ROMANIAN JOURNAL OF PHYSICS

COMPARISON OF DIFFERENT TECHNIQUES USED TO IMPROVE THE SEALANTS ADHESION ON ENAMEL SURFACE A. ROTARU1, 2, T. TESLARU1, I. CHIRAP1, A.M. PRODAN1, 2, N. DUMITRASCU1 1

“Alexandru Ioan Cuza” University of Iasi, Faculty of Physics, Iasi Plasma Advanced Research Center (IPARC), 11 Carol I Avenue, Iasi, 700506 Romania, E-mails: [email protected]; [email protected]; [email protected]; [email protected] 2 “Apollonia” University of Iasi, Faculty of Dental Medicine, 2 Muzicii St., Iasi, 700399, Romania, E-mail: [email protected] Received June 19, 2015 Traditional approaches to treat caries, in a surgical manner, were replaced by new strategies that focus on preventing disease and preserving intact tooth structure. One of the most efficient methods of teeth decay preventions is the sealing. The success of the sealant application technique on the enamel surface depends on the maintenance of tooth isolation, adequate rinsing and efficient photopolymerization and cooperation with patients (children). The purpose of this research is to improve adhesion of sealants on the enamel surface of tooth by various pretreatment techniques. Posterior teeth, recently extracted for orthodontic reasons, free of caries, were treated by acid – etching, UV radiations and plasma. Plasma was generated in a dielectric barrier discharge with asymmetrical configuration of electrodes, working at atmospheric pressure in helium. The control of plasma parameters was assured by electrical and optical diagnosis and the main surface properties of enamel implied in the sealant adhesion, respectively roughness and wettability, were investigated by atomic force microscopy and contact angle measurements. Key words: enamel – sealant interface, acid – etching, UV radiations, plasma treatments.

1. INTRODUCTION

At the beginning, dental caries was considered a disease with irreversible changes, a chronic destructive process that evolves without causing inflammatory phenomena, typical dental hard tissue necrosis and finally, pulp infection [1]. Nowadays, tooth decay is seen as a dynamic process conducted at the interface between plaque and tooth [1]. A review of epidemiological data currently available in several countries shows that there is a significant increase in the prevalence of dental caries. The United States Surgeon General's report that tooth decay is the most common chronic disease of children aged between 5 and 17 years, being five times more common than asthma [2]. Rom. Journ. Phys., Vol. 61, Nos. 3–4, P. 666–675, Bucharest, 2016

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Poor oral health is harmful to children because it affects their nutritional status, growth and development. Caria and its complications affect quality of life, both physically and physiologically, and premature loss of teeth can lead to a variety of consequences, such as gastrointestinal disturbances, aesthetic and psychological. Scientific research continues to make progress in identifying best practices for diagnosing, treating, and preventing tooth decay. Traditional approaches to treat caries in a surgical manner were replaced by new strategies that focus on preventing disease and preserving tooth structure. Since about 50% of school children caries are located mainly in the occlusal surfaces [3] and fluoride treatments are less effective in these areas having sought ways to achieve isolation of teeth from oral environment, sealing is one of them. Sealing is defined as the procedure of application and mechanical joining of a composite material or glassionomer to the demineralized enamel surface, covering the occlusal pit, fissures and isolating them from the environment. The idea of coverage of occlusal surfaces with un even relief dates back about a century, but was considered a serious method of orodental prevention since 1950, when it was discovered and introduced into practice acid – etching. In the dental fissures, as is well known, in the early years after the eruption of the permanent teeth, are well expressed, food retention occurs, which creates favorable conditions to the germs growth. This explains the high frequency of fissure caries in the near tooth after eruption. Attention should be paid to the first permanent molars, which are termed the key occlusion. To achieve sealing of occlusal surface are used many types of materials, but the most popular are resins, photo or self – curing. The polymerization of composites is accomplished by chemical or visible light activation, the last method being the most common. The polymerization is initiated by various sources of radiations, with a wavelength varying between 450 and 490 nm. A 6% reduction in voltage causes a corresponding reduction in intensity, usually 25% in some lamps. The intensity is not uniform for all illuminated areas, being higher in the center and decreasing with the distance from the source. It was found that the change in intensity produces no significant changes in the depth of penetration in enamel. The standard exposure time of radiation is 20 seconds and a source for sealant failure could be the impossibility of polymerization process control on children or uncooperative patients [4]. The composite adhesion onto the enamel surface is the result of the polymer matrix bonding at a rough surface, primarily due to mechanical retention [5]. To maintain an optimal resistance, the surface should be demineralized with 30–50% phosphoric acid, creating mechanical micro-retentions. The adequate connection to enamel depends on the dry carved surface, perfectly isolated from the humidity and the mucoproteins from saliva [6]. Many studies demonstrated that isolation represents a major retentive factor, moreover, in order to assure an optimal adhesion to occlusal surfaces, the sealers must have a high superficial tension and a low viscosity [7].

