The effects of different opacifiers on the translucency of experimental dental composite resins

Abstract

Objective

The aim of this study was to evaluate the effects of different opacifiers on the translucency of experimental dental composite-resins.

Methods

Three metal oxides that are used as opacifiers were tested in this study: titanium oxide (TiO 2 ), aluminium oxide (Al 2 O 3 ) and zirconium oxide (ZrO 2 ). Experimental composite-resins were fabricated containing 25 wt.% urethane dimethacrylate (UDMA)-based resin matrix and 75% total filler including different concentrations of metal oxides (0, 0.25, 0.5, 0.75 and 1 wt.%) blended into silane treated barium-silicate filler. The specimens (15.5 mm diameter and 1 mm thickness) were light-cured and tested in the transmittance mode using a UV/VIS spectrophotometer at wavelengths from 380 to 700 nm under a standard illuminant D65. The color differences (ΔE* ab) between different concentrations of opacifiers were also measured in transmittance mode based on their Lab values.

Results

Statistical analysis by ANOVA and Tukey’s test showed a significant decrease (p < 0.05) in light transmittance with the addition of opacifiers to the experimental composite-resins. There was a linear correlation between different concentrations of TiO 2 and Al 2 O 3 and total transmittance. Total transmittance was also found to be wavelength dependent. The color differences for the concentrations of 0–1 wt.% of the opacifiers were above 1 ΔE* unit, with Al 2 O 3 showing the smallest color shift.

Significance

The type and the amount of the opacifiers used in this study had a significant effect on the translucency of the experimental UDMA-based dental composite resins. The most effective opacifier was TiO 2 , followed by ZrO 2 and Al 2 O 3 in decreasing order, respectively.

Introduction

It has been shown that the appearance of a restoration is influenced by many factors including color, translucency and opacity, light reflectance and transmittance, and surface texture . The inherent translucency of tooth structure and different morphology across the surface contributes to the complexity of achieving a natural looking restoration. Furthermore, it is often challenging for the clinician to mask the dark visual effect of the oral cavity on a class III or class IV restoration, or when trying to mask intense discolorations on the tooth structure. In order to overcome these problems, the opaque shades and dentin shades of dental composite resins have been manufactured. These new shades have higher opacity compared to the standard monochromatic dental composite shades .

According to Ragain and Johnston , a translucent material or a tooth undergoes four optical phenomena when light reaches it: (I) specular transmission of the light flux through the tooth; (II) specular reflection at the surface; (III) diffuse light reflection at the surface; and (IV) absorption and scattering of the light flux within the dental tissues.

The color and translucency of the composite resin are influenced by its shade, thickness and background color ; matrix composition ; filler particle size and content , pigment additions and potentially the initiation component and filler coupling agent . It has been also reported that translucency and color of resin composites are affected by depth of cure , light transmittance , and two wavelength-dependent elements such as absorption coefficient and scattering coefficient .

Scattering of light is an effect of refraction and reflection at the interface between the resin matrix and particles or voids . It has been reported that opacifiers in composite resins can act as scattering centers and therefore, affect their translucency.

Metal oxides such as titanium oxide (TiO 2 ), aluminium oxide (Al 2 O 3 ) and zirconium oxide (ZrO 2 ) are known opacifying agents which are added in minute amounts to the resin mixture. These opacifiers have refractive indices substantially different from the matrix. In addition to the refractive index, it has been shown that the shape and the size of filler particles, also have a significant effect on the light transmittance characteristics and the color of experimental composite resins. Materials that contain smaller size opacifiers with irregular shapes demonstrate higher light transmittance and diffusion angle distribution, in comparison to composites containing spherical-shaped and larger fillers .

An ideal opacifier is the one that is able to mask the unwanted discoloration or background darkness efficiently in minute concentration. Studies on the effects of different pigments and opacifiers at different concentrations on the translucency of dental composite resins are rare. Therefore, the aim of this study was to evaluate the effects of different opacifiers on the translucency of the experimental dental composites. The null hypothesis was that the addition of different opacifiers does not have any significant effect on the translucency of experimental dental composite resins.

Materials and methods

Specimen composition

All the materials used in this study for fabrication of the experimental composites, except for the opacifiers (metal oxides), were supplied by Dentsply (Konstanz, Germany).

Resin matrix was prepared by mixing the following ingredients: UDMA (99.22%), camphorquinone (CQ) (0.3%), dimethylaminobenzoic acid ethyl ester (DMABE) (0.3%), 3,5-di- tert -butyl-4-hydroxytoluene (BHT) (0.12%) and 2-hydroxy-4-methoxybenzophenone (HMBP) (0.06%).

The experimental composite resins were produced by mixing 25 wt.% of resin matrix with 75 wt.% of filler.

The filler used was silane treated barium silicate glass filler (particle size 1.5 μm). Three metal oxides were used as opacifiers: titanium oxide (TiO 2 ), aluminum oxide (Al 2 O 3 ) and zirconium oxide (ZrO 2 )—particle size of all <5 μm, according to manufacturer (Sigma-Aldrich, Dorset, UK).

Specimen groups

13 groups ( Table 1 ) of experimental composite resins were made containing different concentrations of the opacifiers: 0.25, 0.5, 0.75 and 1 wt.%. The metal oxides were blended in the filler mixture, giving the same total filler content of 75 wt.% for all four groups. A control group with no opacifier was also prepared.

