| Abstract|| |
Aim: The aim of this study was to assess the impact of nonthermal atmospheric plasma (NTAP) on the bond strength of composite resin following plasma application at different steps of dentin bonding in total-etch and self-etch adhesive systems.
Materials and Methods: Ninety extracted third molars were taken, and the occlusal surfaces were removed until the exposure of the dentin. Samples were distributed into two main groups: Group T: total-etch adhesive system and Group S: self-etch adhesive system. Groups are further subdivided (n = 10) based on plasma application at different steps of dentin bonding. T1: surface etching with 37% phosphoric acid and bonding agent application. T2: plasma application and bonding agent application. T3: plasma application, etching, and bonding agent application. T4: etching, plasma application, and bonding agent application. T5: etching, plasma application, bonding agent application, and again plasma application. S1: self-etch bonding agent application. S2: plasma application and bonding agent application. S3: Bonding agent application and plasma application. S4: plasma application, bonding agent application, and again plasma application. For all the samples, composite resin buildup was done, and shear bond strength (SBS) was measured. The contact angle was measured at different steps of dental adhesive systems.
Statistical Analysis: Two-way analysis of variance and the post hoc Tukey's test were used for analysis, regarding P < 0.05 as statistically significant.
Results: Among all groups of total-etch and self-etch adhesives, Group T4 (48.81 Mpa) and Group S2 (36.59 Mpa), respectively, have demonstrated significantly greater bond strength values than the corresponding control groups.
Conclusion: NTAP enhanced the composite resin's SBS when plasma treatment was done before bonding agent application and significantly reduced the contact angles of the distilled water.
Keywords: Bond strength; contact angle; nonthermal atmospheric plasma; self-etch adhesive; total-etch adhesive
|How to cite this article:|
Bolla N, Mayana AB, Gali PK, Vemuri S, Garlapati R, Kamal SA. Effect of nonthermal atmospheric plasma on bond strength of composite resin using total-etch and self-etch adhesive systems. J Conserv Dent 2023;26:292-8
|How to cite this URL:|
Bolla N, Mayana AB, Gali PK, Vemuri S, Garlapati R, Kamal SA. Effect of nonthermal atmospheric plasma on bond strength of composite resin using total-etch and self-etch adhesive systems. J Conserv Dent [serial online] 2023 [cited 2023 Jun 5];26:292-8. Available from: https://www.jcd.org.in/text.asp?2023/26/3/292/376909
| Introduction|| |
Long-lasting restorations are essential to the success of adhesive dentistry, and composite restorations commonly lose their adhesion or retention, which is a typical problem. To improve the composite restoration's long-term durability, improved techniques and various adhesive systems have been used to create a solid adhesive–dentin bonding. Dental adhesives are classified as “etch-and-rinse” and “self-etch” systems.
Total-etch adhesives etch the tooth surface using 35–37% phosphoric acid. When the dentinal surface is etched, the acid demineralizes superficial hydroxyapatite, eliminates smear layer and plugs, exposing collagen fibrils, and opens the dentinal tubules to funnel the orifices. Although it is considered a reliable adhesive, bonding to etched dentin poses challenges due to its complicated compositional, histological, as well as morphological characteristics. Hybridization of adhesive resin inside the exposed collagen fibrils is the main driving force behind the adhesion process to dentin. The exposed collagen fibril collapses due to air drying, which may obstruct resin's penetration and reduce bond strength. Therefore, it is believed that surface wetness is necessary for demineralized dentin. Controlling dentin surface moisture and preserving the natural collagen fibril structure are extremely technique-sensitive.
The smear layer can be removed using self-etch adhesives without needing a separate phosphoric acid etching since they include acidic monomers. Instead of eliminating the smear layer, these adhesives make it permeable to monomers. Self-etching adhesive contains water to facilitate ionization of the acidic monomers and solvents such as ethanol and acetone to hasten the water removal process. Although these systems are said to be less technique-sensitive in terms of controlling dentin moisture, there are still factors that depend on the operator when it comes to how much dentin needs to be dried before adhesive bonding.
