Effects of different types of molds on the color difference, translucency, surface roughness, and hardness of a maxillofacial silicone elastomer

Abstract

Purpose: The purpose of the present study was to evaluate the effects of dental stone molds and three dimensional (3D)-printed molds on the color difference, translucency, surface roughness, and hardness of maxillofacial silicones.

Methods: To prepare molds, a rectangular body 20 mm in diameter and 2 mm in thickness, was designed with computer-aided design software. Two different dental gypsum molds were prepared with the lost-wax technique. Silicone mixture was poured into molds and polymerized at room temperature for 24 h. Color parameters were measured using a spectrophotometer. A profilometer was used for measuring surface roughness, and Shore A values were obtained with a durometer.

Results: Color change (ΔE₀₀) values of the 3D-resin group (1.53 ± 0.35) were significantly lower than others. The highest translucency parameter (TP) values belonged to the stainless steel group (12.44 ± 0.65). Surface roughness values (Ra) of the stainless steel group (0.28 ± 0.06) were significantly lower than other groups. The mean Shore A value of the 3D-resin group (23.90 ± 1.37) was significantly higher than the blue gypsum group (21.53 ± 0.93).

Conclusion: Lower color difference and higher Shore A values were examined with 3D-printed resin molds. The highest TP values and lowest Ra values were obtained when stainless steel was used for molding of maxillofacial silicone.

Introduction

Maxillofacial prostheses are used to replace facial parts lost through disease, trauma, or congenital defects [1]. Silicone elastomers are the standard type of polymer used in maxillofacial prostheses due to their acceptable properties [2]. However, conventional manufacturing techniques for silicone prostheses are time-consuming, experiential, and skill based. Furthermore, the lifespan of these prostheses is limited. Environmental factors such as sunlight, air pollution, perspiration, and patient usage including cleaning and wearing frequency, result in color degradation and changes in mechanical properties. Therefore, maxillofacial prostheses should be renewed after a mean of 1.5 to 2 years of use [3,4,5,6].

Traditionally, maxillofacial prosthesis fabrication includes taking an impression and obtaining a gypsum cast of the defect side, modeling a wax pattern of the prosthesis, and trying it on the patient, preparing a stone mold by placing the wax pattern into a flask and melting the wax, and finally packing the individually colored silicone elastomer into the mold [7]. The esthetic result of these maxillofacial prostheses is relevant to many issues such as shape, symmetry, facial harmony, surface texture, and color. A good color match of the prostheses with the skin of the patient has been reported as the most important determinant of the esthetics in maxillofacial prosthodontics [8].

Coloring maxillofacial prosthetic material to reproduce skin shade and translucency has been achieved by adding pigments manually into silicone elastomer. The pigment type and quantity added to the silicone are determined by the prosthodontist subjectively or by a computer-based color-matching system. Each color-matching method has its inherent advantages and deficiencies while the use of a combination of two methods results in excellently color-matched maxillofacial prostheses [9]. Besides pigments added to the silicone prosthetic material, processing conditions affect the color of the prostheses [9,10]. Following pigment loading and verifying a close color-match between silicone and skin, silicone is placed into the mold and processed for polymerization. At this stage, the liquid silicone turns into an elastic material through a chemical reaction called polymerization. Previous studies reported that polymerization temperature, duration, and mold material affected the final color and surface of the polymerized silicone material [9,10,11].

Additive manufacturing technology using three-dimensional (3D) printers was first used in dentistry in 2013 to produce surgical bone models, models to simulate implant surgery, and surgical templates for implant placement. In prosthodontics, 3D printers aim to eliminate the disadvantages of subtractive manufacturing in computer-aided design and computer-aided manufacturing (CAD-CAM) workflow, and have been used in model production, the construction of fixed or removable prostheses, the preparation of wax specimens for casting processes, the production of custom impression trays and occlusal splints, and maxillofacial prostheses [12,13,14,15].

