Degradation of polymeric restorative materials subjected to a high caries challenge

Abstract

Objectives

The aim of this study was to evaluate the degradation of different resin filling materials after a caries challenge, by high performance liquid chromatography (HPLC) and contact angle ( ) measurement.

Methods

Four different polymeric restorative materials (a resin composite, a polyacid-modified resin composite, an ormocer and a resin-modified glass ionomer cement) were tested. Five samples (30 mm × 6 mm × 2 mm) of each material were formed in a Teflon mold, following the manufacturer’s instructions. After pH cycles, the solutions were injected in an HPLC. The was obtained, before and after pH cycle, by a goniometer at 60% air humidity and 25 °C. A distilled water drop (0.006 ml) was put on the material surface, and after 6 min, 10 measures were obtained at 20 s intervals. Each sample received 4 drops, one at a time, on different areas.

Results

HPLC results showed elution of byproducts in all materials. This was greater in the acid medium. Bis-GMA and TEGDMA were detected in TPH Spectrum and Definite residues. Analyses of the contact angle by ANOVA and Student–Neuman–Keuls’s test showed that the surfaces of TPH Spectrum, Dyract AP and Definite were altered, except Vitremer ( p < 0.05).

Significance

All materials tested degraded on a caries simulated medium, suggesting that a great effort should be made to disseminate oral health information, since a high caries challenge environment (low pH) can lead to dental composite degradation, with potential toxic risks to patients.

Introduction

Resin-based restorative materials have been widely used in dentistry, not only because of their esthetic properties but also for the ability to adhere to the tooth structure. Dental composites are complex mixtures of materials that generally consist of reinforcing filler and an organic resin matrix based on different monomers such as Bis-GMA, UDMA and TEGDMA .

However, when a composite is in the oral environment these materials are exposed to plaque acids, foods and salivary enzymes that can cause material softening . In addition, chemical degradation can occur, leading to the release of various components from composite restorations, such as residual monomers, oligomers and other degradation products .

These components can irritate the oral soft tissues, stimulate the growth of bacteria and promote allergic reactions . Several in vitro studies have shown cytotoxic or estrogenic effects and pulp and gingival/oral mucosa reactions to some of the monomers released from composite materials .

Several factors contribute to the process of elution from resin-based dental materials. According to Ferracane , the amount of the released components is directly related to the extent of the polymerization reaction, i.e., the degree of double bond conversion. In addition, it has been shown that the chemistry of the solvent has a significant effect on the elution .

The size and the chemical nature of the released components also play an important role in the degradation process. Smaller molecules are expected to be eluted at a faster rate than larger, bulkier molecules .

Another important aspect that has been analyzed by several authors as being one of the possible reasons for composite resin degradation is the variation of the pH levels in the long term . It has been shown that resin-based restorative materials undergo greater micro-morphological damage following a regimen of acid challenge than after storage in either distilled water or artificial saliva .

Thus, changes in the surface-topography of such materials can cause an increase in surface roughness, which may lead to a greater increase of bacterial plaque on the restorative materials. The bacteria adhere to surfaces that are abraded and wrinkled to a greater extent than on flat surfaces , promoting physico-chemical changes that affect surface energy and wettability of the materials. These changes can be measured by contact angle measurements .

The purpose of this study was to evaluate the degradation effect of the pH cycling process of four composites, simulating a high cariogenic challenge, by contact angle measurements and high performance liquid chromatography (HPLC). The null hypothesis tested was that the pH cycling has no effect on wettability and elution of the different resin-based restorative materials.

Materials and methods

Characterization of resin-based filling materials

The four resin-based filling materials used in this study are shown in Table 1 . Different techniques and equipment were used to characterize the materials, as described in Table 2 .