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The application technique of sealant on the enamel surface is the same for all modern materials as following: rigorous cleaning, tooth isolation which aims to not impede the sealant adhesion due to the persistent moisture, etching, washing and careful drying, applying sealant and polymerization. Laboratory tests have indicated that the most critical moment of the application procedure is the maintenance of tooth isolation, as contamination with saliva, more than 1 min during the acid attack and during the polymerization process, can produce superficial defects after reaching equilibrium by absorbing water. Although the literature mentions that occlusal caries incidence decreased after sealing up to 90%, our attention must focus on the difficulties of classic technique, respectively maintaining isolation, inadequate rinsing allowing phosphoric salts to remain on the enamel surface, interfering as a contaminant of the bond between the sealant and enamel [7], or the lesions of patients who have not good oral hygiene, inefficient curing at uncooperative patients (children). These impediments shape the need to introduce innovative approaches using the latest technologies. The introduction of cold plasma beams as technique in dental practice offers a unique opportunity to improve retention and adhesion of sealants to the enamel surface, decreasing the duration of procedures and preserving the enamel quality. Practically, the direct action of plasma beam onto the enamel surface optimizes adhesion between tooth substrate and restorative materials. Plasma is widely used in various biomedical applications taking into account its known effects such as cleaning, activation and functionalization or etching [8, 9]. In this way different types of implants can be improved, assuring a higher compatibility with the human or biological media [10]. In dental applications plasma is used for surface treatments to enhance the adhesion between resin – based materials and various substrates, such as acrylic denture base resin [11]. In additions, plasma can be used for endodontic disinfections [12] and tooth bleaching [13]. In our paper we present a comparison of different techniques used to improve the sealants adhesion on enamel surface. 2. EXPERIMENTAL 2.1. MATERIALS

In our study we used posterior teeth, recently extracted for orthodontic reasons (premolar sand molars), free of caries. After extraction, the teeth were cleaned with 3% hydrogen peroxide (S.C. Hipocrate SRL, Romania) from the remaining connective tissue and debris, and then stored in 0.9% NaCl (m/v) intravenous infusion (308 mOsm/L osmolarity, Hemofarm) at room temperature. Each tooth was cutted in 4 flattest sections of enamel using a diamond disc superflexibile (Dental Future Systems, Germany) with 0.17 mm thickness and 22 mm diameter. The samples were washed with distilled water and stored in 0.9%

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NaCl (m/v). Prior to acid, UV and plasma treatments, the samples were dried at 37 o C for 30 min. Phosphoric acid type gel was procured from SpofaDental, Czech Republic. The helium (He, 99.999% spectral purity, Messer Gas SRL, Romania) gas was used to generate plasma beam for enamel surface modification. 2.2. METHODS

