|Year : 2021 | Volume
| Issue : 4 | Page : 378-385
Influence of weather in Saudi Arabia on mechanical properties of maxillofacial elastomeric materials: An in vitro study
Mohammed A Mousa1, Bader K Alzarea1, Mohammed G Sghaireen1, Sherif Sultan2, May O Hamza1, Nafij B Jamayet3, Edward Lynch4
1 Department of Prosthetic Dental Sciences, College of Dentistry, Jouf University, Sakakah 42421, Jouf Province, Saudi Arabia
2 Department of Prosthetic Dental Sciences, College of Dentistry, Jouf University, Sakakah 42421, Jouf Province, Saudi Arabia; Fixed Prosthodontics Department, Tanta University, Tanta, Egypt
3 Prosthodontic Unit, School of Dentistry, International Medical University (UMI), Jalan Jalil Perkasa 19, Bukit Jalil, Kuala Lumpur 5700, Malaysia
4 Department of Restorative Dentistry, School of Dental Medicine, University of Nevada, Las Vegas, NV, USA
|Date of Submission||11-Feb-2021|
|Date of Decision||13-Apr-2021|
|Date of Acceptance||22-Apr-2021|
|Date of Web Publication||19-Aug-2021|
Dr. Mohammed A Mousa
Department of Prosthetic Dental Sciences, College of Dentistry, Jouf University, Sakakah 42421, Jouf Province.
Source of Support: None, Conflict of Interest: None
Aim: The aim of this article was to analyze the influence of Saudi Arabia climate on the mechanical characteristics of two room-temperature (RV)- and heat-temperature-vulcanized (HV) medical silicone materials. Materials and Methods: In this in vitro study, 160 specimens of four different elastomeric materials, two RV (A-2000 and A-2186) and two HV (TechSil S-25 and Cosmesil M-511), were exposed to natural weather of Jouf Area in Saudi Arabia for 6 months. The mechanical tests, including tear resistance, tensile strength, and percentage of elongation, were performed using a universal testing machine. A t-test and analysis of variance followed by a post hoc test with α<0.05 were used to analyze the results. Results: The weather statistically influenced the mechanical properties of the silicone materials. A-2000 showed the highest tear resistance (37.34 ± 2.36, P < 0.05). TechSil S-25 showed the highest tensile strength (57.34 ± 1.92, P < 0.05) and percentage of elongation (888.20 ± 13.83, P < 0.05). The least tensile strength and percentage of elongation were found in Cosmesil M-511 among all test specimens (P < 0.05). Conclusion: After 6 months of natural weathering in hot and dry climates, A-2000 exhibited the highest tear strength values among all test specimens, whereas TechSil S-25 achieved the best results for tensile strength and percentage of elongation.
Keywords: Maxillofacial Prostheses, Silicone Elastomers, Tensile Strength, UV Radiation, Weather
|How to cite this article:|
Mousa MA, Alzarea BK, Sghaireen MG, Sultan S, Hamza MO, Jamayet NB, Lynch E. Influence of weather in Saudi Arabia on mechanical properties of maxillofacial elastomeric materials: An in vitro study. J Int Oral Health 2021;13:378-85
|How to cite this URL:|
Mousa MA, Alzarea BK, Sghaireen MG, Sultan S, Hamza MO, Jamayet NB, Lynch E. Influence of weather in Saudi Arabia on mechanical properties of maxillofacial elastomeric materials: An in vitro study. J Int Oral Health [serial online] 2021 [cited 2022 Jan 29];13:378-85. Available from: https://www.jioh.org/text.asp?2021/13/4/378/324138
| Introduction|| |
Maxillofacial prostheses (MFPs) have been used for functional and cosmetic restoration of missing parts of maxilla, mandible, and other facial parts when surgical procedures are unable to restore the missing parts. Chlorinated polyethylene, polyvinyl chloride, polymethyl methacrylate, vinyl plastisols, polyurethanes, and silicone materials are all, but not limited to, materials used to fabricate MFP.,,, An ideal maxillofacial prosthetic material should have a stable surface morphology, good mechanical properties, sufficient hardness, and it should not be affected by local environmental factors, such as solar radiation, thus achieving a satisfactory lifespan for prolonged use.,,,
The most common materials used to fabricate MFP are the elastomeric silicone materials (dimethylsiloxane) owing to their high tear strength, percentage of elongation, and adequate tear resistance. In addition to that, the coloring compatibility with the tissue, the chemical inertness, high elongation, ease of fabrication, and patient comfort render them as the current material of choice for the management of maxillofacial defects.,,, According to the type of chemical reaction, elastomeric silicone materials can be classified into room temperature-vulcanized (RV) and heat temperature-vulcanized (HV). Both types vary in terms of physical and mechanical properties, to a lesser degree in physical properties. The quality of the different types of these materials mainly depends on the polydimethylsiloxane chains and the silica fillers and affects the overall strength and serviceability of these materials. Lontz found that the color stability, thermal properties, and physical properties are superior in HV when compared with RV.
