Assessment of the color stability of two silicone elastomers for maxillofacial prostheses upon exposure to cold beverages

Abstract

Purpose: This study investigated the color stability of different commercially available silicone materials for facial prostheses upon exposure to everyday beverages. It was hypothesized that the beverages would not alter the color of the silicone under conditions simulating daily exposure.

Methods: Sixty specimens were fabricated using two materials – VST-50 and Silfy – and exposed to commercially available cold mineral water, tea, or coffee. Specimen color was evaluated on days 1 and 15. The color was measured with a spectrophotometer based on CIELab system and color differences were calculated as ΔE. Statistical analysis was performed using the Kruskal-Wallis test and Mann-Whitney U test.

Results: The mean ΔE values after 15 days of exposure to mineral water, cold green tea, and coffee were 1.016, 3.480, and 3.636 for VST-50 and 0.440, 0.798, and 1.425 for Silfy, respectively. Both materials showed significant differences in color, and VST-50 showed a greater color change than Silfy, especially for coffee.

Conclusion: Pigmented silicone elastomers have low color stability, leading to an overall color change in silicone prostheses when exposed to pigmented beverages. Color changes in such prostheses can be mitigated by selecting materials with better color stability to extend their longevity.

Introduction

Maxillofacial defects affect physiological functions such as speech and mastication, thus also impacting individuals on a psychological level. Maxillofacial defects can be congenital or the result of injury or disease, especially cancer, which is a major reason for maxillofacial resection [1].

For patients with head and neck cancer who have undergone maxillofacial resection, rehabilitation is needed to address the resulting facial defects [2]. In cases where it is not possible to close the defect through surgical reconstruction, a maxillofacial prosthesis is the most suitable alternative. Such prostheses can restore oral functions, protect underlying tissues, improve esthetics, and be psychologically reassuring to the patient, thus helping to improve quality of life [3].

Compared with other materials such as vinyl plastisols, polyurethane, biphenylene, and polyphosphazines, silicone elastomers have long been regarded as the best choice for maxillofacial prostheses because of their physical properties, including excellent tear and tensile strength over a wide range of temperatures, durability, a skin-like texture, and adequate biocompatibility. These properties make silicone elastomers suitable for use when adaptability and accommodation of underlying tissue movement are needed for contact between the prosthesis and the patient’s soft tissues [4]. Moreover, silicone has a high degree of chemical inertness, a low degree of toxicity, and good thermal and color stability, as well as physical properties comparable to human tissues [5].

The color of its constituent material is an important feature that determines the clinical serviceability of a maxillofacial prosthesis, and several studies have investigated the color stability of silicone materials under a wide range of environmental conditions, including sunlight, temperature, atmospheric moisture, and exposure orientation, all of which can limit the service life of a prosthesis [6,7,8]. Additionally, dentists need to inform patients that certain beverages may discolor a prosthesis, potentially leading to dissatisfaction and additional costs for replacements.

Color changes can be measured visually or using dedicated instruments. Once such instrument is a spectrophotometer, which is a standardized instrument that matches and measures color based on the reflectance curve as a function of the wavelength of light [9]. Spectrophotometers have been extensively used to measure color change in silicone materials, as they offer quick, accurate, and quantifiable readings for color measurement and analysis.

When evaluating the color stability of different silicone materials, it is important to consider the typical diet of prosthesis users, such as those with mid-facial and lip defects.

Previous studies [10,11,12,13] have investigated the properties of silicone elastomers after exposure to artificial daylight or radiation, various outdoor environments, acidic perspiration, and cleaning agents. However, it appears that there has been no investigation of the effect of commonly consumed cold beverages on the coloration of silicone polymers used in maxillofacial prostheses. Therefore, the aim of the present study was to investigate the color stability of two commercially available silicone materials when exposed to cold beverages commonly consumed in Japan. The null hypothesis was that these beverages would not alter the color of these materials during daily use.

Materials and Methods

Sample fabrication

Specimens were made using two commercially available silicone elastomers: VST-50 (Platinum silicone elastomer VST-50, Factor II, Lakeside AZ, USA) and Silfy (Gc Silfy, GC, Tokyo, Japan) (Table 1). Functional Intrinsic coloring agents (Functional intrinsic II, Factor II, Lakeside AZ, USA) were added to the silicones to simulate skin color.

