The roughness, microhardness, and surface analysis of nanocomposites after application of topical fluoride gels

Abstract

Objectives

Application of acidulated phosphate fluoride (APF) gels has long been considered to cause deterioration of composite surfaces. The aims of this study were to demonstrate that nanocomposite surfaces were not affected by some APF gels and to investigate the possible underlying mechanisms.

Methods

The elemental composition and viscosity of 3 acidulated phosphate fluoride (APF) agents (60 Second Taste Gel, Topex, and Zap) and 1 neutral fluoride agent (pH7 Gel) were analyzed. Subsequently, 320 specimens of 3 nanocomposites (Premisa, Filtek Z350, and Grandio) and a microhybrid composite (Estelite Sigma) with 80 specimens for each composite were randomly divided into 5 groups ( n = 16) and treated with 4 fluoride gels as well as distilled water which served as the control. Fluoride gels were applied on composite resin surfaces 4 times, 30 min each time. The roughness and microhardness were measured after treatments. Qualitative examination of the surface degradation of the composites was carried out with Fourier transforming infrared spectroscopy (FTIR) and scanning electron microscopy (SEM).

Results

Topex and Zap did not cause surface changes of composite resins, the possible reason being ascribed to the presence of magnesium aluminum silicate (MAS) clays. In contrast, 60 Second Taste Gel treatments caused significant roughness increase, microhardness decrease, more prominent filler dissolution, and IR spectral changes of Premisa, Filtek Z350, and Grandio. Estelite Sigma was less affected by the 4 fluoride gels.

Significance

The composite surfaces were not affected by Topex or Zap even after extended treatments. These two APF gels may be more suitable for clinical applications.

Introduction

Dental caries represent one of the most common chronic diseases in the world. In managing caries, restoration of cavitated carious lesions and prevention of carious lesions are equally important. The periodic professional topical fluoride application has been demonstrated to result in a significant reduction in dental caries as well as the arrestment of incipient lesions . Therefore, children and adults who are susceptible to caries development are often advised to receive topical fluoride treatments every 6 months. Topical fluoride agents may be acidulated or neutral. Compared to neutral agents, acidulated phosphate fluoride (APF) gel has been found to increase fluoride uptake by enamel to a greater extent and to more efficiently reduce the demineralization of enamel . However, APF gel treatment can contribute adverse effects on resin-based composites such as dissolution of inorganic fillers , surface erosion , increased surface roughness, decreased wear resistance , increased adherence of cariogenic bacteria , influence on color stability , and reduced surface hardness . The deterioration of resin-based composites is not significant if neutral sodium fluoride gel is used .

The underlying mechanism of APF deterioration of resin-based composites has been postulated through three major interaction pathways : interaction of fluoride with reinforcing fillers, filler–matrix coupling agents, or the organic matrix. Dissolution of composite filler particles has been ascribed to the presence of hydrogen and fluoride ions in APF gel that forms hydrofluoric acid and thereby decreases surface hardness . The severity of resin surface damage is related to the acidulated or neutral types of fluoride gel used, the composition and size of filler particles in the composite resin , as well as the entanglement of the resin matrix and inorganic fillers . Barium boroaluminosilicate glass-containing composites are the most susceptible to attack by APF agents , while microfilled materials are the least sensitive to APF gel . Other possible factors contributing to deterioration include the surface roughness of composite materials, which may increase the interfacial surface area, as well as the viscosity and thixotropy characteristics of fluoride gels, both of which can extend the reaction time of fluoride with composites .

Most APF agents are available in gel form for easy application with a cotton swab or tray and with no overflow into the oral cavity (which can cause gagging and ingestion). Another type of APF gel called thixotropic gel has been introduced; it exhibits a stable form at rest but becomes fluid when agitated or shaken. The thixotropy formula liquefies when the tray is being placed on the teeth, and returns to a gel once the tray is seated. Consequently, this material may increase penetration into proximal areas and reduce gagging and ingestion . However, research on the effect of thixotropic gel on resin-based composites has been limited.

Composite resin has been widely utilized for restoration of carious lesion. Traditional hybrid and microfilled composites use colloidal silica particles as inorganic fillers, but these tiny colloidal silica particles tend to agglomerate and increase viscosity, thus producing undue thickening and limited clinical use. To increase filler loading and overcome the viscosity problem, precured fillers (also called organic fillers or prepolymerized fillers) are blended with uncured material to enhance physical properties . Due to great progress in nanoscience, new filler technology of nanocomposites, nanohybrid and nanofill, has recently been developed. Nanohybrid composites contain conventional glass fillers and nonagglomerated discrete silica nanoparticles. The nanometer-sized particles can be dispersed in higher filler concentrations and polymerized into the resin system to increase filler loading of composites . Nanofill composites consist of individual nanosilica particles and nanoclusters. Nanocluster fillers are agglomerates of nano-sized particles and act as a single unit to achieve higher filler loading and strength . It is still not well understood if the nanohybrid and nanofill composite materials can be more resistant to APF agent challenges.

