To Evaluate and Compare the Effect of Indoor and Outdoor Weathering on Mechanical Properties and Color Stability of High Temperature Vulcanized Maxillofacial Elastomer Material: An In Vitro Study

INTRODUCTION

Maxillofacial prosthetics is the department of prosthodontics concerned with the rebuilding and/or substitution of stomatognathic and craniofacial structures with prosthesis which will or may not be expelled on a customary or elective premise.¹

The evolution in surgical techniques and the increasing number of patients requiring service have presented new challenges to the management of these patients and principles of prosthodontic practice. Today, patients with defects that cannot be corrected surgically should be referred to a dentist for their oral defects. The profession’s knowledge and skills can help restore these defects along with cooperative effort with bioengineers and biochemists as part of the team that makes maxillofacial prosthetics successful.²

Materials for maxillofacial applications should have sufficient strength while remaining flexible and soft enough to adapt to facial motion.³ The material’s strength is the most essential physical property in clinical practice. The tear and tensile strengths of maxillofacial prosthesis indicate the extent to which a material can stretch before failure in tension and to endure the development of facial muscles. The maxillofacial silicone’s hardness is also a measure of flexibility and it is essential because it is preferable to use as a material that is similar in texture to the missing facial tissue.⁴

Color stability shows the resistance of the prosthesis to discoloration amid benefit. The mechanical and physical properties of facial prosthetic materials have progressed over a course of time but the color instability of the facial prosthesis still limits the serviceability and is frequently the reason for changing the prosthesis.⁵

In order to produce a prosthesis with all the above-mentioned properties, it should be made with an ideal maxillofacial prosthesis material. Maxillofacial prosthesis is fabricated by variety of materials, namely, polyvinyl chloride, polyurethanes, polymethyl-methacrylate, chlorinated polyethylene and silicones are examples of these materials. Out of these, silicones till date outruns other materials.³ Barnhart first used silicones in the year 1960 for extraoral prosthesis, which were introduced in 1946.⁶ Nowadays, medical grade silicones are perhaps the most commonly utilized materials for facial reconstruction.⁷

Silicones have sufficient tear and tensile strengths, high elongation, ease of production, chemical inertness and suitable bonding to underlying materials, among other characteristics. Fillers are used to boost the polymer’s strength especially elastomers made of polydimethyl siloxane (PDMS) are commonly used for that purpose.⁷

Overtime maxillofacial material is subjected to primary environmental characteristics that cause degradation, such as daylight, temperature, dampness, wind, dust, and toxins.⁵ The optical and mechanical qualities of the maxillofacial silicone are degraded by environmental variables, such as weathering, normal aging, and cleaning solvents.⁸ Therefore, to maintain the efficacy of prosthesis, silicone-based maxillofacial prosthesis requires replacement in every 6–12 months.⁵ Moreover, scientific evidence on the physical properties of maxillofacial material in Asian nations is very constrained, where climatic conditions are exceptionally diverse than the climatic condition where these materials are fabricated.⁹

MATERIALS AND METHODS

This in vitro study was conducted at the Department of Prosthodontics Crown & Bridge and Implantology, Sudha Rustagi College of Dental Sciences and Research, Faridabad, Haryana, India. Data were collected and were analyzed statistically to obtain the result.

RESULTS

Indoor weathering master chart (Table 1). Reading of samples for all four parameters before and after 6 months in indoor dark box.

Outdoor weathering master chart (Table 2). Reading of samples for all four parameters before and after 6 months in outdoor box.

The data were analyzed utilizing Statistical Package for Social Sciences (SPSS) version 21, IBM Inc. Descriptive information (Frequency distribution) was detailed for each variable. Summarized information was displayed utilizing tables. Data were not normally distributed as tested using the ShapiroWilk W test (p-value < 0.05). Mann–Whitney U test (two independent groups) was used for inter-group comparison for tear strength, tensile strength, hardness, and color stability (Table 3). p-value < 0.05 was considered statistically significant.

DISCUSSION

Maxillofacial silicone’s physical properties and esthetics are possible signs of general strength, flexibility, durability, marginal integrity, and longevity in clinical service. The primary goal of maxillofacial prostheses is to enhance the patient’s quality of life by reducing their morbidity and suffering. To achieve these objectives, the tear strength and tensile strength of silicone is the most crucial physical characteristics from a clinical standpoint, especially at the delicate edges of extraoral silicone prosthesis. The thin borders of the facial prosthesis are gently peeled away from the facial tissue. When it is taken off, typically during the night or for cleaning, as a result, the prosthesis is irreparably damaged and needs to be changed. Therefore, these prostheses should be made of a material that has high resistance to tearing.¹²

Similarly, the silicone elastomer’s tensile strength reflects the material’s overall strength, and the consequent elongation shows how flexible is the prosthesis. When removing nose or ocular prosthesis from facial tissue, when it comes to prosthesis, it is preferred to use a material with high elongation at break. This can be achieved through cross-linking a high molecular weight polymer, which results in a highly elastic material and increases the overall strength of the base polymer.⁴

Color and surface changes in maxillofacial silicones overtime, is the most common reasons for replacing a prosthesis, especially changes in appearance that patients can notice with their eyes.¹³ Silicones in comparison with other maxillofacial materials are color stable, durable and resilient while also being soft and flexible (low Shore A Hardness) and more lifelike.

