Effect of nonthermal atmospheric plasma on the shear bond strength of composite resin after using different tooth-whitening systems: An in vitro study.

AIM: We aimed to evaluate the effect of nonthermal atmospheric plasma (NTAP) on the bond strength of composite resin after using different tooth-whitening systems.
METHODS: Eighty maxillary central incisors (n = 80) were divided into two groups based on the tooth-whitening procedure used - Group 1: bleaching (Pola Office, USA), Group 2: microabrasion (Opalustre, Australia). The samples were subdivided into four groups depending on the surface treatments - Group A: control (no surface treatment), Group B: plasma, Group C: antioxidant (sodium ascorbate), and Group D: buffering agent (sodium bicarbonate). After the surface treatments, the specimens were stored in artificial saliva for 24 h and composite resin was bonded to the labial surface of the teeth perpendicular to the long axis. The samples were then subjected to shear bond strength test under the universal testing machine. STATISTICAL ANALYSIS: The results received from shear bond strength analysis were subjected to statistical analysis using a two-way ANOVA test, independent t-test, and Tukey's multiple post hoc tests. The P value set was <0.05.
RESULTS: The highest mean bond strength value was seen in Group 2B (NTAP treatment after microabrasion) followed by Group 1B (NTAP treatment after bleaching). Mean shear bond strength values have suggested a statistically significant difference between all the other groups (P < 0.05) except between Groups 1A, 2A and 1D, 2D.
CONCLUSIONS: Within the limitations of the current in vitro study, bleaching and microabrasion followed by surface treatment using NTAP showed the highest bond strength than other groups. Copyright:

INTRODUCTION

Esthetics play a vital role both socially and professionally among individuals of all age groups, thus increasing the demand for cosmetic dentistry.[1] Some intrinsic and extrinsic enamel defects may reduce the quality of smile.[2] Various treatment methodologies have been employed to overcome those problems and achieve a pleasing smile, which include bleaching, microabrasion, ceramic or composite veneers, as well as crowns.[3] Bleaching and microabrasion have gained popularity, being conservative technique to improve the appearance of discolored teeth.[4] Bleaching of tooth surface involves the release of oxygen, resulting in the oxidation of the pigments causing stains.[5] On the other hand, microabrasion removes the surface stains with an abrasive/acid compound-containing paste of 18% hydrochloric acid and flour of pumice, which involves an insignificant and unrecognizable loss of enamel.[67] Immediate or delayed composite veneering or restoration is considered as the next minimally invasive treatment in certain clinical scenarios when the above tooth-whitening procedures do not give a satisfactory result. Despite being conservative approaches, bleaching and microabrasion resulted in the alteration of surface topography of enamel that might reduce the bond strength of direct or indirect veneers.[1] Several researchers have postulated the presence of active chemicals or free radicals as one of the reasons for reduced bond strength followed by bleaching.[8] Surface treatment either with a biocompatible and neutral antioxidant such as sodium ascorbate or a buffering agent before bonding may reverse this effect and neutralize the acidic pH.[910] In the past few years, nonthermal atmospheric plasma (NTAP) has been widely experimented in biomedical and dental applications, attributed to its ability in altering the surface characterization and enhancing the adhesion.[11]

The current study evaluated the effect of NTAP on the bond strength of composite resin after using different tooth-whitening systems.

METHODS

Specimen preparation

Eighty intact permanent maxillary central incisors, freshly extracted due to periodontal diseases, were selected for the present study. Preference was given to teeth with intact crowns regardless of the root structure. The sample size was calculated using Raosoft online[12] (http://www.raosoft.com/samplesize.html accessed on December 2019) sample size calculator, and the power of the study set was 80%. The teeth with visible fractures, cracks, teeth not stored in any media, and teeth with developmental defects were excluded from the study. The collected teeth were stored in distilled water containing 0.2% thymol solution (Chemigens Research and Fine Chemicals, Mumbai, India). The teeth were then embedded in self-cure acrylic (DPI products, India) resin blocks till cementoenamel junction. The labial enamel surfaces of all samples were cleaned using a prophylactic paste (DPI Propol, India) with a rubber cup attached to slow-speed handpiece (Yoda Greentech, Gujarat, India) to remove any surface debris.