Table 1
Composite materials used in the study.
Name Manufacturer Class Bath number
TPH Spectrum Dentsply, Rio de Janeiro, Brazil Hybrid composite resin 61365/2
Definite Degussa, Hanau, Germany Ormocer 0996226
Dyract AP Dentsply, Rio de Janeiro, Brazil Compomer 0102000767
Vitremer 3M/ESPE, St. Paul, MN, USA Resin-modified glass ionomer cement 20010522

Table 2
Techniques and equipment used for the characterization of resin-based filling materials.
Technique Equipment
H NMR Varian Mercury 300 Device (Palo Alto, CA, USA)
FTIR Perkim Elmer 1700 (Waltham, MA, USA)
TGA Thermogravimetric Analyzer Q 500 (New Castle, DE, USA)
HPLC Shimadzu Cromatograph (Columbia, MD, USA)
Contact angle Ramé Hart Goniometer (Netcong, NJ, USA)

The organic composition of each material was characterized using nuclear magnetic resonance ( H NMR). Samples were dissolved in deutered chloroform at room temperature and analyzed with a Varian Mercury 300 device (Palo Alto, CA, USA) with a frequency of 300 MHz. Reference standards of Bis-GMA, TEGDMA and UDMA were analyzed in the same conditions. From the samples’ spectra, the main chemical displacements were assigned and integrated, allowing estimation of the relationship among them for each sample.

The degree of conversion was evaluated by an FTIR spectrometer Perkin Elmer 1700 (Waltham, MA, USA), with 40 scans at 2 cm −1 resolution. The degree of conversion was based on the rate between the absorbance values at 1637 cm −1 and at 1610 cm −1 , representing the absorbance of the double bond of the aliphatic and aromatic carbons, respectively.

The thermogravimetric analysis (TGA) was carried out with a Thermogravimetric Analyzer, model Q500 (New Castle, DE, USA), with a temperature scan of 30–700 °C, at 10 °C/min, under a nitrogen atmosphere.

Specimen preparation

Specimens were built in a 30 × 6 × 2 mm Teflon mold, following the manufacturer’s instructions. The mold was positioned on a Mylar strip lying on a glass slide. The samples were built up in three increments. After inserting the materials into the mold, another Mylar strip was placed on top of them to avoid oxygen-inhibition of the surface layer. Additionally, a glass slide was used in order to flatten the surface. Each increment was photoactivated for 40 s with a halogen unit (Optilux 400, Demetron, Kerr Corp., Orange, CA, USA) with 500 mW/cm 2 intensity.

From each test material, four groups were prepared with five specimens each. Samples were submitted to pH cycle variations to simulate a high caries challenge, as described by Paraizo et al. . The solutions were adjusted for demineralized (pH = 4.3) and buffer (pH = 7.0) specimens, respectively. Each specimen was maintained in a glass flask containing 10 ml of demineralized solution for 6 h and then was washed with 4 ml of distilled water, dried with absorbent paper and transferred to another flask containing 10 ml of buffer solution. The specimens remained in this step for 17 h at 37 °C and then were again transferred to a flask containing 10 ml of demineralized solution. This procedure was conducted successively during 14 days at 37 °C. After the pH cycle, the incubated solution was freeze dried (−70 °C and 6 Pa). The remaining residue was dissolved in methanol and the resultant solutions were centrifuged. For HPLC analysis, 10 μL from each solution was used.

High performance liquid chromatography (HPLC)

The HPLC analysis was carried out in a Shimadzu chromatograph (Columbia, MD, USA) equipped with a UV–vis detector, automatic injector and a Merck LiChrosorb reverse phase C-18 silica column measuring 4.6 × 250 mm × 5 μm. The mobile phase conditions were the following: water (phase A)/acetonitrile (phase B), with variation of 3–70% of phase B in 50 min. Detection was performed at a wavelength of 22 nm. Calibration curves were plotted relating the elution peak to identify the Bis-GMA, TEGDMA and UDMA monomers. The abscissa represented the elution time. In chromatographic analysis, the elution time of the monomer can be taken from the maximum of the peak. Standard chromatograms of Bis-GMA, TEGDMA and UDMA are shown in Fig. 1 .

Fig. 1
Standard chromatograms of: (a) Bis-GMA (top), (b) TEGDMA (center) and (c) UDMA (bottom).

Contact angle measurements

The contact angles were measured with a Ramé Hart goniometer (Netcong, NJ, USA), using the sessile drop method at 23 °C and 50% relative humidity. Drops were formed using a 6-μl fixed volume micropipette. Four drops of double-distilled water were measured on different areas of the surface of the polymerized specimen after 6 min (10 measurements, with 20-s intervals for each drop).

Materials and methods

Characterization of resin-based filling materials

The four resin-based filling materials used in this study are shown in Table 1 . Different techniques and equipment were used to characterize the materials, as described in Table 2 .