A total of 60 specimens (VST-50, n = 30; Silfy, n = 30) were fabricated. Each specimen was a square measuring 20 mm × 20 mm, with a thickness of 2 mm to simulate the thickness of maxillofacial prostheses. The base and catalyst of the silicone were mixed at a ratio of 10:1 by hand for 30 s, followed by vacuum mixing for 60 s. The silicone was then loaded into a syringe to reduce voids as it was dispensed into preformed moulds. The silicone specimens were maintained at room temperature for 24 h to vulcanize.

After vulcanization, the Silfy and VST-50 specimens were each randomly allocated to three groups of 10 specimens each (one group for each test beverage) and immersed in the test beverages for 15 days. The following commonly consumed beverages were selected: mineral water (Nagano Azumino, Famimaru, Tokyo, Japan), green tea (Iemon green tea, Suntory, Tokyo, Japan), and coffee (Craft Boss Black, Suntory) (Table 1). The silicone specimens were placed in individual containers and 20 mL of each test beverage was added. The containers were then stored at 37℃ to simulate the conditions of the oral cavity and kept away from sunlight and other weathering conditions so that no factors other than the beverages would affect the color of the specimens. The solution was replaced daily.

Table 1 Details of the two silicone materials, VST-50 and Silfy, and the cold beverages tested in this study

Material	Brand name	Manufacturer	Composition	Lot number
Silicone elastomer	Gc Silfy	GC, Tokyo, Japan	part A: Vinyl polysiloxane, silicic anhydride, platinum catalyst part B: Vinyl polysiloxane, silicic anhydride	2207061
	Platinum silicone elastomer VST-50	Factor II.Inc., Lakeside AZ, USA	part A: Polymethylvinylsiloxanes, polymethylhydrogensiloxanes, silica part B: Polymethylvinylsiloxanes, platinum complex, silica	S4107156
Beverages	brand name	manufacturer
Mineral water	Natural mineral water	Famimaru, Tokyo, Japan
Cold green tea	Iemon green tea	Suntory, Tokyo, Japan
Cold coffee	Craft Boss Black	Suntory.

Color measurement

The color of the specimens was evaluated on day 1 and again on day 15, and this period was considered to simulate exposure to the amount of the beverage that an average person would consume over a period of approximately 6 months (14 h to simulate the weekly exposure time with beverages or foods, 2 h × 7 days) [14]. Measurements were performed with a spectrophotometer (CM-23d, Konica Minolta, Tokyo, Japan) on a white background and the measured colors were characterized according to the Commission International d’Exchange (CIE) L* a* b* system, a color order space with coordinates for white-black (L*), red-green (a*), and yellow-blue (b*). After first calibrating the spectrophotometer according to the manufacturer’s instructions, the mean values of L*, a*, and b* were automatically calculated by the spectrophotometer and recorded in the CIELab color system.

Statistical analysis

The change in color (ΔE) was calculated using the equation ΔE = ([ΔL*] 2 + [Δa*] 2 + [Δb*] 2)^1/2, where ΔL*, Δa*, and Δb* represent the change in L*, a*, and b*, respectively, between day 1 and day 15. The data were compiled in a spreadsheet (Excel, Microsoft, Redmond WA, USA) and subjected to statistical analysis using statistical software (SPSS ver. 28.0, IBM, Chicago, USA). Descriptive statistics, including ΔE with respect to L*, a*, b* of the specimens as a whole and according to group, are shown. Statistical analysis was performed using the Kruskal-Wallis test and the pairwise comparison Mann-Whitney U test with Bonferroni correction. For most statistical tests, differences at P < 0.05 were considered statistically significant. The α error and β error were set at 5% and 20%, respectively, giving the study a power of 80%.

Results

Color changes were observed for all combinations of specimen type and beverage. For VST-50 exposed to mineral water, L* increased while both a* and b* decreased. For VST-50 exposed to green tea, there was an increase in both L* and b*, whereas a* decreased. VST-50 exposed to coffee showed a decrease in both L* and a*, whereas b* increased. Silfy exposed to mineral water showed a decrease in a*, while both L* and b* increased. For Silfy exposed to green tea, there was an increase in b*, whereas L* and a* decreased. For Silfy exposed to coffee, there was a decrease in both L* and a*, whereas b* increased (Fig. 1). The results of inter- and intra-group comparisons of ΔE between day 1 and day 15 for the VST-50 and Silfy groups are shown in Fig. 2 and Tables 2,3,4.