Maximum contact between the adhesive and the adherend surface is necessary for good adhesion. To achieve this, the substrate must be optimally wettable, or the adhesive substance must spread impulsively throughout the adherend surface. To promote adhesion between the substrates, surface modification for wettability enhancement is required. Many in vitro investigations have shown that surface alteration of the dentin substrate yields acceptable outcomes.,,, The substrate has been changed using a variety of techniques to improve the surface properties such as wettability, permeability, adhesive ability, and biocompatibility.
The surface properties can be improved using a variety of conventional techniques, such as antioxidants, wet chemical treatment, mechanical abrasion, resin matrix reinforced using remineralizing agents or fillers, collagen cross-linking agents, laser treatment, protease inhibitors, and by modifying the bonding protocol. Regardless of their widespread usage, they show adverse effects.
Nonthermal atmospheric plasma (NTAP) is a recent potent surface modification tool gaining attention in adhesive dentistry. Due to its ability to modify surface characterization and improve adherence, NTAP has recently undergone extensive experimentation in biomedical and dental applications. The highly reactive plasma binds to various substrates, etches, or combines to form a thin layer of plasma that modifies the surface characteristics.
NTAP was used previously for surface treatments to increase bonds between resin-based materials and different substrates, such as glass fibers, acrylic denture base resin, glass ceramics, fiber-reinforced composite posts, tooth bleaching, polycrystalline ceramics, and for disinfection of root canals. NTAP application on dentin substrate is a promising mode of increase in bond durability. According to Ritts et al., short-term plasma treatment can alter the chemical structure of the exposed collagen fibrils and increase hydrophilicity of the dentinal surface, enabling a better adhesive penetration into collagen fibrils and significantly increasing the bond between composite and dentin. Thus, treatment with NTAP allows dentin surface modification, providing a unique opportunity to improve bond between the dentinal surface and composite restoration.
According to the available literature, relatively few studies have deeply assessed the effects of NTAP application at different steps of total-etch and self-etch bonding agent application in restorative dentistry.
Thus, the study aimed to assess the effects of NTAP application at different steps of dentin bonding in total-etch and self-etch adhesive systems.
| Materials and Methods|| |
Nonthermal atmospheric plasma application
A plasma jet was generated using compressed He gas and a pulsing head of 2 GHz at 3.25 L. NTAP delivery was done using <2 Pascal pressure and 2 Watt power. Application was made in a closed isolated chamber for 30 s with 5 s intervals by maintaining a 5 mm distance from the tooth surface [Figure 1]. The parameters were controlled by a microwave generator-connected module AcXys plasma technologies, France. The temperature changes were noticed on the dentinal surface using a thermocouple which ranged from 27°C to 29.4°C, endured by the dental pulp.
Ethical clearance for the study was obtained from the Institutional Ethics Committee (Pr. 182/IEC/SIBAR/2018). Ninety (n = 90) extracted third molars devoid of caries, old restorations, and fractures were collected and kept in distilled water until use. Diamond discs at low speed and water coolant were used for the removal of the occlusal surface until the exposure of dentin and were embedded in plastic molds until the cementoenamel junction using self-cure acrylic resin (DPI, India). Depending on the adhesive system employed, the samples were divided into Group T (n = 50): total-etch adhesive system, and Group S (n = 40): self-etch adhesive system. Groups are further subdivided (n = 10) based on NTAP application at different steps of dentin bonding.
T1 (E + BA + C): Surface etching using 37% phosphoric acid, bonding agent application, and composite placement.
T2 (P + BA + C): NTAP application, bonding agent application, and composite placement.
T3 (P + E + BA + C): NTAP application, etching, bonding agent application, and composite placement.
T4 (E + P + BA + C): Etching, NTAP application, bonding agent application, and composite placement.
T5 (E + P + BA + P + C): Etching, NTAP application, bonding agent application, again NTAP application, and composite placement.
S1 (BA + C): Self-etch bonding agent application and composite placement.
S2 (P + BA + C): NTAP application, self-etch bonding agent application, and composite placement.
S3 (BA + P + C): Self-etch bonding agent application, NTAP application, and composite placement.
