cavity preparation and material choice for difficult-to-treat cervical caries. Although composite resin also has better adhesion than alloys due to its flowable property, which allows it to penetrate into dentinal tubules, the repeated changes in temperature within the oral cavity can cause microleakages in restorative material margins and retention failure (Helvatjoglu-Antoniades et al., 2004; Sarrett, 2005; Lavigueur and Zhu, 2012). Much effort has been put into developing new composite resins by improving adhesion and restorative techniques (Bayne, 2005; Lavigueur and Zhu, 2012). These challenges and the criteria used for evaluating restorative substances depend on many factors, including polymerization contraction, thermal changes, and occlusion forces that result in debonded stress at the marginal interfaces (Xu et al., 1999, 2002; Silva et al., 2013). Currently, commercially available composite resins suffer from microleakage because the hydrophilic monomers polymerization often used for composite resins are easily affected by the water content of the oral cavity in patients, causing crevices and leakages (Xu et al., 1999; Bayne, 2005). Furthermore, thermal cycling from changes in oral temperature provides conditions for degraded bond strength in an aqueous environment as well as microleakages (Xu et al., 2002). The formation of microleakages between the margin of restorative sites and teeth causes complications due to bacterial growth. Eventually, bacterial growth may acidify dentin and restorative substances, thereby causing the resin interface to dissolve. Secondary caries may result, and, if untreated, may cause restoration fracture (Sarrett, 2005; Miglani et al., 2010). Thus, composite resin is continuously being researched and improved upon to meet different clinical needs and dental restoration purposes (Dickens-Venz et al., 1994; Skrtic et al., 1996; Dickens-Venz et al., 2003; Chen, 2010; Vouvoudi and Sideridou, 2012). For example, the basic reinforced fillers formula and types for coupling agents are being studied. Most especially, changing reinforced fillers have a significant effect on the properties of composite resin and is the commonly used strategy (Klapdohr and Moszner, 2005; Cramer et al., 2011). For example, when fillers are modified with whiskers capped on its surface, the adhesion between the composite resin matrix and the fillers increases significantly, overcoming the shrinkage caused by polymerization (Xu et al., 1999, 2002; Klapdohr and Moszner, 2005). Furthermore, the ions automatically released by calcium phosphate fillers may be beneficial for decalcified dentin from remineralization and controls microleakage from thermal cycling
effects (Skrtic et al., 1996; Dickens-Venz et al., 2003). Although the aforementioned calcium phosphate bioceramic has the ability to induce remineralization of dentin in vitro (Thomann et al., 1990; Chen et al., 2013a), relevant studies have not supported the expected results of restorative materials against thermal cycling. The current research uses technology previously developed in-house by capping a layer of nanocrystals and salinization with resin matrix to form a compound of composite resin (Chen et al., 2013b; Chen and Wu, 2014). In vitro studies have shown that reinforced fillers with treated with nanocrystals increase the mechanical strength of composite resins. The hydroxyl group within the phosphate composite increases the chemical bond between the resin and dentin. Furthermore, the release of calcium and phosphate ions induces apparent remineralized precipitates on the sample surfaces in vitro (Klapdohr and Moszner, 2005; Chen, 2010; Cramer et al., 2011). Although initial studies are promising, more research must be performed to study the clinical applications of composite resins and prove that such resins could be quantified for treating cervical carious lesions. To simulate fillings in the clinical setting, composite resins including Filtek Z250 composite resin (A3shade, 3 M/ESPE), Filtek Z350 flowable resin (A3 shade, 3 M/ESPE) as the comparison group, and a clinicallyavailable glass ionomer Fuji-II filling (GIC) as the control group were filled into bovine teeth and put through thermal cycling. The mechanical properties and SEM observations were studied. This study hypothesizes that, after thermal cycling, composite resins using calcium phosphates as fillers have less debonding force decay than other fillers such as ZrO2 and glass ionomers. This study is based on the aforementioned premises that study how thermal cycling processes cause shear force decay and the bonding differences between the composite material and dentin to simulate composite resins used clinically.
2.
Materials and methods
2.1.
Preparation of composite resins
The DCPA filler (CaHPO4, Alfa Aesar GmbH & Co. KG, Karlsruhe, Germany) powder ranged from 1 μm to 3 μm in particle distribution size, and powder with 98% was used. Nanocrystal formation behavior was determined during the monitored treatment according to previous studies (Chen et al., 2013b; Chen and Wu, 2014). Briefly, 5 g of DCPA powder was mixed in 40 mL solution with a constant calcium-to-phosphate atomic ratio of 2.0 and a pH of 5.0 for 20 min at room temperature. The particles were filtered, washed, and then dried. A 100 mL colloidal solution of cyclohexane solvent with 4% (v/v) (3Mercaptopropyl) trimethoxysilane and 2% (v/v) n-propylamine from Alfa Aesar GmbH & Co., KG (Karlsruhe, Germany) was used to cap the particle surface. Up to 5 g per batch of DCPA powder with nanocrystal modification was added to the colloidal solution under rapid agitation for 30 min at room temperature and heated to 60 1C. Then, the solvent was removed by drying the samples in a vacuum for 2 h.
journal of the mechanical behavior of biomedical materials 38 (2014) 105 –113
The chemicals used in custom-made resins were all purchased from Sigma-Aldrich. The monomers were composed of bisphenol A diglycidyl methacrylate (48.975% Bis-GMA) and triethylene glycol dimethacrylate (48.975% TEGDMA) with a light initiator camphoroquinone (1.0% CQ, SIGMA-ALDRIC Co., Buchs, Switzerland), an accelerator of dimethylaminoethyl methacrylate (1.0% DMAEMA), and butylated hydroxyl toluene (0.05% BHT) photostability. To form a resin, the compositions of organic matrix and inorganic fillers of DCPA with treated fillerbased composite resin at 30% and 50% weight percentage ratios of filler/(filler and matrix) (respective abbreviated names of w/ NP/Si 30 wt% and w/NP/Si 50 wt%) were prepared in a dark room and mixed using a magnetic stirrer until a colloid was formed.