The use of 3D-printing technology for the fabrication of facial prostheses includes printing a model of the defect side, wax pattern of the prosthesis, molds, and direct 3D manufacture of silicone prostheses [16,17,18]. Because of the layered, vascularized, pigmented, and complex structure of human skin, directly printing the prosthesis using 3D-printed silicones is not common. Currently, the use of 3D-printed molds that are designed by CAD applications of 3D-printing technology on maxillofacial prosthodontics are being considered. 3D-printed molds eliminate laboratory procedures of stone mold preparation while allowing the clinician to place regionally colored silicone into the mold according to the layered structure of the skin [18]. Molds of the maxillofacial prostheses are printed using polymer filament or liquid resin-based 3D-printers [17,18]. Despite these advantages, no study has evaluated the effects of 3D-printed molds on the color and surface properties of silicone elastomers compared with stone molds.

The aim of the study was to assess the color difference, translucency, surface roughness, and hardness of a silicone elastomer after being molded in 3D-printed resin and stone molds, which are prepared from different colored dental stones. The null hypothesis was that the mold type would not affect the color difference, translucency parameter (TP), surface roughness values (Ra), and Shore A hardness of the material.

Materials and Methods

A platinum-catalyzed, room-temperature-vulcanized maxillofacial silicone (Technovent Ltd., Bridgend, UK) was used in the study. To evaluate the silicone elastomer’s color difference, translucency parameter, surface roughness, and hardness when polymerized in a 3D-printed resin mold, Type III blue dental stone mold and Type IV light brown dental stone mold, disk-shaped specimens were prepared. The materials used in the study are listed in Table 1.

The 3D molds were designed using CAD software (Onshape; PTC, Rockwell Automation, Boston, MA, USA), as a rectangular body 20 mm in diameter and 2 mm in thickness, and the obtained data was exported in the Standard Tessellation Language (STL) digital file format. The design was printed using the photopolymer resin (Model Resin; Powerresins, Istanbul, Türkiye) and a digital light processing (DLP) 3D printer (DentaFab; Istanbul, Türkiye). The thickness of each printing layer was set to 100 µm, and a printing support structure with cone shaped, 0.25 mm in radius, and 1 mm in length was attached to the bottom of the model. For the fabrication of stone molds, wax patterns of the specimens (20 mm in diameter and 2 mm in thickness) were prepared to create negatives in dental stone. Wax patterns were embedded in two different dental gypsums; Type IV (Moldastone CN; Heraus Kulzer, Hanau, Germany) with a light brown color and Type III (Hera Moldano; Heraus Kulzer) with a blue color. Also, stainless steel molds were fabricated from the STL file which was used to print resin molds to serve as a control group.

The base and the catalyzer of the silicone elastomer were mixed at a ratio of 9:1 according to the manufacturer’s instructions. Once combined silicone components were thoroughly mixed, intrinsic skin shades were added to simulate a skin color and translucency as described in a previous study [19]. The colored silicone was poured into the disk-shaped molds. The molds were maintained at room temperature for 24 h for polymerization under constant pressure. Polymerized silicone disks were removed from the molds and evaluated under magnification (Loupe opt-on; Orange Dental, Biberach, Germany) for porosity. Excess material at the edges of specimens was trimmed using scissors and cleaned in an ultrasonic cleaner (Electrosonic Type 7 Profi; Electrosonic GmBH, Düsseldorf, Germany) in distilled water for 10 min to remove the residue. The sample size was determined based on previous studies [20,21,22,23] so each group included 10 specimens (n = 10) and a total of forty silicone elastomer specimens were prepared.

Color measurements of specimens polymerized in stainless steel, 3D-printed resin, light brown gypsum, and blue gypsum molds were performed with a spectrophotometer (CM 3600 D; Konica Minolta, Tokyo, Japan).