Table 1
Composition of the filler and opacifiers in different experimental composite resins.
Silica filler (wt.%) TiO 2 (wt.%) Al 2 O 3 (wt.%) ZrO 2 (wt.%)
Composition 1 74.75 0.25 0 0
Composition 2 74.50 0.5 0 0
Composition 3 74.25 0.75 0 0
Composition 4 74 1 0 0
Composition 5 74.75 0 0.25 0
Composition 6 74.50 0 0.5 0
Composition 7 74.25 0 0.75 0
Composition 8 74 0 1 0
Composition 9 74.75 0 0 0.25
Composition 10 74.50 0 0 0.5
Composition 11 74.25 0 0 0.75
Composition 12 74 0 0 1
Composition 13 75 0 0 0

As the silica filler varied in minute amounts for the four groups to give the same total content of 75 wt.% of filler, an additional group was tested in a pilot study containing no opacifier and 1 wt.% reduction of glass filler and compared with the control group to evaluate whether varying only these minute concentrations of silica filler would significantly affect the translucency. No significant differences in optical properties were seen and therefore, only one control group was used for the purpose of this study (75 wt.% of filler).

Specimen fabrication

The ingredients were measured for the desired weight using an analytical balance (Mettler AJ100, Greifensee, Switzerland) and then were mixed by hand in small flexible plastic containers. Once mixed to a homogeneous paste, the experimental resin was ready to be placed into the molds.

A polycarbonate sheet of 1.5 mm thickness, containing six holes of 15.5 mm diameter, was made to act as mold for the specimens. Each group of unpolymerized resin composite specimens was packed into the six molds over a glass plate using a condenser, making sure no bubbles were created. Another glass plate was placed over the polycarbonate sheet and firm pressure was applied for twenty seconds. The specimens were then light-cured from both sides in three different locations for a total of 90 s. The light source unit (QHL 75, Dentsply) had an intensity setting of 450 mW/cm 2 .

Of the six polymerized specimens, three were chosen based on homogeneity and lack of porosities. The other three were discarded. A total of thirty-nine specimens were selected for the study (N = 39).

The specimens were ground using a silicon carbide grinding paper (Buehler-Met ® II, Buehler UK, Coventry) P400 to the thickness of 1.3 mm, and subsequently polished with a P1200 to the thickness of 1 mm (±0.05 mm) for a smooth finish. This was carried out on a grinder-polisher machine (Buehler Metaserv, Buehler UK) rotating at 200 rpm speed. A micrometer was used to check thickness of the specimens in five different locations (one at the center and four at the corners). A bright light source was used to check for porosities. Specimens that showed inappropriate thickness and/or porosities were discarded and replaced.

Each specimen was then rinsed with water, dried and stored with the other two specimens of the same group in a dry environment in a self-sealing small poly bag.

Measurement of optical properties

Optical properties data were collected using a computer-controlled spectrophotometer (Lambda 2, PerkinElmer, Massachusetts, USA) with integrating sphere accessors. Transmittance (total, diffuse and total direct) was measured in the wavelength range of 380–700 nm under standard illuminant D65 at 1 nm intervals. Color coordinates, L* (lightness), a* (red-green chromaticity index), and b* (yellow-blue chromaticity index) were determined from the total transmittance data using Pecol color software (PerkinElmer, USA).

For total transmittance and diffuse transmission, measurements were taken for every wavelength from 380 nm to 700 nm, resulting in 321 readings. For total transmittance measurement, a specimen was placed in the transmission port (entry port) of the spectrophotometer and a white reference material was placed in the reflectance port ( Fig. 1 ).

Fig. 1
Schematic diagram of the mechanism of light transmittance detection by spectrophotometer.

For diffuse transmission, a light trap needs to exist in the reflectance port. The light trap absorbs the direct transmission, and therefore only scattered light is measured. A light trap can be either a black background or an open port. In this study, an open port was chosen as a light trap.

For direct transmittance, the values of total transmittance were subtracted from diffuse transmittance, to measure light passing through the samples without scattering.

Color measurements were taken using CIE Lab values in total transmittance mode. Color difference (ΔE*) was measured using the following equation:

ΔE* ab = [(ΔL*) 2 + (Δa*) 2 + (Δb*) 2 ] 0.5

Statistical analysis

Statistical analysis of the data was carried out by one-way ANOVA followed by Tukey’s analysis, as well as regression analysis using the Minitab statistical analysis software.

Materials and methods

Specimen composition

All the materials used in this study for fabrication of the experimental composites, except for the opacifiers (metal oxides), were supplied by Dentsply (Konstanz, Germany).

Resin matrix was prepared by mixing the following ingredients: UDMA (99.22%), camphorquinone (CQ) (0.3%), dimethylaminobenzoic acid ethyl ester (DMABE) (0.3%), 3,5-di- tert -butyl-4-hydroxytoluene (BHT) (0.12%) and 2-hydroxy-4-methoxybenzophenone (HMBP) (0.06%).

The experimental composite resins were produced by mixing 25 wt.% of resin matrix with 75 wt.% of filler.

The filler used was silane treated barium silicate glass filler (particle size 1.5 μm). Three metal oxides were used as opacifiers: titanium oxide (TiO 2 ), aluminum oxide (Al 2 O 3 ) and zirconium oxide (ZrO 2 )—particle size of all <5 μm, according to manufacturer (Sigma-Aldrich, Dorset, UK).

Specimen groups

13 groups ( Table 1 ) of experimental composite resins were made containing different concentrations of the opacifiers: 0.25, 0.5, 0.75 and 1 wt.%. The metal oxides were blended in the filler mixture, giving the same total filler content of 75 wt.% for all four groups. A control group with no opacifier was also prepared.

Nov 22, 2017 | Posted by in Dental Materials | Comments Off on The effects of different opacifiers on the translucency of experimental dental composite resins
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