During use, the color and mechanical properties of silicone prosthetics may deteriorate due to the material’s reaction to sunlight, humidity, and temperature, resulting in patient’s dissatisfaction. These changes occur due to photooxidative degradation.

The following events may take place during a photooxidative degradation:

• Initiation: Free radicals are formed through a radical chain process that can be initiated by the dissociation caused by the collision of a photon with sufficient energy with a polymer molecule or the presence of impurities, such as trace metals from the polymerization catalyst.

• Propagation: When free polymer radicals react with oxygen, it leads to the generation of polymer oxy- and peroxy-radicals, as well as secondary polymer radicals. This process ultimately causes chain scission.

• Termination: When different free radicals react with each other, it results in further cross-linking. The main changes in irradiated polymers are found in their molecular weight distribution. This is due to main chain scission, cross-linking, and end linking. Additionally, there is production of volatile degradation products. These phenomena tend to modify the physical properties of the material.³

In 2018, Rahman et al. conducted a systematic review and meta-analysis which found limited scientific evidence regarding the physical properties of maxillofacial material in Asian countries. This is particularly noteworthy given that the climatic conditions where these materials are manufactured are vastly different from those in Asian countries.¹⁴ This current study evaluated widely used maxillofacial material in India, Technovent Z004 Platinum Silicone which is a medical grade silicone imported from the United Kingdom. The climate in India and the UK is different, therefore, there might be more degradation in the mechanical properties and color stability when subjected to different weathering conditions. This was in accordance with a study done by Shetty et al. in 2020 where the authors subjected silicone (UK) samples to Indian climatic conditions and concluded that outdoor aging decreased the color stability and mechanical strength of the silicone.¹⁵

In the current study, difference in tear strength, tensile strength, hardness, and color stability of maxillofacial silicone (Technovent, UK) were evaluated and compared before and after outdoor and indoor weathering conditions. According to the ISO, standard tear strength of silicone material used in current study must be 18–53 N/mm and tensile strength 2.0–7 N/mm².¹⁰ For hardness, Castleberry and Lewis in 1980 stated that 25–35 Shore A units were ideal but values within 10–45 range were considered acceptable. Conroy et al. in 1979 considered 25–55 Shore A units as the correct range for hardness.¹¹ For color stability, the perceptible thresholds is ΔE = 0.7, as demonstrated by Paravina et al. in 2015, >1 by Lemon et al., Kiat-Amnuay et al., Haug et al., 2 by Beatty et al., Polyzois et al., and 3 by Kantola et al. has been considered perceptible to the human eye.¹⁶ Therefore, ΔE values less than equal to 3 can be considered as acceptable for evaluating color stability.

The findings of the current study showed the mean value of tear strength at baseline as recommended by manufacturer was 21.57 N/mm and after 6 months values decreased to 20.22 N/mm for indoor weathering and 16.93 N/mm for outdoor weathering. In the case of tensile strength, the mean value at baseline as per manufacturer was 4.24 N/mm² and after 6 months of weathering it showed decrease in values for indoor to 3.61 N/mm² and outdoor to 2.77 N/mm². Additionally, the mean value of hardness at baseline was 35 SHA and after 6 months of weathering, it showed increase in values, with 37.25 SHA for indoor weathering and 39.5 SHA for outdoor weathering. As per the results, the mean value of ΔE in color stability at baseline was 0.59, and after 6 months, the values increased to 0.72 for indoor weathering and 1.81 for outdoor weathering. The values for all parameters were calculated with level of significance of p < 0.05 was set to be statistically significant.

It was also observed that after 6 months of outdoor weathering, values for tear strength, tensile strength, and color stability were decreased, with an increase in the hardness values as compared with samples kept indoor. Maxillofacial silicone elastomer therefore with time becomes less elastic, less resistant to tear, changes color visible to naked eye (i.e., darker over time) and becomes harder. This may be due to post-polymerization cross-linking from photoirradiation energy causing changes in the polymer network structure. These modifications will include the number of polymer electronic devices, the connectivity between them, and their position in space, likely altering the light transmitted by the maxillofacial device and the degradation of the polymer’s shade.

In addition to this, the mechanical, physical, and optical properties degraded after 6 months of outdoor and indoor weathering but still remained within acceptable range showing that the material used has good mechanical and optical properties. The variations in characteristics were influenced by the type of materials used. These changes could be attributed to modifications in the chemicals present, which include dispersed fumed silica particles in platinum-catalyzed silicone fluid with vinyl termination. Using a silica filler that has a high surface area can enhance the interactions between the polymer and filler leading to more flexible network which ultimately increases mechanical strength.