Nonthermal atmospheric plasma jet apparatus

Compressed helium gas with a pulse head of 2 GHz at 3.25 standard liter was used to generate the plasma jet. The delivery pressure was maintained <2 pascals with a mean power of 2 Watt. Each application was carried for a total period of 15 s in the installments of 5 s each with a rest period of 5 s in between. The distance between the applicator tip and the tooth surface was maintained at 5 mm. All the parameters were controlled by a microwave generator-connected module (Acxys Smart Systems, Germany). The temperature changes at the enamel surface were monitored using a thermocouple and found to be in the range of 27°C–29.4°C which is well tolerated by dental pulp.[13] All the plasma exposures were done in an isolated chamber to prevent any possible environmental contamination.

Experimental procedure

The teeth were randomly divided into two main groups depending on the tooth-whitening system: Group 1 – bleaching and Group 2 – microabrasion. In Group 1, the labial surface of the teeth was blot dried using blotting paper. Contents from the Pola Office syringe (SDI limited, Australia) containing 35% hydrogen peroxide were released into the pot containing bleach powder (73.26% thickeners, 26.2% catalysts, 0.04% dye, and 0.5% desensitizing agents) and mixed immediately using a brush applicator until the gel is homogeneous. A uniform layer of gel was applied to the labial surface of the teeth and left for 6–8 min [Figure 1a] and was later rinsed off using distilled water.

Figure 1

(a) Bleaching on the labial surface of tooth (Pola Office). (b) Microabrasion on the labial surface of tooth (Opalustre). (c) Plasma treatment on the labial surface of tooth. (d) Antioxidant solution and buffering solution. (e) Contact angle measurement. (f) Instron machine testing for shear bond strength

In Group 2, a fine grit (TC-11EF/20–30 μm) water-cooled diamond bur (Mani, Inc., Japan) was used to superficially abrade the labial surface in two to three gentle strokes to initiate microreduction. A plastic White Mac Tip (Ultradent, USA) was attached to the Opalustre syringe and a uniform layer was applied to the labial surfaces. A slow-speed handpiece (Yoda Greentech, Gujarat, India) was used at 250 RPM, with the Opal prophy cups (Ultradent, USA) [Figure 1b] directly on the enamel surfaces for 60 s and later rinsed off with distilled water.

Both the groups were further divided into four subgroups depending on the surface treatment performed:

Group A: Control group – No surface treatment was done (1A and 2A)

Group B: Plasma group – The labial surface of the samples was subjected to NTAP for 15 s [Figure 1c] (1B and 2B)

Group C: Antioxidant group – The labial surface of the teeth was treated with 10% sodium ascorbate liquid (Avi Chem Industries, India) for 30 s by applying gentle pressure using a brush and then rinsed off [Figure 1d] (1C and 2C)

Group D: Buffering group – The labial surface of the teeth was treated with 10% sodium bicarbonate liquid (Avi Chem industries, India) for 30 s by applying gentle pressure using a brush and then rinsed off [Figure 1d] (1D and 2D).

All the specimens were stored in artificial saliva for 24 h at 37°C in an incubator (Nabertherm lab incubator, India).

Contact angle measurement

Six of the samples with near-flat enamel surface from each group were selected and contact angle has been evaluated with the help of a goniometer (Rame-Hart Contact angle goniometer). Bidistilled water was used as the contact medium in the process [Figure 1e].

Composite restoration

Self-etch adhesive (Single Bond Universal adhesive, 3M) was applied on the labial enamel surfaces of all the above specimens as suggested by the manufacturer using an applicator tip. The first coating of the bonding agent was applied, left for 10 s, and was air-dried gently to allow the evaporation of any excess material. The second coat of bonding agent was applied in the same manner and photo-polymerized (LED Curing Light, Woodpecker – 5W high power blue light LED, 420–480 nm, 650 mW/cm2 –800 mW/cm2). A custom-made cylindrical plastic matrix of 2 mm diameter and 2 mm height was used as a mold to pack the composite material. The plastic mold was placed on the labial surface of the tooth perpendicularly followed by incremental layering (1 mm each) of composite material using a plastic filling instrument (Ivoclar, USA). Each increment was photo-polymerized for 20 s. After polymerizing, the plastic matrix was separated carefully. All the specimens were stored in artificial saliva for 24 h at 37°C in an incubator (Nabertherm lab incubator, India) to simulate the oral environment after the composite restoration.