2.2.
Measurements
Composite resins were studied using Filtek Z250 composite resin (A3shade, 3 M/ESPE), Filtek Z350 flowable resin (A3 shade, 3 M/ESPE) as the comparison group, and a clinically-available glass ionomer Fuji-II filling (GIC) as the control group. The etching agent used on bovine teeth was 37% phosphoric acid and the bonding agent was mainly composed of acrylic monomer; both products are from SDI Limited, Australia. First, the specimens were prepared by incubation in normal saline for 24 h. Then, it was taken out for tissue sectioning using a high-speed drilling device for dental use to drill cavities with consistent dimensions of 3 mm in length, 0.5 mm in width, and 1 mm in thickness. The drilling process was performed in a water environment to mimic the inside of an oral cavity. Manufacturer
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instructions were followed when the composite resins were filled. The bovine cavities were etched for 20 s, washed, and adhesives were applied onto the surface of the cavity. Then, the cavities were cured with light for 15 s followed by being filled with resin. After filling, the composite resin was cured with light from above and two sides for 40 s each. A dental curing light machine (Demetron Optilux 401) was used. Light intensity was maintained between 580 and 600 mW/cm2. The optical head faced the specimens at a working distance of 1.5 mm. The samples were divided into two groups: one comprised of as-prepared dried samples from curing, which were tested immediately, and the other comprised of wet samples obtained by further immersing the cured specimen in deionized water at 37 1C beyond the initial 24 h immersion, followed by thermal cycling (600 cycles, 5 1C/55 1C, 2 min/ cycle) (Long Wha Enterprise Co., Kaohsiung, Taiwan) (Monteiro et al., 2011). The samples were obtained through thermal cycling testing using specimens immersed in pure water baths (1 g sample in 10 mL deionized water) to prevent ion effects. Then, the testing of different specimens was immediately recorded using a desktop mechanical tester (LLOYD instruments, Tokyo, Japan) at a crosshead speed of 1.0 mm/min. The resin from the bovine was tested, and their push-out force (N) and working distances (mm) were recorded. Five duplicate specimens were prepared and analyzed for each group (n ¼5). The push-out shear tests were performed as shown in Fig. 1. The fractured specimens were collected and tagged as bovine teeth and composite resin margins for SEM scanning (Scanning Electron Microscope, SEM, Hitachi S-3000N, Hitachi, Tokyo, Japan) to observe the fracture at the interface between the resin and the teeth.
Fig. 1 – Schematic diagram and photo displaying methods for bovine teeth restoration with composite resins and push-out debonding force tests.
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journal of the mechanical behavior of biomedical materials 38 (2014) 105 –113
Fig. 2 – The debonding force tests of different groups of restored bovine teeth analyzed by one-way ANOVA and Tukey's HSD. Labels in capital letters represent groups after thermal cycles. Groups with the same letters represent no significant difference whereas different letters represent statistical significance.
2.3.
Statistical analysis
To study the highest push-out shear forces following thermal cycling processes, the results were analyzed using one-way ANOVA and Tukey's HSD to compare significant differences among each group. JMP 9.0 software (SAS Institute, Inc., Cary, NC, USA) was used for statistical analysis. In all cases, the results were considered statistically different at po0.05.
integrated area under the curve represents the total energy absorption of toughness. These results demonstrate that w/NP/Si 50 wt% and 3 M-Z350 absorbed the deformation energy with the same pattern and magnitude (Fig. 3). Furthermore, the plastic deformations of the composite resins were not affected by thermal cycling. However, the push-out debonding force and energy absorption did decrease significantly within the strain hardening zone.
3.2.
3.
Results
3.1.
Push-out testing of debonding forces
The results of the push out debonding tests of each group before and after thermal cycling are shown in Fig. 2. The material ultimate forces of 3 M-Z250 and w/NP/Si 50 wt% are very similar while 3 M-Z350 is similar to w/NP/Si 30 wt% of which w/NP/Si 50 wt% and Z250 were the strongest in terms of debonding forces at 78.14 N and 69.58 N, respectively. One way ANOVA and Tukey's HSD analysis reveal that 3 M-Z250 was significantly stronger than 3 M-Z350, GIC, and w/NP/Si 30 wt% before thermal cycling (po0.01). No significant differences were observed between 3 M-Z250 and w/NP/Si 50 wt% (p40.05) (Fig. 2). After thermal cycling, the values of all groups declined in push-out debonding forces except for w/NP/Si 30 wt%, which showed no significant differences compared to the specimen before thermal cycling. Notably, although w/NP/Si 50 wt% showed a significant decline in debonding forces after thermal cycling, it still maintained the highest debonding force (50.55 N) among all groups. Furthermore, statistical analysis shows that w/NP/Si 50 wt% is significantly higher in debonding force compared to 3 M-Z350 and GIC (po0.01). The flat portion of the stress–strain curves represents the total plastic deformation and the strain hardening zone, where the
Microscopic structure analysis of bovine teeth
Microscopic analysis of the bovine teeth after push-out debonding tests with SEM showed that composite resins flowed into the bovine dental tubules in the w/NP/Si 30 wt% and w/NP/Si 50 wt% groups (Fig. 4). Furthermore, the observed de-bonding fracturing surfaces did not all occur at the interface between the cavities of bovine tooth and the composite resin filling. Instead, some fractures occurred directly on the bovine teeth, suggesting that the interface toughness is higher than the bovine teeth itself. However, the commercially available composite resin/3 MZ250 did not flow into the dentine tubules as much as the flowable composite resin/3 M-Z350, which were similar to w/NP/Si 30 wt% and w/NP/Si 50 wt%, as observed by SEM (Fig. 5). Notably, the images of the fracture surface did not reveal any of the commercially available GIC filling the dentinal tubules in Fig. 5, suggesting that glass ionomer restorative materials are not suitable for repairing cervical lesions.
