2.1.

Sample Preparation

We obtained samples that were produced by way of the above techniques of ocular iris button manufacture and divided them into six groups (Table 1). We made 60 samples simulating ocular prostheses with brown irises. The manufacturing techniques for the ocular iris’ button were the conventional technique (pressing the acrylic resin onto a painted disk of paper, PE), the technique involving a prefabricated cap (the base of the ocular cap positioned over the painting of the paper disk, CA) and inverted painting (the base of the painted ocular iris, PI). We made 20 samples with each technique and applied protective varnish to half of the samples made by each technique (PEV, PIV, and CAV), leaving the other half unvarnished (PE, PI, and CA) and producing 10 samples in each final group.

Table 1

Analysis of variance (ANOVA) of the color changes after polymerization.

We made the samples using modified metal matrices according to the technique established by Goiato et al.²³ These matrices were rectangular, measuring 30 mm length, 20 mm width, and 2 mm thickness, with a 13-mm-diameter circular demarcation in the center (Fig. 1). We positioned colorless ocular caps onto the center of the matrices, on top of the demarcation. The metal matrix and colorless cap unit was placed in their own muffle for polymerization in a microwave oven.

Fig. 1

A schematic representation of the dimensions of the metal matrix showing the ocular cap’s position from above (a) and from the side (b).

Shortly after the final press of the plaster, we withdrew each unit from the muffle, and we filled the region corresponding to the prefabricated ocular cap (Artigos Odontológicos Clássico Ltda., SP, Brazil) with extra-hard silicon (Zhermack, Italy) and filled the region corresponding with the matrix with thermopolymerizable N.1 acrylic resin (Artigos Odontológicos Clássico Ltda., SP, Brazil) and placed the unit in a hydraulic press (Midas Dental Products Ltda., SP, Brazil) with a force of 1.2 kgF for 2 min. We polymerized the resin in a microwave oven (Brastemp, SP, Brazil) at 60% maximum power (1400 W) for 3 min.^23,24

After polymerizing the resin, we opened the muffle again and removed the silicon that had filled the mold of the ocular cap to be pressed into each artificial iris button. We produced the artificial iris by painting a brown-colored oil paint (Acrilex Tintas Especiais SA, SP, Brazil) onto disks of black cardboard or onto the planar surface of the ocular cap (inverted painting), both with a diameter of 13 mm, during the same period and under the same lighting conditions, using an n.0 ref. 175 brush (Tigre S.A., SP, Brazil). To accelerate drying of the oil paint, we added a cobalt drying agent (Acrilex Tintas Especiais SA, SP, Brazil) corresponding with 30% in weight of the quantity of oil paint used. We conducted the final drying by using direct exposure to a 250-W, 130 E26-V infrared light source (Empalux Ltda., PR, Brazil) at a 30-cm distance from the disks for 2 h.¹¹ After completing this process, we applied three light layers of waterproof paint-protecting varnish spray (Acrilex Tintas Especiais SA, SP, Brazil) to the artificial irises from the PEV, CAV, and PIV groups.

To obtain the artificial eye buttons for the samples from groups PE and PEV, we pressed acrylic resin onto the painted surface of the paper disk. For this process, we fixed the painted disks with a liquid adhesive (J-305, Monopoly Syrup; Factor II Inc., Lakeside, Arizona)¹⁸ onto the center of a thermopolymerizable N.1 acrylic resin plate (Artigos Odontológicos Clássico Ltda., SP, Brazil). We pressed a colorless acrylic resin specifically made for ocular prostheses onto the disks. We inserted the resin into the molds contained in the muffles, so we could then press them in a hydraulic press with a force of 1.2 kgF. We polymerized the resin in a microwave oven (Brastemp, SP, Brazil) at 60% maximum power (1400 W) for 3 min.¹¹

We obtained the buttons of the artificial irises from the samples in groups CA and CAV by fixing the painted area of the cardboard disks onto the base of the ocular caps, using liquid adhesive (J-305, Monopoly Syrup; Factor II Inc., Lakeside, Arizona).¹⁸ Next we fixed the iris buttons using the same adhesive to the center of a thermopolymerizable N.1 acrylic resin plate. On the interface between the iris button and the N.1 acrylic resin plate, we pressed on thermopolymerizable colorless acrylic resin, and polymerized it in a microwave oven under the same conditions described above.¹¹

We obtained the artificial iris buttons from the samples in groups PI and PIV by painting the iris on the base of the ocular cap (inverted painting technique), then fixing it with liquid adhesive (J-305, Monopoly Syrup; Factor II Inc., Lakeside, Arizona) 20 onto the center of the N.1 acrylic resin plate. On the interface between the iris button and the thermopolymerizable N.1 acrylic resin plate, we pressed and polymerized thermopolymerizable colorless acrylic resin, as previously described.¹¹

2.2.

Spectrophotometric Analysis of Color Change

Once this procedure was completed, initial color readings were taken using visible ultraviolet reflect spectrophotometer, across the diameter of the sample. For the measurement of a color difference, the uniformed color space color system, which is closer to the human sensation, was used to show the color difference quantitatively. Chromatic and whiteness/brightness changes were evaluated by refraction spectrophotometer (UV-2450; Shimadzu Corp, Kyoto, Japan), with wavelength ranging from 780 (at the beginning) to 380 (at the end); and color changes being calculated according to the International Commission on Illumination (CIE) standards using the L*a*b* system with the standard illuminant D65.^2,11–13

The spectrophotometer (UV-2450; Shimadzu Corp, Kyoto, Japan) has a ceramic emission source in which the light beams are transmitted toward the sample. In the refraction spectrometry, the monochromatic color focus on the surface of sample and part of the light beam is absorbed and the other is reflected.¹¹ The amount of reflected light is captured by a photocell, which translates its wavelength into electrical signals, and is further captured by the computer system (UVPC Optional Color Analysis software package, Shimadzu Corp, Kyoto, Japan). This color measurement software provides the CIE L*a*b* color systems, where L* (or $\mathrm{L}$) shows the lightness, and the other two parameters (a* and b*; or $\mathrm{a}$ and $\mathrm{b}$) show the chromaticity.¹³

The CIE L*a*b* system allows color perception to be specified in three-dimensional (3-D) space (Fig. 2) by comparing the color of the surface of sample with the control group of the equipment, through a wavelength versus refraction index. In this 3-D color space, the three axes are namely L*, a*, and b*. The “$\mathrm{L}$” value is known as whiteness or brightness of an object and ranges from 0 (black) to 100 (perfect white).¹³ The “$\mathrm{a}$” value represents the quantity of red (positive values) and green (negative values), while the “$\mathrm{b}$” value represents the quantity of yellow (positive values) and blue (negative values). The “$a$” and “$b$” parameters coexist on the same plane within this 3-D space (Fig. 2). Color variation between two points was calculated using the CIE L*a*b* system, based on the following formula: ${\mathrm{\Delta }E=[(\mathrm{\Delta }\mathrm{L})2+(\mathrm{\Delta }a)2+(\mathrm{\Delta }b)2]}^{1/2}$. The samples were positioned in the same way using a black covered metal device that was created by the authors and adapted to the spectrophotometer (UV-2450; Shimadzu Corp, Kyoto, Japan) for use in this study; this device was designed to standardize the reading area and prevent color variation.¹³

Fig. 2

Schematic diagram of the CIE L*a*b* system.

2.3.

Statistical Analysis

We performed two-way analyses of variance (ANOVAs) to verify whether outcomes were affected by technique or the presence of varnish. We used the Tukey-Kramer’s HSD test ($\alpha =0.05$) to compare the values obtained from the tests performed.