It was hypothesized that there may be some changes in the color and surface properties of the silicone prosthesis when gypsum is used as a mold material during the production of maxillofacial prosthesis. Therefore, the color change (ΔE₀₀), TP, and Ra of the silicones were evaluated. The color parameters of silicone specimens polymerized in the stainless steel mold were considered as the control and used as the first (reference) measurements in the calculation of the color difference according to the CIEDE2000 formula. The ΔE₀₀ of silicones polymerized in the 3D-printed resin, light brown gypsum, and blue gypsum molds from the silicone polymerized in the stainless steel mold were calculated using the following equation [1]:

ΔE₀₀ = [(ΔL՛ /k_LS_L)² + (ΔC՛ /k_CS_C)² + (ΔH՛ /k_HS_H)² + R_T(ΔC՛ /k_CS_C)(ΔH՛ /k_HS_H)]^0.5

The spectrophotometer was calibrated for each group. Each value was measured 3 times and the average of L՛, a՛, b՛, c՛, and h՛ values were used to calculate the ΔE₀₀. ΔL′ represent lightness, ΔC′ chroma and ΔH′ hue of CIELAB color system. k_L, k_C, and k_H are compensation coefficients for experimental conditions. The weighting functions S_L, S_C, and S_H are utilized to modify the total color difference. The rotation function RT is associated with the interaction of chroma and hue differences in the blue region [7].

Measurements were taken using the standard illuminant (D65) on a white and black background for calculating the TP using the following equation [1]:

TP = [(LB* − LW*)² + (aB* − aW*)² + (bB* − bW*)²]^½

Hardness measurements were made using a Shore A durometer (PCE Instruments GmbH, Meschede, Germany). Three hardness measurements were made from each specimen as Shore units, and the average results were noted as the final Shore A value.

Ra was measured with a profilometer (Mahr Perthometer; Mahr GmbH, Goettingen, Germany) three times for each specimen and the mean value was calculated.

Table 1 Materials used in the study

Material	Properties	Brand name-producer	Lot number
Maxillofacial silicone	high temperature vulcanizing (HTV) and room temperature vulcanizing (RTV)	Techsil S-25, Technovent Ltd., Bridgend, UK	B20H
Maxillofacial color pigment	white:203 light grey:202 red:204 ochre:211	QuickWeigh LSR, Spectromatch Ltd., Bath, UK	231 209 204 206
Dental stone	color blue gypsum type III	Hera Moldano, Heraus Kulzer, Hanau, Germany	5651861
Dental stone	color light brown gypsum type III	Moldastone CN, Heraus Kulzer	5251532
Resin	3D model resin	Power resins, Istanbul, Türkiye	02663

Statistical analyses

The data of the tested parameters was analyzed using software (IBM Corp. Released 2011. IBM SPSS Statistics for Windows, Version 20.0. Armonk, NY, USA). The descriptive and comparative statistics of the results were presented with mean, standard deviation, minimum, and maximum values.

The normality of the data was tested using the Shapiro-Wilk test. The homogeneity of variances of the data was tested using the Levene test. When the data were confirmed to be normally distributed and homogeneous of variance, one-way analysis of variance (ANOVA) was used for the statistical analyses. Welch correction was used for the Ra values (P = 0.006), and pairwise comparisons were made using the Games Howell test. The Tukey HSD was used to assess the TP (P = 0.842), Shore A (P = 0.346), and ΔE₀₀ (P = 0.687) values. The results were considered significant for P < 0.05.

Results

The Shapiro-Wilk test of normality was conducted to determine whether ΔE₀₀, TP, Ra, and Shore A data is normally distributed. The ΔE₀₀, TP, Ra, and Shore A values of the experimental groups were evaluated with one-way ANOVA, and it was found that the mold type was significantly effective on the tested parameters (P < 0.001).

The Shapiro-Wilk test of normality was conducted and 3D-resin, blue and light brown groups P values of ΔE₀₀ were found to be P = 0.622, P = 0.560, and P = 0.396 respectively. The ΔE₀₀ values of the three groups were compared in Table 2 and statistically significant differences were found among the groups as a result of the one-way ANOVA (P = 0.002). When two groups were compared with the Tukey test, the mean value of the 3D-resin group was 1.53 ± 0.35, significantly lower than the mean value of the other two groups (P < 0.05). The mean ΔE₀₀ of the blue and light brown groups showed no statistically significant differences (P = 0.998).

The ΔE₀₀ values of all the experimental groups were lower than the acceptability threshold (2.1); however, the values for the gypsum mold groups were slightly lower than the threshold value.