S4 (P + BA + P + C): NTAP application, self-etch bonding agent application, again NTAP application, and composite placement.
The tooth surface was etched with 37% phosphoric acid (D-Etch, D-Tech, India) for 30 s and rinsed in the total-etch adhesive system. Then, the surface was dried using absorbent papers and was exposed to an NTAP jet intermittently. After NTAP application, the dentinal surfaces were coated with a total-etch adhesive (Adper Single Bond 2, 3M ESPE, USA) and polymerized using conventional light-emitting diode curing unit (Mediplus, India) with an output power of 550 mW/cm2 for 10 s.
In the self-etch adhesive system, the tooth surfaces were intermittently exposed to an NTAP jet for 30 s. After NTAP application, the dentinal surfaces were coated with a self-etch adhesive (Adper Single Bond Universal, 3M ESPE, USA) and polymerized using conventional light-emitting diode curing unit (Mediplus, India) with an output power of 550 mW/cm2 for 10 s.
Composite placement for all the samples was done immediately after the bonding procedure. 2 mm × 2 mm plastic matrix was prepared and placed on the occlusal surfaces. Placement of the composite (Spectrum, Dentsply, USA) was done incrementally and polymerized each increment for 30 s. The samples were stored in distilled water for 24 h. Bond strength testing was done using the universal testing machine (UTM) (Instron Hydraulics). Each sample was held in UTM so that leading edge of the plunger was directed at the interface of the occlusal surface and the composite resin at 1 mm/min speed.
Contact angle measurement
Surface wettability at different steps of the dentin bonding procedure to distilled water was determined with static contact angle measurements. The Rame-Hart Goniometer was used to measure contact angle of the dentinal surface [Figure 2]. A predetermined quantity of distilled water syringe tip was placed above the sample to dispense. Each time, 0.5 μL of distilled water droplets were dispensed on the surface. Contact angle (ø) and droplet arc at the interface were recorded after water application on the surface for 5 s. The contact angle was also recorded before NTAP exposure.
|Figure 2: The evaluation of contact angle using the Rame-Hart Goniometer on dentin surface at different steps of total-etch and self-etch adhesive systems with and without NTAP application. NTAP: Nonthermal atmospheric plasma. a) before etching b) after etching c) after NTAP application without etching d) NTAP followed by etching e) etching followed by NTAP application f) total-etch adhesive application g) NTAP followed by total-etch adhesive application h) self-etch adhesive application i) NTAP followed by self-etch adhesive application|
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Version 22.0 of SPSS (Statistical Package for the Social science, IBM Corp. Ltd., Armonk, New york, USA). (Statistical Package for Social Science, IBM Corporation) software was used to analyze the data. A two-way analysis of variance test was used to determine the overall significant values for all the groups, and the post hoc Tukey's tests were used to compare within and intergroup with a significance value of P < 0.05*.
| Results|| |
Bond strength evaluation
Results have shown that Group T4 (E + P + BA + C; 48.81 Mpa) showed significantly higher shear bond strength (SBS), and Group S4 (P + BA + P + C; 15.87 Mpa) showed lower values of SBS.
When comparing the subgroups T, Group T4 (E + P + BA + C) showed significantly higher SBS values (48.81Mpa) than Group T1 (E + BA + C; 34.22 Mpa), Group T2 (P + BA + C; 29.82 Mpa), and Group T5 (E + P + BA + P + C; 27.98 Mpa). No significant difference was seen between Group T4 and Group T3 (P + E + BA + C; 44.38 Mpa). The results for Group T3 (P + E + BA + C; 44.38 Mpa) were significantly higher than those for Groups T1, T2, and T5. No significant difference was seen between Groups T1, T2, and T5. Compared to the other subgroups, Group T5 (E + P + BA + P + C; 27.98 Mpa) had the lowest mean values for SBS [Table 1].