The findings of the present research were consistent with other researchers like Hatamleh et al, in 2011 who evaluated the effect of outdoor weathering on physical properties of Techsil high temperature vulcanized silicone (HTV) silicone and concluded that while hardness increased, tear strength and tensile strength dropped. Authors concluded that human and environmental factors have a negative impact on the mechanical characteristics of maxillofacial elastomers.¹⁷ In 1972, Sweeney et al. discovered that the weatherometer-induced artificial weathering of several maxillofacial silicones resulted in considerable changes in the material’s tensile strength, percent elongation, tear resistance, hardness, and elastic modulus. Similarly, Eleni et al. in 2009, carried out a study with same parameters but different silicone material was taken and determined that all properties of the maxillofacial materials deteriorated over time in the outdoor weathering in a year. Similar results were put forward by Haug et al. in the year 1992.¹⁸ M G Kheur et al. in 2012 compared the hardness of room temperature vulcanized silicone (RTV) and HTV silicone after 9 months of outdoor weathering and concluded that hardness increased after 9 months for both the silicones but more so in HTV silicone owing to more complete polymerization in HTV as compared with RTV with time and as it progressed, the hardness also increased in the same proportionality.³

The results for color stability showed that all samples became darker over time in both indoor and outdoor weathering expressed in terms of ΔE. The mean ΔE values of outdoor and indoor samples were more after 6 months as compared with baseline readings. The color stability of the samples decreased drastically over 6 months of weathering with more instability noted in outdoor weathering suggesting that the prosthesis needs to be replaced after 6 months of use. This outcome could be the result of post-polymerization cross-linking brought by the energy from light irradiation, altering the structure of the polymer network affecting the transmission of light through maxillofacial material, thus resulting in unacceptable color change over time. These changes may have involved the quantity of units in the polymer chains, their connection to one another, and their angular arrangement in space.⁸

The results of this study were followed by other researchers, such as Al-Harbi et al. found that pigmented Techsil s25, a-2186,and med-210 silicone elastomers have poor color change due to weather conditions.⁸ Kantola et al., in 2013, examined the color stability of maxillofacial elastomer over time and reported a significant shift and concluded that polymeric materials often have poor thermal stability and minimal solar radiation protection. The UVA-UVB irradiation (sunlight), moisture, wind, dust, temperature, and pollution are some of the elements that have an impact on color stability.⁵ Babu et al., in 2020, evaluated the effects of chemical disinfection and accelerated aging on two types of maxillofacial silicone: A2186 and Cosmesil. They measured the changes in color stability, Shore A hardness, and surface roughness and found that both types of silicone experienced deterioration in their overall properties.¹⁰

In contrast to favoring studies, many authors gave different conclusions. Takamata et al. reported that when maxillofacial silicone elastomer (RTV and HTV) is kept in dry and dark environment for 1 year, the silicone elastomer shows a change in the color and concluded that aging rather than exposure to daylight leads to most of the color change observed in the RTV and HTV polymers which could be due to the chemistry of the elastomers and no further detailed reasoning of the result was done by the authors.¹⁹ GL Polyzois et al. in a study presented that when silicone specimens were aged for a period, that is, 1.5 years of clinical service (perspiration and simulated sebum at 37°C), it showed clinically insignificant results with respect to the tear strength, tensile strength, hardness, weight, and color change, the authors concluded that this could be due to the propagation of the cross-linking reaction of the silicone elastomer which does not alter the internal chemistry of elastomer on aging and on exposure to sebum or perspiration.²⁰

Considering statistical analysis and facts proposed by various authors, this study revealed that the mechanical elements of the maxillofacial prosthetic material were negatively impacted by 6 months of indoor and outdoor weathering, with more degradation observed in outdoor weathering. In pigmented HTV, weathering also brought about undesirable color changes which is visible to the naked eye. This could be because of continued polymerization as a function of aging process which increases even more when subjected to exposure of light, and can result in changes to the structure of the polymer network.

CONCLUSION

Within the realm and preview of the current study, the following conclusions were made for maxillofacial silicone:

• The tear strength decreased after 6 months of indoor and outdoor weathering with more significant results on exposure to sunlight, rain, wind, and dust. The values were in acceptable range.

• The tensile strength decreased after 6 months of indoor and outdoor weathering with more significant results on exposure to sunlight, rain, wind, and dust. The values were in acceptable range.

• The hardness increased after 6 months of indoor and outdoor weathering with more significant results on exposure to sunlight, rain, wind, and dust. The values obtained were still in acceptable range.

• The color stability decreased after 6 months of indoor and outdoor weathering with more visually detectable results on exposure to sunlight, rain, wind, and dust. The pigmented samples became dark in color over time.

Silicone elastomers are thus materials that have ideal physical properties suitable for making prosthesis to replace lost facial structures. But the esthetic quality still requires further research, especially long-term clinical trials, to determine the amount of biological and mechanical deformation.

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