Descriptive and comparative statistics of the TP values of the experimental groups are shown in Table 3. The Shapiro-Wilk test of normality was conducted and metal, 3D-resin, blue and light brown groups P values of TP were found to be P = 0.562, P = 0.874, P = 0.303, and P = 0.053 respectively. When the TP variable between the two groups was examined using the Tukey test, the mean value for the stainless steel group was found to be 12.44 ± 0.65, which was significantly higher than the other groups (P < 0.05). While there was no difference between the mean values of the 3D-resin and light brown groups (P = 1), the mean TP value of the blue group was significantly lower than these two groups. (P < 0.05).

Descriptive and comparative statistics of the Ra values of the experimental groups are shown in Table 4. The Shapiro-Wilk test of normality was conducted and metal, 3D-resin, blue and light brown groups P values of Ra were found to be P = 0.265, P = 0.471, P = 0.311, and P = 0.087 respectively. When the two groups were compared by the Games-Howell test, the mean value of the stainless steel group was 0.28 ± 0.06, which was significantly lower than the other three groups (P < 0.05). There were no statistically significant differences among the mean values of 3D-resin, blue, and light brown groups (P > 0.05) (Table 2).

Descriptive and comparative statistics of the Shore A values of the experimental groups are shown in Table 5. The Shapiro-Wilk test of normality was conducted and metal, 3D-resin, blue and light brown groups P values of Shore A were found to be P = 0.444, P = 0.854, P = 0.325, and P = 0.855 respectively. Significant differences were found among the Shore A values of four groups according to the results of the one-way ANOVA. The mean value of the stainless steel group (23.00 ± 0.64) was found statistically higher than the mean value of the blue group (21.53 ± 0.93) when the Shore A variable was compared using the Tukey test between the two groups (P < 0.05). The mean value of the 3D-resin group (23.90 ± 1.37) was found to be significantly higher than the blue group (21.53 ± 0.93) and the light brown group (22.52 ± 0.93) (P < 0.05).

Table 2 Comparison of ΔE₀₀ values between groups with one-way ANOVA

ΔE2000 (n = 10)	Mean ± SD	Median (min-max)
3D-resin	1.53 ± 0.35a	1.55 (1.04-2.26)
Blue	2.08 ± 0.35b	2.13 (1.53-2.74)
Light brown	2.07 ± 0.38b	2.12 (1.50-2.55)
	P < 0.001

Groups with different superscript letters showed significant differences from each other.

 

Table 3 Means, medians and standard deviations of all groups TP values

TP (n = 10)	Mean ± SD	Median (min-max)
Stainless steel	12.44 ± 0.65a	12.27 (11.31-13.72)
3D-resin	8.05 ± 0.76b	8.15 (6.71-9.21)
Blue	7.14 ± 0.68c	7.08 (6.01-8.65)
Light brown	8.06 ± 0.60b	8.30 (7.32-8.84)
	P < 0.001

Groups with different lowercase letters had significant differences from each other.

 

Table 4 Means, medians and standard deviations of all groups Ra values

Ra (n = 10)	Mean ± SD	Median (min-max)
Stainless steel	0.28 ± 0.06a	0.27 (0.21-0.39)
3D-resin	1.10 ± 0.25b	1.15 (0.75-1.50)
Blue	1.36 ± 0.26b	1.43 (0.78-1.69)
Light brown	1.18 ± 0.20b	1.16 (0.93-1.43)
	P < 0.001

Different lowercase letters indicate significant differences between groups in terms of Ra values.

 

Table 5 Means, medians and standard deviations of all groups Shore A values

Shore A (n = 10)	Mean ± SD	Median (min-max)
Stainless steel	23.00 ± 0.643a,c	23.08 (22.00-23.83)
3D-resin	23.90 ± 1.37a	23.83 (21.83-26.67)
Blue	21.53 ± 0.93b	21.25 (20.50-23.33)
Light brown	22.52 ± 0.93b,c	22.58 (21.00-24.17)
	P < 0.001

Groups with different lowercase letters had significant differences from each other in terms of Shore A values.