In this study we have proposed three methods to improve adhesion of sealants on the enamel surface as follow: acid etching, UV and plasma beam treatments. 37.5% phosphoric acid type gel was applied on the enamel surface for 60 s, subsequently washed with water for 60 s and dried at room temperature. UV source was a type Curing light UV lamp (SMACO, model SLC – III, China) with wavelength between 430 and 485 nm, 5 W power, 50–60 Hz frequency, working in a.c. mode. The time of UV radiation exposure was 60 s and maximum intensity of the source was maintained at a distance of 1 mm from the enamel surface. A dielectric barrier discharge (DBD) with asymmetrical configuration of electrodes (Fig. 1), working at atmospheric pressure in helium, was used to generate a plasma beam for enamel surface treatment. Helium flow rate was fixed at 3 L/min using an electronic flow – meter (MKS, type 1179B) and a flow controller (MKS, type 247). The band gap between the power and ground electrodes was 1cm and the diameter of plasma beam was 2 mm. High voltage pulses are applied on HV electrode using a digital waveform generator (Tabor Electronics, WW5064) and a high voltage amplifier (Trek Model, PD07016). The waveform of applied signal was a square type, 5 kV amplitude and 2 kHz frequency. The duration of plasma beam pretreatment was 10 s. The control of plasma beam was assured by electrical and optical diagnosis giving a surety related to the reproducibility of the effects onto the enamel surface. The waveform of applied voltage and discharge current were recorded using a digital oscilloscope (Tektronix TDS5034B). The optical emission spectrum of light emitted by DBD discharge was collected by the use of an optical fiber and a monochromator (Triax 550) with CCD detector. Atomic Force Microscopy (AFM), Water Contact Angle Measurements (CAM), and UV-Vis Spectroscopy were used to characterize the surface properties of enamel. The enamel surface morphology was studied by an atomic force microscope (NT-MDT, Solver Pro-M) working in the tapping mode and using standard silicon nitride tips with a radius of approx. 10 nm (NT-MDT, NSG03). The topography and phase images were recorded for each sample, on several scanned areas, from 10 µm × 10 µm to 3 µm × 3 µm. The root mean square roughness (Rrms) and average roughness (Ra) were calculated for each scanned sample by the use of Nova software.

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Fig. 1 − Dielectric barrier discharge set-up used for pretreatment of enamel surface.

The evaluation of enamel surface wettability was made by contact angle measurements based on the sessile drop method (1 µL, volume of drop). It used an optical system with a telescope, 20 × magnification and a photo camera (Optika Microscopy Digital USB camera), and all measurements were performed at room temperature. ImageJ (version 1.40g) software was used to evaluate the contact angles and the standard deviation for each set of values was ± 2o. The thermodynamic work of adhesion ( Wa ) was calculated by the use of Young’s equation, i.e., Wa   lv 1 cos  

(1)

where:  lv – is the surface tension of the test liquid (for water  lv is 72.8 mN/m) and  – contact angle of the test liquid drop onto the enamel surface [14]. The UV – Vis spectra of teeth before and after various treatments were recorded in the range of 400–800 nm using an Analytik Jena AG Specord 210. 3. RESULTS AND DISCUSSIONS

During the enamel surface treatment, the applied voltage and discharge current were maintained constantly at 5 kV and 11 mA, respectively. Optical emission spectrum of the DBD discharge in helium (Fig. 2) shows the presence of excited species of “pure” helium. In addition to helium lines (at 501.5 nm, 587.5 nm, 667.8 nm, 706.5 nm, 728.1 nm), other species specific to impurities were identified in the discharge volume, such as atomic oxygen (at 777.4 nm), molecular nitrogen (at 315.9 nm, 337.1 nm, 353.6 nm, 357.6 nm),

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nitrogen molecular ion (N2+) (at 391.4 nm 427.8 nm, and 470 nm) and free radical (OH) (at 308.9 nm) [15, 16].

Fig. 2 – Optical emission spectra of light emitted by DBD discharge.