4.
Discussion
The null hypothesis of this study was partially robust because w/NP/Si 30 wt% group had no debonding force decay in the push-out tests among composite resins. However, the w/NP/
journal of the mechanical behavior of biomedical materials 38 (2014) 105 –113
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Fig. 3 – Typical push-out debonding force-extension profiles of different bonding materials before (dried debonding forces) and after thermal cycling processes (wet debonding forces).
Si 50 wt% group apparent declined in ultimate push-out forces compared with other composite resins even though the force was higher than other groups after the thermal cycling processes. The present study uses bovine teeth to simulate teeth dental caries filled with composite resin and found that commercially available 3 M-Z250 and the w/NP/Si 50 wt% group produced in-house had the best push-out forces before thermal cycling. Specimens in all groups after thermal cycling decreased in ultimate push-out forces, suggesting that thermal cycling does indeed affect interface bonding between restorative substances and tissues. Although the bonding forces of w/NP/Si 50 wt% decreased by approximately 20% after thermal cycling, it still maintained the highest bonding forces among all groups. However, the push-out forces of w/NP/Si 30 wt% did not significantly change after thermal cycling. The thermal cycling weakens the interface possibly due to the volume changes in situ from the cycled heating and cooling. Therefore, microcracks are created between the resin and the bovine teeth tubules. Once shear force is acted on the restorative material in the interfacial junction, fractures propagate from the microcracks, which have been found in previous studies where all specimens became weaker after thermal cycling (Helvatjoglu-Antoniades et al., 2004; Fischer et al., 2010). Furthermore, morphologies by SEM analysis revealed that some resin flowed into the tubules. The composite resin in this study uses DCPA as filler particles. DCPA is a major component in hydroxyapatite (HA). It also hydrolyzes to become HA when placed in neutral water for a prolonged period of time. Accordingly, as previously mentioned, enamel, dentine, and alveolar bone tissue are mostly composed of HA. Thus, DCPA, through the mineralization mechanism, exchanges Ca2þ and PO4 3 ions to strengthen the bonding, thus raising the shear force (Hossein et al., 2008; Ishikawa et al., 1999). Moreover, Xu et al. (2002) mentioned that whiskered filler patterns result in higher mechanical properties because the high surface contact area of the reinforced fillers are anisotropic, which offsets the volume changing effects of thermal cycling (Xu et al., 2002). This phenomenon is similar to the results in the present study due to the anisotropic strengthening provided by fillers.
The resin seen flowing into the dentinal tubules suggests that these composite resins have the potential to be used clinically. They can also serve as relief material against sensitive teeth. Sensitive teeth are due to dentin hypersensitivity, caused by the chemical, physical, and temperature stimulation of exposed dentinal tubules. These stimuli would change osmolality and result in discomfort (Miglani et al., 2010). Currently, this phenomenon is not well studied, and no mechanism for the cause of the disease has been proven. The most acceptable proposed theory is the fluid dynamics theory by Bräennströem and Aströem, 1964, which states that the fluid inside the dentinal tubules flow and excite the peripheral nerves of the pulp cavity, thereby causing soreness, pain, and discomfort (Bräennströem and Aströem, 1964). Therefore, the current strategy for treating hypersensitivity is occluding dentinal tubules, which decreases permeability and limits the contact of the tubules to prevent stimulus from the external environment. Additionally, research by Stojanac et al. showed that, after 12 and 24 months, microfilled composites (Esthet.X/Dentsply/De Trey, Konstanz, Germany, and Prime&Bond NT/Dentsply/De Trey), nanohybrid composites (TetricEvoCeram/Vivadent, Schaan, Liechtenstein, and AdheSE/Vivadent), and compomers (Dyract eXtra/Dentsply/ De Trey and Xeno III Dentsply/De Trey) used in the restoration of noncarious cervical lesions all showed promising results (Stojanac et al., 2013). Under appropriate adhesive systems and properly implemented restorative procedures, the study gave satisfactory results after a two-year evaluation period. Originally, calcium phosphate fillers in the composite resins were expected to have the ability to release ions through thermal cycling processes. The DCPA used in the present study is stable under acidic environments through the release of Ca2þ and phosphate ions (Xu et al., 2006; Mandel and Tas, 2010; Sun et al., 2010). The ions can form a apatite-like phase precipitation, making this more effective interlocking the dentinal tubules (Mandel and Tas, 2010). Thus, suppressing against shear force decay is better than ZrO2 and glass ionomers through the remineralized mechanism. Although the potential for generating a barrier through the mineralization mechanism enabled the possible positive effect of avoiding micro-leakages, this study still did not
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Fig. 4 – SEM analysis of fractured specimen from push-out tests and the relative locations of specimen are shown in schematic diagram of Fig. 1. The observed de-bonding fracturing portions did not all occur at the interface ( apparent in the 2nd and 4th rows of w/NP/Si 50 wt% and 30 wt% groups, respectively) between the dentine tubules and the composites. The elongated resin morphology as indicated by the white-arrows of w/NP/Si 30 wt% and 50 wt% groups shows the toughening effects.