Discussion

This study examined the effect of the mold type on the color difference, translucency, hardness, and surface roughness of a maxillofacial silicone elastomer. Three types of molds were tested including 3D-printed, light brown, and blue dental stones. The 3D-printed mold revealed a smaller color difference compared with stone molds, and differences were detected in TP, Ra, and Shore A properties of the material. Therefore, the null hypothesis was rejected.

A close color match is the most important feature of a natural-looking maxillofacial prosthesis [24]. Color-matching between silicone elastomer and skin has been conventionally achieved by mixing the pigments into silicone according to the clinician’s experiences [25]. Contemporary computerized color-matching systems have been developed which quantify the skin color and pigmentation using a colorimeter or spectrophotometer and then establishes a pigment formulation using this data. These systems provided an accurate match between human skin and maxillofacial prosthesis regarding color and translucency, independently from the individual skills of the clinician [26,27]. However, variations in processing steps affect the color of the maxillofacial silicone. In the literature, vulcanization temperature and time, the type of mold, and separating media have been suggested as processing factors affecting the silicone color [9,10,11]. Sethi et al. [9] investigated the effect of mold and separating media on the color during the polymerization process. They used white and green dental stone, and orange die stone as molding materials, and coated these molds with three different separating media namely an alginate-based medium, soap solution, and a resin-based die hardening material. Die stones with orange color showed the highest color change (ΔE = 3.19) among the molding materials and differences were also found between separating materials. In another study, the color differences of a maxillofacial silicone polymerized in five different colored dental stone molds including yellow, green, white, blue, and reddish-brown were investigated. It was reported that the polymerization temperature of the silicone and the color of the dental stone used to make the mold affected the final color of the silicone elastomer [9]. In the present study, maxillofacial silicone was polymerized in two different dental stone molds, and a 3D-printed resin mold. The color differences of polymerized silicones in each mold group and a stainless steel mold serving as a control group were calculated. Blue and light brown dental stone molds revealed significantly higher color differences compared with 3D-printed resin molds. This may result from the invasion of coloring pigments of dental stone into the silicone during the polymerization reaction in which liquid silicone elastomer transforms into an elastic form. The color difference of polymerized silicone in 3D-printed resin molds was lower than dental stone molds and also lower than the acceptable color difference values for maxillofacial prostheses [28]. This finding may lead clinicians to focus on 3D-printed molds for maxillofacial prosthesis fabrication, in addition to the ease of use.

The effects of mold material on the color of maxillofacial silicone elastomers were evaluated using the CIEDE₀₀ formula, which introduces corrections to the ΔE value to better reflect human perception [29]. A CAD-CAM produced stainless steel mold was used to serve as a control group because of the unreactive and stable structure of the material and the smooth surface. The mean values of color measurements of ten specimens were used as the initial measurement for all mold groups and ΔE₀₀ calculated using the color measurement of each mold group as the second measurement. For 3D-printed resin mold, ΔE₀₀ was 1.53 and for blue and light brown dental stone molds, 2.08 and 2.07, respectively. All the values obtained in this study were higher than the threshold value that the human eye can perceive [28]. This indicates a significant color change happened during polymerization of the silicone [7]. On the other hand, ΔE₀₀ perceptibility and acceptability thresholds for light specimens were 0.7 and 2.1, respectively [28], and the color difference values of silicone elastomers polymerized in different mold materials did not exceed acceptability thresholds.

A separating medium was not applied to the specimen gaps in the molds to investigate the effects of mold material directly on the silicone elastomers. Stainless steel molds were manufactured in a milling machine and silicone elastomers polymerized in these molds were considered as a reference for color difference and a control group for evaluated parameters. Çifter et al. [7] reported that vulcanization temperature also affected the color of the silicone elastomer as fewer color degradations were detected in the silicone elastomer when vulcanized at room temperature even in different molds compared with silicones polymerized at 100°C. In the present study, a silicone elastomer that can be cured with heat or at room temperature was selected and curing is achieved at room temperature for 24 h. All the molds were prepared and kept available for silicone packing to fabricate ten specimens for each group. An adequate amount of silicone and pigment mixture for all specimens was prepared at once and placed in the molds. This eliminated possible minor color differences which may result from pigment measuring, silicone mixing, etc.