|Table 1: Comparison of interactions between the five subgroups (T1, T2, T3, T4, and T5) with the mean shear bond strength using Tukey's multiple post hoc test in Group T|
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When comparing the subgroups S, Group S2 (P + BA + C) showed significantly higher SBS values (36.59Mpa) than Group S1 (BA + C; 27.23Mpa), Group S3 (BA + P + C; 25.12 Mpa), and Group S4 (P + BA + P + C; 15.87 Mpa). Group S1 showed a significantly higher SBS value than S4. No significant difference in the bond strength values was seen between Group S1 and Group S3. Group S3 (25.12 Mpa) showed significantly higher SBS values than Group S4 (15.87 Mpa). Group S4 (P + BA + P + C) recorded the lowest mean SBS values compared with other subgroups [Table 2].
|Table 2: Comparison of interactions between the four subgroups (S1, S2, S3, and S4) with the mean shear bond strength using Tukey's multiple post hoc test in Group S|
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Contact angle measurement
The average contact angle values at different dentin bonding steps in total-etch and self-etch adhesive systems are given in [Figure 2]. The contact angle was drastically reduced when NTAP was applied after etching the dentin surface. Whereas there was lowered contact angle after NTAP application compared to before NTAP application at other dentin bonding steps.
| Discussion|| |
The evaluation of NTAP influence in different steps of both total-etch and self-etch adhesive systems helped us selectively utilize the application to maximize the bonding. In the present study, in the total-etch adhesive protocol, NTAP application before the bonding agent (Group T4) showed significant bond strength. Plasma consists of chemically reactive species, which include electronically excited neutrals, ionic species, high-energy electrons, and free radicals. These active species react with the surface and modify the surface chemistry. The introduction of new functional groups into the surface through modification might alter the surface chemistry. Ritts et al. stated that the carbonyl group number was increased on the dentin surface after NTAP treatment, and these groups are likely to occur on the collagen fibrils of dentin. These carbonyl groups cause the collagen fibers to partially separate into smaller fibrils or even into individual fibrils. This might significantly increase the interface area, adhesive penetration into collagen fibrils, and consequently, enhance the interface bond strength.
Results of the present study were in accordance with previous studies. Dong et al. assessed the effect of plasma on composites and concluded that plasma application modifies the dentinal surface and increases the interfacial bond strength between the adhesive and dentinal surface. Kong et al. stated that plasma brush treatment modifies the dentinal surface and thus increases the bonding between dentin and adhesive. This results from the introduction of bonds that depends on surface chemistry rather than surface porosity. Surface treatment with NTAP following bleaching and microabrasion has enhanced the SBS of composite resin., Yavirach et al. reported that plasma application to fiber-reinforced and resin composite showed an increased tensile SBS compared to traditional core buildup. Garlapati et al. reported that the push-out bond strength of the resin-based and bioceramic sealers has been increased with the application of NTAP.
Kim et al. stated that NTAP application effectively removes the residual water from the surface of dentin without collapsing the demineralized collagen matrix. In addition, the chemically reactive species present in the NTAP can promote the polymerization of the monomers used in the adhesive and improve interaction between infiltrating adhesive and dentinal substrate. However, the application of NTAP again after bonding agent application, i.e., in Groups T5 and S4, resulted in catastrophic failure of bonding, lower than the control groups T1 and S1. Y. Liu reported that excessive NTAP application damages the dentinal collagen and thus decreases the bond strength.
The next favorable increase in bond strength was observed when NTAP was applied even before etching the dentinal surface, i.e., in Group T3. Although NTAP enables a greater penetration of etchant resulting in better micro irregularities, it appears that the efficacy of NTAP was reduced by the etchant. To evaluate the possibility of NTAP alone to replace the etching, the complete step of etching was eliminated in one of the total-etch study groups, i.e., in Group T2. However, it was found that NTAP alone cannot compensate for etching, as evidenced by the bond strength. Increased micro irregularities by the etchant are most favorable for the effect of NTAP by aiding in increasing the quality of resin tags and depth of penetration. NTAP would remain active and prompt cleavage of the vinyl double bonds of the adhesive monomer and are less dependent on water content. This is evident by the result of the total-etch group, where NTAP was applied immediately after etchant.