observe the phenomenon of remineralization. However, the major and significant differences between groups of commercial 3 M-Z250 and Z350 with 70% ZrO2 filler groups and inhouse w/NP/Si with 30% and 50% filler groups are the fluid penetration depths into the dentinal tubules (Figs. 4 and 5), which led to the energy required for tough fracture to take place (Fig. 3). The apparent difference of push-out strength and strain curve shown in Fig. 3 was the characteristic
changes of the tough fracture. The areas under the curves between 3 M-Z250 and w/NP/Si 50 wt% are comparable. The resin of 3 M-Z250 with 70% ZrO2 filler is reasonably more brittle than the resin of w/NP/Si with 50% ZrO2 fillers. However, the penetration depth into the dentinal tubules of w/NP/Si with 50% ZrO2 is larger than that of the Z250 group. Furthermore in 2011, Mitchell et al. suggested that nano-sized bioactive glass decreases fluid dynamics in dentinal tubules,
journal of the mechanical behavior of biomedical materials 38 (2014) 105 –113
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Fig. 5 – The fractured surfaces of 3 M-Z250, 3 M-Z350, and GIC groups are shown. The 3 M-Z250 did not flow into the dentine tubules (clearly shown in the 3rd and 4th rows) as much as the 3 M-Z350. The fracture surfaces of GIC did not show any exposure of dental tubules, which revealed that the fracture occurred on the matrix of restorative materials but not on the restorative interfaces. The fracture morphology as indicated by black-arrows of Z250 and Z350 groups show the brittle cleavage type of fractures.
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leading to possible shallower penetration depth by 3 M-Z250/ Z350 with 70% nano-sized ZrO2 fillers (Mitchell et al., 2011). Aside from the more ductile fracture patterns of the in-house w/NP/Si with 30% and 50% composites, this result is meaningful and the data is shown in Figs. 4 and 5. The fracture patterns indicate that the 3 M-Z250 and Z350 are either more brittle or penetrate only a shallow depth compared with w/NP/Si with 30% and 50% composites and the comparison data of tough fracture is shown. Furthermore, Class V restorations also represent one of the less durable types of restorations and have a high index of loss of retention, marginal linkage, and secondary caries. These issues increase the possibility of material failure, which are caused by the difficulties in clinical practice, including isolation moisture control and gaining access to subgingival margins, inserting materials, contouring, and finishing and polishing procedures (Perez Cdos et al., 2012). These procedures were studied with ex vivo standardized test specimens, making comparisons and simulations of in vivo conditions difficult.
5.
Conclusion
Although many methods reduce the decay in the shear bonding force of the material from thermal cycling, our study showed that the incorporation of nanocrystals that grow on DCPA surfaces and further salinization come close to the properties of commercially available products. The decrease in debonding force and dentinal tubule permeation after thermal cycling were better than that of commercially available products as well. This procedure could be a potential solution for further improving the bonding forces between teeth cervical lesions and the restoration material in clinical applications. As a result, the more the performance of similar surface modified calcium phosphates in composite resins as a filler is understood, the more potential these materials can are proven to have and the closer they are to being used to restore teeth cervical lesions and reduce restoration failures.
Acknowledgments The authors acknowledge the assistance of Ms. Ya-Shun Chen, Dr. Chun-Cheng Hung and Dr. Jen-Chyan Wang. We also appreciate the major funding support of the Ministry of the Science and Technology, Taiwan under contract NSC99-2314B037-051-MY3 and MOST103-2622-E-035-006-CC2 and the partial support plan provided by Alliance Global Technology Co., Ltd., in Kaohsiung Medical Device Special Zone in Southern Taiwan Science Park (EG-32-09-16-101 and AZ-10-08-24-102) are likewise appreciated.
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www.elsevier.com/locate/jmbbm
Research Paper
Thermal cycling effects on adhesion of resin–bovine enamel junction among different composite resins Wen-Cheng Chena,n, Chia-Ling Koa,b, Hui-Yu Wua,b, Pei-Ling Laib, Chi-Jen Shihc,1 a Advanced Medical Devices and Composites Laboratory, Department of Fiber and Composite Materials, College of Engineering, Feng Chia University, Taichung, 40724 Taiwan, ROC b Dental Medical Devices and Materials Research Center, College of Dental Medicine, Kaohsiung Medical University, Kaohsiung 807, Taiwan, ROC c Department of Fragrance and Cosmetics Science, Kaohsiung Medical University, Kaohsiung 807, Taiwan, ROC
art i cle i nfo
ab st rac t
Article history:
Thermal cycling is used to mimic the changes in oral cavity temperature experienced by
Received 29 April 2014
composite resins when used clinically. The purpose of this study is to assess the thermal
Received in revised form
cycling effects of in-house produced composite resin on bonding strength. The dicalcium
26 June 2014
phosphate anhydrous filler surfaces are modified using nanocrystals and silanization
Accepted 1 July 2014
(w/NP/Si). The resin is compared with commercially available composite resins Filtek Z250,
Available online 9 July 2014
Z350, and glass ionomer restorative material GIC Fuji-II LC (control). Different composite
Keywords:
resins were filled into the dental enamel of bovine teeth. The bond force and resin–enamel
Composite resin
junction graphical structures of the samples were determined after thermal cycling
Surface modification
between 5 and 55 1C in deionized water for 600 cycles. After thermal cycling, the w/NP/Si
Calcium phosphates
30 wt%, 50 wt% and Filtek Z250, Z350 groups showed higher shear forces than glass
Shear force
ionomer GIC, and w/NP/Si 50 wt% had the highest shear force. Through SEM observations,
Thermal cycling
more of the fillings with w/NP/Si 30 wt% and w/NP/Si 50 wt% groups flowed into the enamel tubule, forming closed tubules with the composite resins. The push-out force is proportional to the resin flow depth and uniformity. The push-out tubule pore and resin shear pattern is the most uniform and consistent in the w/NP/Si 50 wt% group. Accordingly, this developed composite resin maintains great mechanical properties after thermal cycling. Thus, it has the potential to be used in a clinical setting when restoring noncarious cervical lesions. & 2014 Elsevier Ltd. All rights reserved.