The purpose of this study was to evaluate the structural changes, roughness (Ra), hardness, and morphological changes of 4 composite resins (Premisa, Filtek Z350, Palfique Sigma, and Grandio) after application of 4 fluoride agents (60 Second Taste Gel, Topex, Zap, and pH7 Gel). The composition and viscosity of fluoride agents were also analyzed to investigate the mechanism of induced changes.

Materials and methods

Materials

Four types of composite resin including a nanohybrid (Grandio), a complex nanohybrid (Premisa), a nanofill (Filtek Z350), and a microhybrid (Estelite Sigma) were tested ( Table 1 ). Four types of fluoride agents including 3 acidulated APF gels and 1 neutral fluoride gel were tested ( Table 2 ).

Table 1
Composite resins used in the study.
Products (batch number) Filler (wt%) Type Filler composition Filler size Manufacturer
Premisa (3001513) 84% Complex nanohybrid Silica nanoparticles 20 nm Kerr Corp., Orange, CA, USA
Barium glass 0.4 μm
Prepolymerized fillers 30–50 μm
Filtek Z350 (8RT) 78.5% Nanofill Non-aggregated nanosilica filler Aggregated zirconia/silica nanoclusters (with primary zirconia/silica particles) 20 nm 3M ESPE, St. Paul, MN, USA
0.6–1.4 μm
(5–20 nm)
Estelite Sigma (E686) 82% Microhybrid Prepolymerized spherical silica-zirconia particles 0.1–0.3 μm Tokuyama Dental Corp, Taitu-Ku, Tokyo, Japan
Grandio (0847480) 87% Nanohybrid Silica nanofiller 20–50 nm Voco, Cuxhaven, Germany
Barium-alumina borosilicate microfiller 0.1–2.5 μm

Table 2
Fluoride agents used in the study.
Products Batch number Fluoride concentration Composition pH value Manufacturer Suggested mode of use
60 Second Taste Gel 011310 1.23% 1.23% fluoride ion from NaF, phosphoric acid added 3.5 Pascal, Bellevue, WA, USA 60 s or up to 4 min. 30 min contact time.
Topex 0811080827 1.23% 1.23% fluoride ion from 2.73% NaF and HF 3.5 Sultan Dent Products, Englewood, NJ, USA A minimum of 60 s to a maximum of 4 min. 30 min contact time.
Zap Z081022 1.23% 1.23% fluoride ion from 2% NaF and phospholated HF 3.5 Crosstex, Hauppauge, NY, USA 1 min (80% effectiveness) or 4 min (100% effectiveness).
pH7 Gel 041310 0.9% NaF 2.0%, pH adjust to 6.0–8.0 with sodium phosphate 7 Pascal, Bellevue, WA, USA 1–4 min. 30 min contact time.

Elemental analysis

Four fluoride agents: 60 Second Taste Gel, Topex, Zap, and pH7 Gel, were analyzed without further purification. They were diluted around 10,000 (v/v) times with 2% HNO 3 solution, which had been prepared by diluting HNO 3 (Panreac Quimica Sua, 65%) with D.I. water. The resulting sample solutions were stored in plastic bottles so as to avoid sample reactions with glass. Blank solutions of 2% HNO 3 were also prepared by dilution of 65% HNO 3 with D.I. water. Inductively coupled plasma mass (ICP-Mass) spectrometric analysis was performed on a Perkin-Elmer Elan-6000. A flow capillary nebulizer with a solution uptake rate of <1 mL/min was used.

Viscosity measurement

The viscosity of fluoride gels was measured with a Brookfield viscometer (R/S-CPS Rheometer, Brookfield Engineering Labs., Inc., USA) over the shear rate range 0–100 s −1 at both 25 °C and 37 °C.

Sample preparation

Four slot-shaped cavities, 12 mm × 4 mm × 2 mm, were cut into each of eighty acrylic molds. Then each cavity in a mold was filled with 1 of the 4 different brands of composite. Therefore, each composite was represented by 80 specimens. Forty specimens were used for surface roughness tests and forty specimens were used for microhardness tests. The molds were randomly divided into 5 groups ( n = 16) and treated with one of the following fluoride gels: 60 Second Taste Gel, Topex, Zap, pH7 Gel which is a neutral fluoride gel, distilled water which serves as control ( Table 2 ).