Translucency is another important optical property for maxillofacial prosthodontic material as human skin is a topographically multilayered structure. Pigment content, vascularity, and thickness of each layer provide the resulting skin tone and translucency. Therefore, TP should also be compatible with the skin translucency of the patient. TP of the silicone prostheses primarily depends on pigment content loaded during intrinsic coloring and other factors such as polymerization conditions, and surface topography also affect the translucency of the material [8]. In the present study, TP values of control specimens that were polymerized in stainless steel molds showed significantly higher TP than other molds. This finding may be attributed to the smooth, glossy surface of the mold. Compared to other molds, the blue stone mold showed the lowest TP, which might be associated with the highest surface roughness of silicones cured in that mold. These findings may lead to the use of smoother molds when the patient has more translucent skin.

The surface roughness of the elastomeric materials is known as a good indicator of mechanical properties, because deep irregularities on the surface can act as initiation sites for cracks, corrosion, and bacterial contamination [30,31]. In the present study, stone molds and 3D-printed resin molds revealed similar surface roughness for the silicone specimens while silicones cured in stainless steel molds showed significantly lower Ra values. To reduce the surface roughness of the silicone elastomers, a smaller thickness for each printing layer can be used in the 3D printing process of maxillofacial prosthesis molds. Optimal printing parameters for 3D-printed maxillofacial prosthesis molds require further research. For gypsum-based molds, Khalaf et al. [31] recommended coating the mold with a clear acrylic spray.

Clinically favorable Shore A hardness values for maxillofacial silicones range from 25 to 35 [32,33]. Within these hardness values, the prostheses are soft enough to adapt to tissue movements and are resistant to forces causing tearing [34]. In the present study, the hardness values of the silicones, even those polymerized in different molds, changed from 21 to 27 Shore A. This finding revealed that the mold type had no negative effect on the hardness of the silicone elastomer.

This in-vitro study has some limitations. In the present study, a model resin in a DLP-type printer was used for mold fabrication. Currently, many different types of 3D-printing materials and printers are available in the dental and medical fields. Future studies should be performed to test various materials and printers as 3D-printed molds revealed promising results concerning color degradation of maxillofacial silicones compared with the use of dental stone molds. Furthermore, printing parameters and post-processing procedures are important factors for the accuracy and surface properties of the molds. In future studies, surface analyses, polymerization degree, and the mechanical properties of silicones polymerized in resin molds fabricated with different parameters should be studied.

In conclusion, silicone polymerized in blue and light brown dental stone molds revealed significantly higher color differences compared with 3D-printed resin molds. TP values of silicone polymerized in stainless steel molds were significantly higher than silicone polymerized in blue stone, light brown stone, and 3D-printed resin molds. Ra values of silicone polymerized in stainless steel molds showed the lowest Ra compared to silicone polymerized in blue stone mold, light brown stone mold, and 3D-printed resin mold. Shore A hardness of silicone polymerized in 3D-printed resin mold was the highest while all groups were close to clinically acceptable hardness values.

Abbreviations

ANOVA: analysis of variance; CAD: computer aided design; CAM: computer aided manufacturing; CIEDE: the international commission on illumination; DLP: digital light processing; Max: maximum; Min: minimum; Ra: surface roughness parameter; SD: standard deviation; SPSS: statistical package for the social sciences; STL: standard tessellation language; TP: translucency parameter; 3D: three dimensional; ΔE₀₀: color change

Ethical Statements

None

Conflicts of Interest

The authors declare no conflicts of interest.

Funding

None

Author Contributions

CBİ: investigation, methodology, data curation, writing; MBG: methodology, formal analysis, writing, review, and editing; BTB: methodology, formal analysis, review, and supervision; SKN: conceptualization, methodology, writing, editing, and supervision.

ORCID iD

¹⁾CBİ*: [e-mail], https://orcid.org/0000-0001-6573-7976

²⁾MBG: [e-mail], https://orcid.org/0000-0002-4002-6390

²⁾BTB: [e-mail], https://orcid.org/0000-0001-7825-712X

²⁾SKN: [e-mail], https://orcid.org/0000-0001-8836-0673

Data Availability Statements

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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