n Correspondence to: Advanced Medical Devices and Composites Laboratory, Department of Fiber and Composite Materials, Feng Chia University, 100, Wenhwa Rd., Seatwen, Taichung, 40724, Taiwan (ROC). Tel.: þ886 4 24517250x3413; fax: þ886 4 24514625. E-mail addresses: [email protected], [email protected] (W.-C. Chen). 1 Equal contribution to correspondence.
http://dx.doi.org/10.1016/j.jmbbm.2014.07.003 1751-6161/& 2014 Elsevier Ltd. All rights reserved.
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1.
journal of the mechanical behavior of biomedical materials 38 (2014) 105 –113
Introduction
Composite resins have replaced silver amalgams in being the dominant material used in dental fillings because it can be adjusted to fit natural teeth color (Chen and Wu, 2014). Dentists are also able to closely match the material according to the esthetics and functional needs of patients. Furthermore, composite resin has better coloring and gloss without releasing ions or heavy metals from alloys (Lavigueur and Zhu, 2012). Contemporary dentists have many therapy treatment options for clinically defective restorations (Kuper et al., 2012). Among the different types of defects, clinicians must carefully evaluate enamel cavity preparation and material choice for difficult-to-treat cervical caries. Although composite resin also has better adhesion than alloys due to its flowable property, which allows it to penetrate into dentinal tubules, the repeated changes in temperature within the oral cavity can cause microleakages in restorative material margins and retention failure (Helvatjoglu-Antoniades et al., 2004; Sarrett, 2005; Lavigueur and Zhu, 2012). Much effort has been put into developing new composite resins by improving adhesion and restorative techniques (Bayne, 2005; Lavigueur and Zhu, 2012). These challenges and the criteria used for evaluating restorative substances depend on many factors, including polymerization contraction, thermal changes, and occlusion forces that result in debonded stress at the marginal interfaces (Xu et al., 1999, 2002; Silva et al., 2013). Currently, commercially available composite resins suffer from microleakage because the hydrophilic monomers polymerization often used for composite resins are easily affected by the water content of the oral cavity in patients, causing crevices and leakages (Xu et al., 1999; Bayne, 2005). Furthermore, thermal cycling from changes in oral temperature provides conditions for degraded bond strength in an aqueous environment as well as microleakages (Xu et al., 2002). The formation of microleakages between the margin of restorative sites and teeth causes complications due to bacterial growth. Eventually, bacterial growth may acidify dentin and restorative substances, thereby causing the resin interface to dissolve. Secondary caries may result, and, if untreated, may cause restoration fracture (Sarrett, 2005; Miglani et al., 2010). Thus, composite resin is continuously being researched and improved upon to meet different clinical needs and dental restoration purposes (Dickens-Venz et al., 1994; Skrtic et al., 1996; Dickens-Venz et al., 2003; Chen, 2010; Vouvoudi and Sideridou, 2012). For example, the basic reinforced fillers formula and types for coupling agents are being studied. Most especially, changing reinforced fillers have a significant effect on the properties of composite resin and is the commonly used strategy (Klapdohr and Moszner, 2005; Cramer et al., 2011). For example, when fillers are modified with whiskers capped on its surface, the adhesion between the composite resin matrix and the fillers increases significantly, overcoming the shrinkage caused by polymerization (Xu et al., 1999, 2002; Klapdohr and Moszner, 2005). Furthermore, the ions automatically released by calcium phosphate fillers may be beneficial for decalcified dentin from remineralization and controls microleakage from thermal cycling
effects (Skrtic et al., 1996; Dickens-Venz et al., 2003). Although the aforementioned calcium phosphate bioceramic has the ability to induce remineralization of dentin in vitro (Thomann et al., 1990; Chen et al., 2013a), relevant studies have not supported the expected results of restorative materials against thermal cycling. The current research uses technology previously developed in-house by capping a layer of nanocrystals and salinization with resin matrix to form a compound of composite resin (Chen et al., 2013b; Chen and Wu, 2014). In vitro studies have shown that reinforced fillers with treated with nanocrystals increase the mechanical strength of composite resins. The hydroxyl group within the phosphate composite increases the chemical bond between the resin and dentin. Furthermore, the release of calcium and phosphate ions induces apparent remineralized precipitates on the sample surfaces in vitro (Klapdohr and Moszner, 2005; Chen, 2010; Cramer et al., 2011). Although initial studies are promising, more research must be performed to study the clinical applications of composite resins and prove that such resins could be quantified for treating cervical carious lesions. To simulate fillings in the clinical setting, composite resins including Filtek Z250 composite resin (A3shade, 3 M/ESPE), Filtek Z350 flowable resin (A3 shade, 3 M/ESPE) as the comparison group, and a clinicallyavailable glass ionomer Fuji-II filling (GIC) as the control group were filled into bovine teeth and put through thermal cycling. The mechanical properties and SEM observations were studied. This study hypothesizes that, after thermal cycling, composite resins using calcium phosphates as fillers have less debonding force decay than other fillers such as ZrO2 and glass ionomers. This study is based on the aforementioned premises that study how thermal cycling processes cause shear force decay and the bonding differences between the composite material and dentin to simulate composite resins used clinically.