Shear bond strength test

After completion of incubation, the shear bond strength of composite to the tooth surface was determined using a Universal Testing Machine (Instron Universal Testing Machine, Model 8801). A chisel-edge plunger was mounted onto the movable crosshead of the testing machine with the leading edge aimed at the composite and labial surface interface at a crosshead speed of 1 mm/min. The force required to de-bond the restorative material was measured, and the shear bond strength was calculated by dividing the peak load values and the restorative material base area (N/mm2) [Figure 1f].

RESULTS

The results obtained were then analyzed statistically using the Statistical Package for the Social Sciences (SPSS) (SPSS software version 20.0; SPSS Inc., Chicago, IL, USA). Data were analyzed using a two-way ANOVA test for overall significance, independent t-test, and Tukey's multiple post hoc tests for inter- and intragroup comparison, respectively. The highest mean bond strength values were seen in Group 2B (microabrasion followed by plasma), followed by Group 1B (bleaching followed by plasma). The lowest mean bond strength values were seen in Group 2A and Group 1A [Table 1 and Figure 2]. Mean shear bond strength values have suggested a statistically significant difference between other groups (P < 0.05) except between Groups 1A, 2A and 1D, 2D.

Table 1

Comparison of interactions of two main groups (1 and 2) and four sub groups (A, B, C, D) with shear bond strength by Tukeys multiple posthoc procedures

Groups	Group 1 with sub group A	Group 1 with sub group B	Group 1 with sub group C	Group 1 with sub group D	Group 2 with sub group A	Group 2 with sub group B	Group 2 with sub group C	Group 2 with sub group D
Mean	17.8460	30.2866	23.1562	22.4222	15.0090	35.1395	27.1649	23.8410
SD	2.3598	1.8023	2.3935	1.3914	2.8181	1.8227	3.6096	1.7697
Group 1 with sub group A	-
Group 1 with sub group B	P=0.0001*	-
Group 1 with sub group C	P=0.0002*	P=0.0001*	-
Group 1 with sub group D	P=0.0011*	P=0.0001*	P=0.9968	-
Group 2 with sub group A	P=0.1371	P=0.0001*	P=0.0001*	P=0.0001*	-
Group 2 with sub group B	P=0.0001*	P=0.0005*	P=0.0001*	P=0.0001*	P=0.0001*	-
Group 2 with sub group C	P=0.0001*	P=0.0722	P=0.0064*	P=0.0007*	P=0.0001*	P=0.0001*	-
Group 2 with sub group D	P=0.0001*	P=0.0001*	P=0.9979	P=0.8745	P=0.0001*	P=0.0001*	P=0.0437*	-

Figure 2

Comparison of interactions of two main groups (1 and 2) and four subgroups (A, B, C, and D) with shear bond strength

DISCUSSION

Plasma is portrayed as the fourth state of matter and is extremely a reactive material comprising strong electric field, radicals, and charged particles.[14] NTAP generated at atmospheric pressure, when used as an adjuvant increased the outcome of the bonding technique. For the above reasons, it has recently gathered attention not only in industrial sector but also in the medical and dental fields.[15]

Esthetic reconstructions, laminate and composite veneers, are often necessary after the tooth-whitening procedures.[16] Titley et al. explained the primary mechanism of bleaching as ionic dissociation of hydrogen peroxide and increased formation of free radicals such as nascent oxygen and the hydroxyl radical on the enamel surface.[17] Although bleaching has proven to be a safe and effective treatment, it may present specific antagonistic effects on the enamel, such as surface morphological changes, compositional changes, alterations in the surface microhardness, and surface roughness.[18] Furthermore, there is evidence that tooth bleaching led to a reduction in bond strength of composite resin applied to previously bleached enamel.[17] Glasspole et al. stated that the reduced bond strength was due to the prevention of resin infiltration and polymerization because of oxygen-free radicals.[19] Various additional surface treatments after subjecting the tooth to tooth-whitening procedures were indicated lately to reverse the effect of such methods on the bonding properties of the tooth.[20]