Unlike in total-etch, NTAP application before a self-etch bonding agent application, i.e., in Group S2, has resulted in a significant increase in bond strength compared to the control group S1. Dong et al. experimented and concluded that nonthermal plasma application improved interface bonding of dentin/adhesive for composite restorations using a self-etching system. These results attributed that plasma application partially opened the dentinal tubules, thus enhancing deeper penetration of adhesive, forming longer resin tags and thicker hybrid layer, consequently improving the bond strength. Hirata et al. stated an increase in bond strength of self-etch adhesive system when applied to plasma-treated dentinal surfaces, and these effects of plasma application were product dependent. However, the application of NTAP after self-etch bonding agent, i.e., in Group S3, neither has enhanced nor damaged the bonding in terms of bond strength. Even though the NTAP application before the bonding agent appeared to be beneficial, there has been a significant reduction in bond strength when NTAP was applied twice, i.e., before and after the self-etch bonding agent as seen in Group S4. By breaking the C-C and C-H bonds, NTAP is reported to be able to etch surfaces and eliminate organic matter. NTAP can disintegrate polymeric surface matrix into loosely bound oligomers and low-molecular-weight oxidized material.
The dentin surface has become super wettable when etched and subjected to NTAP, as suggested by the contact angle measurements in the current study. The water contact angle measurement is a useful indicator for surface tension evaluation and reflects the substrate wettability by water. High contact angle value entitles hydrophobic properties of the surface, and these values depend on the substrates' chemical composition. Water contact angle value significantly dropped after exposure to NTAP following the etching protocol as shown in [Figure 2], which was in accordance with the previous studies. Chen et al. concluded that contact angle values decreased considerably after plasma application and were close to super hydrophilic properties of the surface. The atomic percentage of carbon (C) was reduced, indicating that the NTAP application eliminated the C-containing materials, which is why the NTAP was able to lower the water contact angles. Rafat et al. stated that the collagen surface contact angle was reduced significantly, after NTAP application. According to Lehmann et al., the surface wettability with ethylene glycol and water significantly increased after plasma application. The reason for increased surface wettability was due to the reduction in the carbon (C) compound amount, caused by chemical reactions by NTAP application.
It was observed that the contact angle with the substrate had reduced almost in all the cases, thereby favoring the positive physical influence of adhesive. The overall performance of total-etch groups in conjunction with NTAP was better than self-etch groups in conjunction with NTAP. The role of water-chasing primers in self-etch bonding agents in determining the NTAP application's efficacy for better bond strength is yet to be confirmed in further studies.
The possible limitations of the current in vitro study could be the lack of possible pulpal pressure, variable hydration levels of dentin during the procedure, and lack of Fourier transform infrared spectroscopy analysis data to confirm the possibility of functional groups in achieving the current observations. The bond durability after subjecting to various cycles of thermal treatment and aging can be done.
| Conclusion|| |
- NTAP application achieved a better bond strength after etching in the total-etch adhesive system
- NTAP application before the bonding agent has resulted in better bond strengths in the self-etch adhesive system
- Total-etch bonding systems' overall performance is better than self-etch in conjunction with NTAP.
- NTAP alone is not an alternative to etching in a total-etch adhesive system.
Scope for the development of a portable plasma device for the application of plasma into the oral cavity to enhance bonding. As the temperature is tolerable and promotes wound healing, NTAP might be a potential device in pulp capping.
We thank the Department of Chemistry, Indian Institute of Space Science and Technology (IIST), Kerala, for the provision of the NTAP jet equipment, and the Department of Conservative Dentistry and Endodontics, and the management of the Sibar Institute of Dental Sciences for their kind support and guidance throughout the study. We thank the Indian Association of Conservative Dentistry and Endodontics (IACDE) for providing Research Grant in 2018.
Financial support and sponsorship
Research grant 2018, IACDE.
Conflicts of interest
There are no conflicts of interest.
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Dr. Aameena Banu Mayana
Department of Conservative Dentistry and Endodontics, Sibarinstitute of Dental Sciences, Takkellapadu, Guntur - 522 509, Andhra Pradesh
Source of Support: None, Conflict of Interest: None
[Figure 1], [Figure 2]
[Table 1], [Table 2]