2.
Materials and methods
2.1.
Preparation of composite resins
The DCPA filler (CaHPO4, Alfa Aesar GmbH & Co. KG, Karlsruhe, Germany) powder ranged from 1 μm to 3 μm in particle distribution size, and powder with 98% was used. Nanocrystal formation behavior was determined during the monitored treatment according to previous studies (Chen et al., 2013b; Chen and Wu, 2014). Briefly, 5 g of DCPA powder was mixed in 40 mL solution with a constant calcium-to-phosphate atomic ratio of 2.0 and a pH of 5.0 for 20 min at room temperature. The particles were filtered, washed, and then dried. A 100 mL colloidal solution of cyclohexane solvent with 4% (v/v) (3Mercaptopropyl) trimethoxysilane and 2% (v/v) n-propylamine from Alfa Aesar GmbH & Co., KG (Karlsruhe, Germany) was used to cap the particle surface. Up to 5 g per batch of DCPA powder with nanocrystal modification was added to the colloidal solution under rapid agitation for 30 min at room temperature and heated to 60 1C. Then, the solvent was removed by drying the samples in a vacuum for 2 h.
journal of the mechanical behavior of biomedical materials 38 (2014) 105 –113
The chemicals used in custom-made resins were all purchased from Sigma-Aldrich. The monomers were composed of bisphenol A diglycidyl methacrylate (48.975% Bis-GMA) and triethylene glycol dimethacrylate (48.975% TEGDMA) with a light initiator camphoroquinone (1.0% CQ, SIGMA-ALDRIC Co., Buchs, Switzerland), an accelerator of dimethylaminoethyl methacrylate (1.0% DMAEMA), and butylated hydroxyl toluene (0.05% BHT) photostability. To form a resin, the compositions of organic matrix and inorganic fillers of DCPA with treated fillerbased composite resin at 30% and 50% weight percentage ratios of filler/(filler and matrix) (respective abbreviated names of w/ NP/Si 30 wt% and w/NP/Si 50 wt%) were prepared in a dark room and mixed using a magnetic stirrer until a colloid was formed.
2.2.
Measurements
Composite resins were studied using Filtek Z250 composite resin (A3shade, 3 M/ESPE), Filtek Z350 flowable resin (A3 shade, 3 M/ESPE) as the comparison group, and a clinically-available glass ionomer Fuji-II filling (GIC) as the control group. The etching agent used on bovine teeth was 37% phosphoric acid and the bonding agent was mainly composed of acrylic monomer; both products are from SDI Limited, Australia. First, the specimens were prepared by incubation in normal saline for 24 h. Then, it was taken out for tissue sectioning using a high-speed drilling device for dental use to drill cavities with consistent dimensions of 3 mm in length, 0.5 mm in width, and 1 mm in thickness. The drilling process was performed in a water environment to mimic the inside of an oral cavity. Manufacturer
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instructions were followed when the composite resins were filled. The bovine cavities were etched for 20 s, washed, and adhesives were applied onto the surface of the cavity. Then, the cavities were cured with light for 15 s followed by being filled with resin. After filling, the composite resin was cured with light from above and two sides for 40 s each. A dental curing light machine (Demetron Optilux 401) was used. Light intensity was maintained between 580 and 600 mW/cm2. The optical head faced the specimens at a working distance of 1.5 mm. The samples were divided into two groups: one comprised of as-prepared dried samples from curing, which were tested immediately, and the other comprised of wet samples obtained by further immersing the cured specimen in deionized water at 37 1C beyond the initial 24 h immersion, followed by thermal cycling (600 cycles, 5 1C/55 1C, 2 min/ cycle) (Long Wha Enterprise Co., Kaohsiung, Taiwan) (Monteiro et al., 2011). The samples were obtained through thermal cycling testing using specimens immersed in pure water baths (1 g sample in 10 mL deionized water) to prevent ion effects. Then, the testing of different specimens was immediately recorded using a desktop mechanical tester (LLOYD instruments, Tokyo, Japan) at a crosshead speed of 1.0 mm/min. The resin from the bovine was tested, and their push-out force (N) and working distances (mm) were recorded. Five duplicate specimens were prepared and analyzed for each group (n ¼5). The push-out shear tests were performed as shown in Fig. 1. The fractured specimens were collected and tagged as bovine teeth and composite resin margins for SEM scanning (Scanning Electron Microscope, SEM, Hitachi S-3000N, Hitachi, Tokyo, Japan) to observe the fracture at the interface between the resin and the teeth.
Fig. 1 – Schematic diagram and photo displaying methods for bovine teeth restoration with composite resins and push-out debonding force tests.
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journal of the mechanical behavior of biomedical materials 38 (2014) 105 –113
Fig. 2 – The debonding force tests of different groups of restored bovine teeth analyzed by one-way ANOVA and Tukey's HSD. Labels in capital letters represent groups after thermal cycles. Groups with the same letters represent no significant difference whereas different letters represent statistical significance.
2.3.
Statistical analysis
To study the highest push-out shear forces following thermal cycling processes, the results were analyzed using one-way ANOVA and Tukey's HSD to compare significant differences among each group. JMP 9.0 software (SAS Institute, Inc., Cary, NC, USA) was used for statistical analysis. In all cases, the results were considered statistically different at po0.05.
integrated area under the curve represents the total energy absorption of toughness. These results demonstrate that w/NP/Si 50 wt% and 3 M-Z350 absorbed the deformation energy with the same pattern and magnitude (Fig. 3). Furthermore, the plastic deformations of the composite resins were not affected by thermal cycling. However, the push-out debonding force and energy absorption did decrease significantly within the strain hardening zone.