It was found that NTAP increased the bond strength at the dentin–composite interface by 60%.[2122] This interface-bonding enhancement was reported to significantly improve composite performance, durability, and longevity.[22] In the current study, a significant increase in the bond strength was seen in the groups that were subjected to plasma irrespective of the tooth-whitening procedure used. Plasma enhances the hydrophilicity of the tooth surface, surface energy, and wettability and changes the chemical structure of exposed collagen fibrils by temporarily uncovering the functional groups and partially dispersing them.[23] In case of dentin, it has been proven that the surface modification plays a lesser role compared to alteration of the surface chemistry due to the presence of higher organic content and more collagen fibrils. In the present study, since the evaluated area is mostly on the enamel, alteration of surface properties seems to play a major role than surface chemistry which is evident by reduced contact angle. Plasma-treated samples have shown the lowest contact angle compared to all the study groups[23] [Figure 1e].

When NTAP was used along with hydrogen peroxide, increased molecular breakdown and superior bleaching ability were observed.[2425] However, there is minimal or no literature evaluating the effect of plasma on the bond strength after treating the tooth surface with bleaching or microabrasion. The present study evaluated the same.

It was found that application of plasma after microabrasion significantly improved the bond strength compared to plasma treatment after bleaching. The superior bond strength might be due to the nonselective etching, enamel demineralization, and increased surface roughness by the aggressive hydrochloric acid. The further application of plasma enhanced the adhesive-tooth bond strength.[26] However, the efficacy of plasma is reduced due to the residual free radicals after bleaching resulting in inferior bond strength compared to microabrasion.[19]

Several authors postulated that the application of antioxidants on the bleached tooth surface caused the reversal of detrimental effects.[272829] In the current study, the use of sodium ascorbate showed increased bond strength values compared to the control group similar to a previous study conducted by Kaya et al.[30] This might be due to the potential of antioxidant agents to prevent the interference of residual radicals with the polymerization of adhesives, which in turn showed the improvement of the bond strength of resin composites to bleached enamel.[3132] After the bleaching procedure, increased free radicals are effectively reduced by antioxidant treatment resulting in better bond strength compared to the control group. However, after the microabrasion procedure, the released free radicals are minimal and also the available collagen content is more compared to bleached tooth surface due to the greater surface depth. Further attributed to the collagen-cross-linking ability of the antioxidants, in the current study, the bond strength of the composite in Group 2C (microabrasion and antioxidant) is greater than Group 1C (bleaching and antioxidant).

On the other hand, various studies evaluated the effect of surface treatment using a buffering agent on the bleached enamel and its influence on composite resin bonding.[10] It was reported that there was an improvement in the bond strength values after using a buffering agent. The outcomes of the present study are in conjunction with the previous literature stating that the application of a 10% sodium bicarbonate solution after bleaching procedure could be an alternative prerestorative treatment.[33] The mechanism of action of sodium bicarbonate built on its high pH (approximately 8.67) can disrupt the peroxide molecule and result in its decomposition and inactivation.[10] Buffering agent in combination with microabrasion performed better than bleaching. This is evident as there is no much reduction in the pH after bleaching procedures as compared to microabrasion revealing the effectiveness of buffering agent. However, there is no statistically significant difference in the bond strength between both the groups (Group 1D and 2D).

The plasma-treated group showed the best results which could be due to the improvement in the interface properties and modification of the tooth surface, thus increasing the tooth/adhesive interfacial bonding. Still, further studies are required to clinically evaluate the effect of plasma on bond strength, microabrasion, and bleaching in the near future. It is of valuable interest to know if the application of plasma after using antioxidant or buffering agent enhances the bonding further in the upcoming studies.

CONCLUSIONS

Within the limits of the study, it can be concluded that

Irrespective of the tooth-whitening procedure performed, surface treatment using plasma showed a significant increase in the mean bond strength values

No significant difference in the mean bond strength values was seen between bleaching and microabrasion without any surface treatment

Application of sodium ascorbate and sodium bicarbonate showed an increase in mean bond strength values compared to control groups.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.