The composites put into the slots were covered with mylar strips, cured with LED light (Elipar FreeLight2, 3M ESPE, MN, USA) for 80 s, and stored in distilled water for 24 h. Subsequently, the surface layer was finished and polished using a polishing machine (Grinder/Polisher, Phoenix Alpha, Buehler Ltd., Lake Bluff, IL, USA). The finishing and polishing procedures were carried out with sandpaper discs (silicon carbide water-proof abrasive paper, 3M ESPE, MN, USA) of nos. 400, 600, 800, 1000, 1200, 1500 and 2000, each for 1 min at 360 rpm and 2 lb with running water irrigation. The polished samples were stored in distilled water for 24 h before testing.

When topical fluoride agents are applied clinically, the tooth surfaces have to be dried and then the fluoride tray is placed into patient’s mouth. The patient is instructed to close the mouth and bite the fluoride tray lightly for 4 min. A minor chewing motion is suggested to promote coverage of interproximal surfaces by fluoride gel. At the end of treatment, the tray is removed and the patient is instructed to expectorate any excess gel. The patient should not eat, drink, or rinse for 30 min following the treatment. To simulate clinical application procedures, 2 ml of fluoride gel was applied to the surfaces of composites for 4 min. During the 4 min, fluoride gels were slightly stirred by hand once every minute for about 1 s. The fluoride gels were then kept in contact with the composite surfaces for 26 min. Subsequently, the specimens were cleaned with tap water followed by ultrasonic vibration for 10 min. The APF application and washing procedures were repeated 4 times.

Analysis of structural changes using attenuated total reflectance-Fourier transforming infrared spectroscopy (ATR-FTIR)

The structural changes of composite resins after application of fluoride agents were studied with ATR-FTIR (FTIR-4200, Jasco International Co., Ltd., Tokyo, Japan). FTIR spectra were recorded by pressing the samples against the ZnSe ATR crystal with slow scan and normal slit width. The wavelength used was in the range of 4000–400 cm −1 to evaluate the functional groups in the specimens.

Surface roughness test

Average surface roughness (Ra) was measured using a surface profilometer (Surfcorder SE 3500, Kosaka, Japan). The tracing diamond tip was 2 μm with tracing speed of 0.2 m/s, force of 4 mN, tracing length of 1.25 mm, and cutoff value of 0.25 mm. Three tracings were performed at different locations on the surfaces of each specimen. An average surface roughness value (Ra) of eight specimens was calculated.

Microhardness test

The Vickers hardness number (VHN) was determined using a microhardness tester (Shimadzu HMV-2, Japan). Indentations were made using a diamond indenter with 98.07 mN load and 15 s dwell time. Five measurements were made on the surface of each specimen and the average value was taken as the hardness of that particular specimen.

Statistical analysis

Statistical analysis was performed using the Statistical Package for Social Science SPSS, version 12.0. Tests were two-tailed with significance level of 0.05. Descriptive statistics for continuous variables were calculated and reported as a mean ± standard deviation (SD). To detect differences in the means of Ra data and microhardness values among different groups, a two-way analysis of variance (ANOVA) with fluoride gel treatments and composite resins as factors was used to analyze the raw data. Multiple comparisons were performed by Scheffe post hoc tests with a significance level of 0.05.

Scanning electron microscopy (SEM) observation

Observation of the morphology of composite degradation was carried out with SEM. All specimens were mounted on aluminum stubs, sputter-coated with ∼20 nm of gold/palladium and finally examined at 2000× and 20,000× magnification using a Hitachi SEM (Model S-3500N, Tokyo, Japan) with an accelerating voltage of 15 kV.

Materials and methods

Materials

Four types of composite resin including a nanohybrid (Grandio), a complex nanohybrid (Premisa), a nanofill (Filtek Z350), and a microhybrid (Estelite Sigma) were tested ( Table 1 ). Four types of fluoride agents including 3 acidulated APF gels and 1 neutral fluoride gel were tested ( Table 2 ).

Table 1
Composite resins used in the study.
Products (batch number) Filler (wt%) Type Filler composition Filler size Manufacturer
Premisa (3001513) 84% Complex nanohybrid Silica nanoparticles 20 nm Kerr Corp., Orange, CA, USA
Barium glass 0.4 μm
Prepolymerized fillers 30–50 μm
Filtek Z350 (8RT) 78.5% Nanofill Non-aggregated nanosilica filler Aggregated zirconia/silica nanoclusters (with primary zirconia/silica particles) 20 nm 3M ESPE, St. Paul, MN, USA
0.6–1.4 μm
(5–20 nm)
Estelite Sigma (E686) 82% Microhybrid Prepolymerized spherical silica-zirconia particles 0.1–0.3 μm Tokuyama Dental Corp, Taitu-Ku, Tokyo, Japan
Grandio (0847480) 87% Nanohybrid Silica nanofiller 20–50 nm Voco, Cuxhaven, Germany
Barium-alumina borosilicate microfiller 0.1–2.5 μm
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Nov 28, 2017 | Posted by in Dental Materials | Comments Off on The roughness, microhardness, and surface analysis of nanocomposites after application of topical fluoride gels
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