The data were found to be nonparametric, so statistical analysis was performed using the Kruskal-Wallis test and Mann-Whitney pairwise comparison test. A color change was observed when the groups were analyzed statistically. Inter-group comparison of the test beverages with Silfy and VST-50 performed using the Kruskal-Willis test (P < 0.05) revealed a significant difference in the values between the groups for each beverage (P < 0.001). The median color change of VST-50 with mineral water, cold green tea, and coffee was 1.016, 3.480, and 3.636 and that of Silfy was 0.440, 0.798, and 1.425, respectively. The least color change was seen with mineral water, followed by green tea, and the greatest change was seen with coffee.

Inter-group comparisons were performed using the Kruskal-Wallis test (P < 0.05) for the VST-50 group, and the results showed a strongly significant difference in ΔE values for mineral water, green tea, and coffee (H = 16.312, P < 0.001). The Mann-Whitney pairwise comparison test (P < 0.05) between mineral water, green tea, and coffee in the VST-50 group showed significant differences between mineral water and green tea, (z = −3.175, P = 0.001) and between mineral water and coffee (z = −3.704, P < 0.001); although a difference was found between green tea and coffee, it was not significant (z = −0.529, P = 0.597), as shown in Fig. 2, Tables 2 and 3.

A comparison of beverages with Silfy using the Kruskal-Wallis test showed a significant change in color (H = 22.872, P < 0.001). The Mann-Whitney U pairwise comparison test (P < 0.05) performed to compare differences between mineral water, green tea, and coffee in the Silfy group revealed significant differences between mineral water and green tea (z = −3.099, P = 0.002), between mineral water and coffee (z = −3.780, P < 0.001), and between green tea and coffee (z = −3.553, P < 0.001), as shown in Fig. 3, Tables 2 and 3.

VST-50 and Silfy were also compared for each beverage and analyzed using the Mann-Whitney pairwise comparison test (P < 0.05). The results revealed a significant difference across mineral water and green tea (z = −3.704, P < 0.001) but not with coffee (z = −2.041, P = 0.041), as shown in Fig. 4 and Table 4.

Fig. 1 CIELab color values of the VST-50 silicone elastomer, before and after immersion in cold beverages

L*1, a*1 and b*1 correspond to values at day 1 and L*15, a*15 and b*15 correspond to values at day 15.

Fig. 2 CIELab color values of the Silfy silicone elastomer, before and after immersion in cold beverages

L*1, a*1 and b*1 correspond to values at day 1 and L*15, a*15 and b*15 correspond to values at day 15.

Table 2 Results of the overall comparison of color change (ΔE) for VST-50 and Silfy exposed to cold beverages, using the Kruskal-Wallis test for total samples on days 1 and 15

VST-50
Kruskal-Wallis H	16.312
P value	<0.001
Silfy
Kruskal-Wallis H	22.872
P value	<0.001

*P < 0.05, Kruskal-Wallis Test. There was a significant change in color for both VST-50 and Silfy among the beverages.

 

Table 3 Results of intergroup comparison of ΔE for VST-50 and Silfy exposed to cold beverages, using the Mann-Whitney U test

	Mineral Water V/S Cold Green Tea	Mineral Water V/S Cold Coffee	Cold Green Tea V/S Cold Coffee
VST-50
Mann-Whitney U	8.000	1.000	43.000
z	−3.175	−3.704	−0.529
P value	0.001	<0.001	0.597
Silfy
Mann-Whitney U	9.000	0.000	3.000
z	−3.099	−3.780	−3.553
P value	0.002	<0.001	<0.001

*(P < 0.05), Mann-Whitney U pairwise comparison test

 

Table 4 Results for intergroup comparison of ΔE for VST-50 and Silfy exposed to cold beverages, using the Mann-Whitney U test

	Mineral water	Cold Green Tea	Cold Coffee
Mann-Whitney U	1.000	1.000	23.000
z	−3.704	−3.704	−2.041
P value	<0.001	<0.001	0.041

*(P < 0.05), Mann-Whitney U pairwise comparison test

 

Fig. 3 Color change (ΔE) of the silicone elastomers with exposure to the cold beverages tested across VST-50 *Indicates a significant difference

Fig. 4 Color change (ΔE) of the silicone elastomers with exposure to the cold beverages tested across Silfy

*Indicates a significant difference

Discussion

The color of the two silicone materials changed with exposure to the cold test beverages and thus the null hypothesis was rejected. The material itself and the surrounding environment are factors involved in the discoloration of silicone elastomers [15]. Color changes were observed in all groups in the present study, with significant changes observed for both Silfy and VST-50. It was evident that the ΔE value remained consistently above zero for all samples, indicating a change in color. Previous studies have suggested that this shift in color may be due to both inherent and external factors. Intrinsic factors include natural material discoloration with alterations in the matrix, while extrinsic factors, such as the absorption and adsorption of staining agents, can also contribute to discoloration [16,17,18]. Color variations were evident across all groups, with VST-50 exhibiting more pronounced changes than Silfy, especially with coffee.