A good dental adhesion entails a strong contact between the adhesive and the adherent surface. In general a hydrophobic material does not provide a supreme contact to a hydrophilic surface [17, 18]. Therefore surface modification for wettability enhancement is required to improve adhesion of sealant on the enamel surface. The water contact angle changes, of enamel surface after different treatments, are shown in Table 1. Our results show that the wettability of enamel surface is modified by each method, the highest adhesion work being obtained in the case of acid – etching pretreatment. If we compare with the control, also the plasma beam induces a benefic increase of the adhesion work, as a measure of the surface capability to enhance the sealant adhesion. Table 1 Water contact angle measurements, θ, onto enamel surface before and after the three methods of pretreatment Sample

Control

60s acid

60s UV

10s plasma

70° ± 2° 97.7

32° ± 2° 134.5

56° ± 2° 113.5

46° ± 2° 123.3

     Wa [mN/m]

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Surface roughness of tooth enamel is another important factor to evaluate the surface modifications. The acid etching method increases substantially the roughness of enamel surface (Rrms = 296.8 nm) compared with untreated one (Rrms= = 37.9 nm) (Fig. 3).

a)

b) 2

Fig. 3 – AFM images 3D and 2D, 10 × 10 µm of untreated enamel surface: (a) morphology (Rrms = 37.9 nm); (b) phase image.

The enamel surface treated 10 s with plasma beam presents a roughness (Fig. 4e) which can be considered favorable for sealant adhesion. No significative changes were observed for tooth enamel exposed for 60 sec to UV radiations (Table 2). Table 2 Average roughness (Ra) and the root mean square roughness (Rrms) of the enamel surface before and after different treatments Sample

Control

Ra [nm] 10 × 10 µm 21.9 3 × 3 µm 8.0

60s UV 10s plasma 60s acid Rrms Ra Rrms Ra Rrms Ra Rrms [nm] [nm] [nm] [nm] [nm] [nm] [nm] 37.9 242.6 296.8 47.1 63.7 117.8 154.2 11.2 97.3 123.8 21.5 27.0 47.1 69.2

The optical transmission spectra of enamel samples (Fig. 5) were measured between 400 and 800 nm. It was observed that the transmittance for all samples increased gradually with wavelength. The height value of transmittance was obtained for acid etching. No significant changes for UV radiation and plasma were observed compared with untreated one. Thus it was demonstrated that by acid etching, the enamel loose dental tissue while UV radiations and plasma treatment does not affect the enamel structure.

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b)

c)

d)

e)

f) 2

Fig. 4 – AFM images 3D and 2D, 10 × 10 µm of treated enamel surface with: (a, b) acid (Rrms = 296.8 nm); (c, d) UV radiation (Rrms = 63.7 nm); (e, f) plasma (Rrms = 154.2 nm).

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Fig. 5 – Optical transmission spectra of enamel sample treated by three methods.

4. CONCLUSIONS

By this study we compare the effects induced by three techniques of pretreatment of enamel surface for the subsequent sealant adhesion. Thus, we observed that our method based on a plasma beam, due to its energies and special composition, has a higher efficiency in the increase of roughness and wettability, without provoking undesired effects related to chemical modifications or destruction of enamel layer just to dentin. Furthermore, 10 seconds of treatment appear to be enough to assure an increased roughness and wettability, as main conditions assuring a better adhesion of sealant onto the tooth. In our opinion, a plasma beam working at atmospheric pressure in helium or other gases could be implemented in dental practice as method aiming to improve the sealant adhesion at the enamel surface. However, in vivo tests will be necessary to prove the stability of sealant coatings onto the teeth in the oral cavity. Acknowledgment. This work was supported by the strategic grant POSDRU/159/1.5/S/137750, Project Doctoral and Postdoctoral programs support for increased competitiveness in Exact Sciences research cofinanced by the European Social Found within the Sectorial Operational Program Human Resources Development 2007–2013.

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