3.2.
3.
Results
3.1.
Push-out testing of debonding forces
The results of the push out debonding tests of each group before and after thermal cycling are shown in Fig. 2. The material ultimate forces of 3 M-Z250 and w/NP/Si 50 wt% are very similar while 3 M-Z350 is similar to w/NP/Si 30 wt% of which w/NP/Si 50 wt% and Z250 were the strongest in terms of debonding forces at 78.14 N and 69.58 N, respectively. One way ANOVA and Tukey's HSD analysis reveal that 3 M-Z250 was significantly stronger than 3 M-Z350, GIC, and w/NP/Si 30 wt% before thermal cycling (po0.01). No significant differences were observed between 3 M-Z250 and w/NP/Si 50 wt% (p40.05) (Fig. 2). After thermal cycling, the values of all groups declined in push-out debonding forces except for w/NP/Si 30 wt%, which showed no significant differences compared to the specimen before thermal cycling. Notably, although w/NP/Si 50 wt% showed a significant decline in debonding forces after thermal cycling, it still maintained the highest debonding force (50.55 N) among all groups. Furthermore, statistical analysis shows that w/NP/Si 50 wt% is significantly higher in debonding force compared to 3 M-Z350 and GIC (po0.01). The flat portion of the stress–strain curves represents the total plastic deformation and the strain hardening zone, where the
Microscopic structure analysis of bovine teeth
Microscopic analysis of the bovine teeth after push-out debonding tests with SEM showed that composite resins flowed into the bovine dental tubules in the w/NP/Si 30 wt% and w/NP/Si 50 wt% groups (Fig. 4). Furthermore, the observed de-bonding fracturing surfaces did not all occur at the interface between the cavities of bovine tooth and the composite resin filling. Instead, some fractures occurred directly on the bovine teeth, suggesting that the interface toughness is higher than the bovine teeth itself. However, the commercially available composite resin/3 MZ250 did not flow into the dentine tubules as much as the flowable composite resin/3 M-Z350, which were similar to w/NP/Si 30 wt% and w/NP/Si 50 wt%, as observed by SEM (Fig. 5). Notably, the images of the fracture surface did not reveal any of the commercially available GIC filling the dentinal tubules in Fig. 5, suggesting that glass ionomer restorative materials are not suitable for repairing cervical lesions.
4.
Discussion
The null hypothesis of this study was partially robust because w/NP/Si 30 wt% group had no debonding force decay in the push-out tests among composite resins. However, the w/NP/
journal of the mechanical behavior of biomedical materials 38 (2014) 105 –113
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Fig. 3 – Typical push-out debonding force-extension profiles of different bonding materials before (dried debonding forces) and after thermal cycling processes (wet debonding forces).
Si 50 wt% group apparent declined in ultimate push-out forces compared with other composite resins even though the force was higher than other groups after the thermal cycling processes. The present study uses bovine teeth to simulate teeth dental caries filled with composite resin and found that commercially available 3 M-Z250 and the w/NP/Si 50 wt% group produced in-house had the best push-out forces before thermal cycling. Specimens in all groups after thermal cycling decreased in ultimate push-out forces, suggesting that thermal cycling does indeed affect interface bonding between restorative substances and tissues. Although the bonding forces of w/NP/Si 50 wt% decreased by approximately 20% after thermal cycling, it still maintained the highest bonding forces among all groups. However, the push-out forces of w/NP/Si 30 wt% did not significantly change after thermal cycling. The thermal cycling weakens the interface possibly due to the volume changes in situ from the cycled heating and cooling. Therefore, microcracks are created between the resin and the bovine teeth tubules. Once shear force is acted on the restorative material in the interfacial junction, fractures propagate from the microcracks, which have been found in previous studies where all specimens became weaker after thermal cycling (Helvatjoglu-Antoniades et al., 2004; Fischer et al., 2010). Furthermore, morphologies by SEM analysis revealed that some resin flowed into the tubules. The composite resin in this study uses DCPA as filler particles. DCPA is a major component in hydroxyapatite (HA). It also hydrolyzes to become HA when placed in neutral water for a prolonged period of time. Accordingly, as previously mentioned, enamel, dentine, and alveolar bone tissue are mostly composed of HA. Thus, DCPA, through the mineralization mechanism, exchanges Ca2þ and PO4 3 ions to strengthen the bonding, thus raising the shear force (Hossein et al., 2008; Ishikawa et al., 1999). Moreover, Xu et al. (2002) mentioned that whiskered filler patterns result in higher mechanical properties because the high surface contact area of the reinforced fillers are anisotropic, which offsets the volume changing effects of thermal cycling (Xu et al., 2002). This phenomenon is similar to the results in the present study due to the anisotropic strengthening provided by fillers.