This difference may arise from variations in composition, intrinsic natural material discoloration coupled with modifications in the matrix, and extrinsic factors such as the absorption and adsorption of staining agents. This comparison aids understanding of how different drinks can affect color, emphasizing the need to consider these factors when choosing materials based on preferences for beverages among individual patients. The observed color changes are consistent with the results of Eleni et al. [19], who evaluated three different types of polydimethylsiloxanes after immersion in simulated body fluid and sweat at 37°C and found that all materials exhibited significant color changes. Also, in two different studies investigating the pigmentation of maxillofacial silicones under different aging conditions, the duration of exposure and the type of silicone were found to be significant factors affecting color stability [20,21]. In addition, water sorption and solubility were found to dramatically affect stain resistance, dimensional stability, and other physical and mechanical properties [22]. The silicone materials in maxillofacial protheses are exposed to saliva during clinical use and may also be exposed to water and cleaning agents when not in use, during which time plasticizers and other soluble substances may leach out over extended periods while water is absorbed, until an equilibrium is reached [23].

The perception of color is based on three fundamental components: hue, brightness, and chroma. When the silicone samples were exposed to mineral water and green tea, their luminosity (L*) increased and their surface appeared lighter, but they appeared darker when exposed to coffee. Meanwhile, the red-green (a*) values decreased, and the yellow-blue (b*) values increased, causing the specimens to have a more greenish and yellowish appearance when wet. The human eye can detect differences in hue (red, yellow, green, blue, etc.), chroma (saturation), and lightness to roughly the same degree. Seghi et al. [24] and Paravina et al. [25] reported that a color difference of ΔΕ > 2 can be visually detected by an observer. According to the National Bureau of Standards, a color change of ΔE < 1 is exceptionally low, between 1 and 3 is clinically acceptable, and >3.39 is clinically noticeable. Based on the above studies, the present study accepted an accessibility threshold of ΔΕ < 2 after 15 days of exposure. It was observed that the color change of VST-50 exposed to green tea and coffee was >2, while that of Silfy was <2. The change in color was greater for VST-50 than for Silfy. Both green tea and coffee exposure affected the color of the two silicone elastomers.

Additional cross-linking caused by continued polymerization of the silicone or by side reactions among impurities present within it may be one reason for the color changes observed [11,12,26]. However, it is also important to consider the actual color changes that occur as a result of direct contact between elastomer prostheses and human oral tissues, and this should be assessed in a clinical study. In the present in vitro study, the silicone samples were colored with pigments. Non-pigmented silicone elastomers may also show some visible color change, and this should be investigated in future studies. Three common cold beverages were tested in this study, and further research should be conducted to investigate the effects of other food items on the coloration of maxillofacial prostheses, so that the properties of silicone materials used can be improved by adding new components with better color stability.

Within the limitations of this in vitro study, the two pigmented silicone elastomers exhibited inherent color instability that resulted in an overall color change in both after exposure to the tested beverages. VST-50 showed the greatest color changes when exposed to the tested beverages, and Silfy demonstrated better color stability overall. The materials used in maxillofacial prostheses need to be selected with due consideration to color stability in order to prolong their service life.

Conflicts of Interest

The authors have no conflicts of interest to declare with respect to the results or funding.

Funding

This research did not receive any specific grants from funding agencies in the public, commercial, or not-for-profit sectors.

Authors Contributions

AC: Data curation, Original draft preparation, Software, Validation; MH: Supervision, Writing Reviewing and Editing; MAA: Visualization, Investigation; YIS: Conceptualization, Methodology; NW: Supervision

ORCID iD

^1,2)AC: [e-mail], 0009-0009-7591-4513

¹⁾MH*: [e-mail], 0000-0003-2802-8016

³⁾MAA: [e-mail], 0000-0002-5617-5390

^4,5)YIS: [e-mail], 0000-0003-3982-8369

¹⁾NW: [e-mail], 0000-0002-0517-6756

Acknowledgments

The authors are grateful to Dr. Murase Mai and Dr. Haraguchi Mihoko for advice and help with specimen fabrication.

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