Table 1
Composite materials used in the study.
Name Manufacturer Class Bath number
TPH Spectrum Dentsply, Rio de Janeiro, Brazil Hybrid composite resin 61365/2
Definite Degussa, Hanau, Germany Ormocer 0996226
Dyract AP Dentsply, Rio de Janeiro, Brazil Compomer 0102000767
Vitremer 3M/ESPE, St. Paul, MN, USA Resin-modified glass ionomer cement 20010522

Table 2
Techniques and equipment used for the characterization of resin-based filling materials.
Technique Equipment
H NMR Varian Mercury 300 Device (Palo Alto, CA, USA)
FTIR Perkim Elmer 1700 (Waltham, MA, USA)
TGA Thermogravimetric Analyzer Q 500 (New Castle, DE, USA)
HPLC Shimadzu Cromatograph (Columbia, MD, USA)
Contact angle Ramé Hart Goniometer (Netcong, NJ, USA)

The organic composition of each material was characterized using nuclear magnetic resonance ( H NMR). Samples were dissolved in deutered chloroform at room temperature and analyzed with a Varian Mercury 300 device (Palo Alto, CA, USA) with a frequency of 300 MHz. Reference standards of Bis-GMA, TEGDMA and UDMA were analyzed in the same conditions. From the samples’ spectra, the main chemical displacements were assigned and integrated, allowing estimation of the relationship among them for each sample.

The degree of conversion was evaluated by an FTIR spectrometer Perkin Elmer 1700 (Waltham, MA, USA), with 40 scans at 2 cm −1 resolution. The degree of conversion was based on the rate between the absorbance values at 1637 cm −1 and at 1610 cm −1 , representing the absorbance of the double bond of the aliphatic and aromatic carbons, respectively.

The thermogravimetric analysis (TGA) was carried out with a Thermogravimetric Analyzer, model Q500 (New Castle, DE, USA), with a temperature scan of 30–700 °C, at 10 °C/min, under a nitrogen atmosphere.

Specimen preparation

Specimens were built in a 30 × 6 × 2 mm Teflon mold, following the manufacturer’s instructions. The mold was positioned on a Mylar strip lying on a glass slide. The samples were built up in three increments. After inserting the materials into the mold, another Mylar strip was placed on top of them to avoid oxygen-inhibition of the surface layer. Additionally, a glass slide was used in order to flatten the surface. Each increment was photoactivated for 40 s with a halogen unit (Optilux 400, Demetron, Kerr Corp., Orange, CA, USA) with 500 mW/cm 2 intensity.

From each test material, four groups were prepared with five specimens each. Samples were submitted to pH cycle variations to simulate a high caries challenge, as described by Paraizo et al. . The solutions were adjusted for demineralized (pH = 4.3) and buffer (pH = 7.0) specimens, respectively. Each specimen was maintained in a glass flask containing 10 ml of demineralized solution for 6 h and then was washed with 4 ml of distilled water, dried with absorbent paper and transferred to another flask containing 10 ml of buffer solution. The specimens remained in this step for 17 h at 37 °C and then were again transferred to a flask containing 10 ml of demineralized solution. This procedure was conducted successively during 14 days at 37 °C. After the pH cycle, the incubated solution was freeze dried (−70 °C and 6 Pa). The remaining residue was dissolved in methanol and the resultant solutions were centrifuged. For HPLC analysis, 10 μL from each solution was used.

High performance liquid chromatography (HPLC)

The HPLC analysis was carried out in a Shimadzu chromatograph (Columbia, MD, USA) equipped with a UV–vis detector, automatic injector and a Merck LiChrosorb reverse phase C-18 silica column measuring 4.6 × 250 mm × 5 μm. The mobile phase conditions were the following: water (phase A)/acetonitrile (phase B), with variation of 3–70% of phase B in 50 min. Detection was performed at a wavelength of 22 nm. Calibration curves were plotted relating the elution peak to identify the Bis-GMA, TEGDMA and UDMA monomers. The abscissa represented the elution time. In chromatographic analysis, the elution time of the monomer can be taken from the maximum of the peak. Standard chromatograms of Bis-GMA, TEGDMA and UDMA are shown in Fig. 1 .

Nov 28, 2017 | Posted by in Dental Materials | Comments Off on Degradation of polymeric restorative materials subjected to a high caries challenge
Premium Wordpress Themes by UFO Themes