The resin seen flowing into the dentinal tubules suggests that these composite resins have the potential to be used clinically. They can also serve as relief material against sensitive teeth. Sensitive teeth are due to dentin hypersensitivity, caused by the chemical, physical, and temperature stimulation of exposed dentinal tubules. These stimuli would change osmolality and result in discomfort (Miglani et al., 2010). Currently, this phenomenon is not well studied, and no mechanism for the cause of the disease has been proven. The most acceptable proposed theory is the fluid dynamics theory by Bräennströem and Aströem, 1964, which states that the fluid inside the dentinal tubules flow and excite the peripheral nerves of the pulp cavity, thereby causing soreness, pain, and discomfort (Bräennströem and Aströem, 1964). Therefore, the current strategy for treating hypersensitivity is occluding dentinal tubules, which decreases permeability and limits the contact of the tubules to prevent stimulus from the external environment. Additionally, research by Stojanac et al. showed that, after 12 and 24 months, microfilled composites (Esthet.X/Dentsply/De Trey, Konstanz, Germany, and Prime&Bond NT/Dentsply/De Trey), nanohybrid composites (TetricEvoCeram/Vivadent, Schaan, Liechtenstein, and AdheSE/Vivadent), and compomers (Dyract eXtra/Dentsply/ De Trey and Xeno III Dentsply/De Trey) used in the restoration of noncarious cervical lesions all showed promising results (Stojanac et al., 2013). Under appropriate adhesive systems and properly implemented restorative procedures, the study gave satisfactory results after a two-year evaluation period. Originally, calcium phosphate fillers in the composite resins were expected to have the ability to release ions through thermal cycling processes. The DCPA used in the present study is stable under acidic environments through the release of Ca2þ and phosphate ions (Xu et al., 2006; Mandel and Tas, 2010; Sun et al., 2010). The ions can form a apatite-like phase precipitation, making this more effective interlocking the dentinal tubules (Mandel and Tas, 2010). Thus, suppressing against shear force decay is better than ZrO2 and glass ionomers through the remineralized mechanism. Although the potential for generating a barrier through the mineralization mechanism enabled the possible positive effect of avoiding micro-leakages, this study still did not
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Fig. 4 – SEM analysis of fractured specimen from push-out tests and the relative locations of specimen are shown in schematic diagram of Fig. 1. The observed de-bonding fracturing portions did not all occur at the interface ( apparent in the 2nd and 4th rows of w/NP/Si 50 wt% and 30 wt% groups, respectively) between the dentine tubules and the composites. The elongated resin morphology as indicated by the white-arrows of w/NP/Si 30 wt% and 50 wt% groups shows the toughening effects.
observe the phenomenon of remineralization. However, the major and significant differences between groups of commercial 3 M-Z250 and Z350 with 70% ZrO2 filler groups and inhouse w/NP/Si with 30% and 50% filler groups are the fluid penetration depths into the dentinal tubules (Figs. 4 and 5), which led to the energy required for tough fracture to take place (Fig. 3). The apparent difference of push-out strength and strain curve shown in Fig. 3 was the characteristic
changes of the tough fracture. The areas under the curves between 3 M-Z250 and w/NP/Si 50 wt% are comparable. The resin of 3 M-Z250 with 70% ZrO2 filler is reasonably more brittle than the resin of w/NP/Si with 50% ZrO2 fillers. However, the penetration depth into the dentinal tubules of w/NP/Si with 50% ZrO2 is larger than that of the Z250 group. Furthermore in 2011, Mitchell et al. suggested that nano-sized bioactive glass decreases fluid dynamics in dentinal tubules,
journal of the mechanical behavior of biomedical materials 38 (2014) 105 –113
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Fig. 5 – The fractured surfaces of 3 M-Z250, 3 M-Z350, and GIC groups are shown. The 3 M-Z250 did not flow into the dentine tubules (clearly shown in the 3rd and 4th rows) as much as the 3 M-Z350. The fracture surfaces of GIC did not show any exposure of dental tubules, which revealed that the fracture occurred on the matrix of restorative materials but not on the restorative interfaces. The fracture morphology as indicated by black-arrows of Z250 and Z350 groups show the brittle cleavage type of fractures.
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leading to possible shallower penetration depth by 3 M-Z250/ Z350 with 70% nano-sized ZrO2 fillers (Mitchell et al., 2011). Aside from the more ductile fracture patterns of the in-house w/NP/Si with 30% and 50% composites, this result is meaningful and the data is shown in Figs. 4 and 5. The fracture patterns indicate that the 3 M-Z250 and Z350 are either more brittle or penetrate only a shallow depth compared with w/NP/Si with 30% and 50% composites and the comparison data of tough fracture is shown. Furthermore, Class V restorations also represent one of the less durable types of restorations and have a high index of loss of retention, marginal linkage, and secondary caries. These issues increase the possibility of material failure, which are caused by the difficulties in clinical practice, including isolation moisture control and gaining access to subgingival margins, inserting materials, contouring, and finishing and polishing procedures (Perez Cdos et al., 2012). These procedures were studied with ex vivo standardized test specimens, making comparisons and simulations of in vivo conditions difficult.
5.
Conclusion
Although many methods reduce the decay in the shear bonding force of the material from thermal cycling, our study showed that the incorporation of nanocrystals that grow on DCPA surfaces and further salinization come close to the properties of commercially available products. The decrease in debonding force and dentinal tubule permeation after thermal cycling were better than that of commercially available products as well. This procedure could be a potential solution for further improving the bonding forces between teeth cervical lesions and the restoration material in clinical applications. As a result, the more the performance of similar surface modified calcium phosphates in composite resins as a filler is understood, the more potential these materials can are proven to have and the closer they are to being used to restore teeth cervical lesions and reduce restoration failures.
Acknowledgments The authors acknowledge the assistance of Ms. Ya-Shun Chen, Dr. Chun-Cheng Hung and Dr. Jen-Chyan Wang. We also appreciate the major funding support of the Ministry of the Science and Technology, Taiwan under contract NSC99-2314B037-051-MY3 and MOST103-2622-E-035-006-CC2 and the partial support plan provided by Alliance Global Technology Co., Ltd., in Kaohsiung Medical Device Special Zone in Southern Taiwan Science Park (EG-32-09-16-101 and AZ-10-08-24-102) are likewise appreciated.
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