Facial prostheses made from maxillofacial silicone elastomers are exposed to a variety of environmental conditions such as sunlight, high temperature, ultraviolet radiation, moisture, wind, dust, and pollutants. Despite the advantages of these silicone elastomers, they show low solar radiation resistance and low thermal stability. These disadvantages lead to degradation, particularly at the edge of the prosthesis, and, accordingly, to the limitation of the service life of silicone elastomeric prostheses.,,,,,,
Various studies on silicone elastomers have inspected the influence of weather on their properties.,,, However, to the best of our knowledge, experience with longevity in terms of mechanical strength of these MFPs in the north area of Saudi Arabia, which has a hot and dry climate on the nominated maxillofacial silicone materials, has not been evaluated. Thus, the performance of elastomeric materials is estimated based on the studies, which evaluated the influence of weather in climates with different thermal and solar radiations and some, but not all, elastomeric materials. This may provide inaccurate data about the real performance of these materials. It is, therefore, essential to understand the mechanical behavior of these materials to allow clinicians to choose the best materials for prostheses. It is assumed that hot and dry weather would not affect the mechanical and physical properties of maxillofacial elastomeric materials (H0). This assumption was examined in the present study (H1).
| Materials and Methods|| |
Specimen selection and preparations
In this study, 160 specimens of four different maxillofacial silicone elastomers were used to study the effect of the weather on their tear, tensile strength, and percentage of elongation. Two materials of them were RV including A-2186 (Code A) and A-2000 (Code B). The other two materials were HV and included Cosmesil M-511 (Code C) and TechSil S-25 (Code T) as shown in [Table 1]. [Figure 1] shows the flowchart of sample distribution and study design. The sample size was based on PS 2009 software, the guidelines for the determination of effective sample size in observational studies,, and previous studies that stated nine samples as sufficient for the detection of differences depending on calculation with 0.05 alpha errors and 0.8 power of the test.,,,,
To fabricate the test specimens, wax patterns (Cavex, The Netherlands) were invested in dental stone with high strength (Dentsply International, Canada) to form the molds in which the mixed silicone elastomers were packed using plastic syringes and a vibrator (Sirio, Italy). For the RV silicone types, the molds were placed in hydraulic press 660 (Silfradent, Italy) and maintained on bench for 24 h for polymerization. For the HV silicones, the flask was placed in a heat oven (Memmert, Germany) at 100°C for 2 h for curing. After complete polymerization for RV, or curing for HV, the test specimens were removed from the molds and examined for any excess or deficiencies to be trimmed with scissors or even remade, if necessary. To test the tensile strength and percentage of elongation, 20 dumbbell-shaped specimens were fabricated for each material according to the International Standards Organization ISO 37 [Figure 2]. For tear strength testing, 20 trouser-shaped specimens (100 × 15 × 2 mm) were fabricated for each material according to ASTM D624 [Figure 3]. For each test, 10 specimens were used as a control and wrapped with coats, placed in suitable plastic containers, and kept in a Stainless Steel waterproof humidity control electronic cabinet (Yunboshi, Jiangsu, China) at 24 ± 1°C and 50 ± 5% relative humidity, and the other 10 specimens were tested after weathering.,
|Figure 2: Steps of dumbbell shape fabrication. A, wax pattern; B, mold fabrication; C, the final shape of dumbbell while A = B = C=25 mm, D=12.5 mm, and E=4 mm|
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|Figure 3: Steps of trouser shape fabrication. A, wax pattern; B, mold fabrication; C, the final shape of trouser while A = B= 51 mm, and C=19 mm|
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Weathering of specimens was conducted in 1–3 days following their preparation, by hanging and keeping them on the rooftop of Jouf dental College for 6 months (May 2019 to October 2019). The specimens were then cleaned for 10 min in bottled water (Aquafina, Aljomeh Water Factory, Boradah, Saudi Arabia), and were subsequently wiped, dried, and prepared for different tests.
Climate and environmental data
The monthly average environmental data including daily temperature, wind, rain, humidity, pressure, and ultraviolet (UV) index scales were collected from World Weather Online (Manchester, UK) and from the study of Mousa that had been done at the same time and in the same region of Saudi Arabia.
Testing tear strength
The computer-controlled Universal Testing Machine (UTM) (GT-C01-1, Gester International Co., Quanzhou, China) was switched on while the length of the testing specimens was gaged with a ruler to adjust the distance between the grips through up/down button. The test specimens were then placed and clamped in axial alignment for a pull direction. The test program was selected, including test method (pull direction), test speed, end method, and test steps. The tearing force was set at a constant rate of 5 mm/min until each test specimen broke. At the point of breakage, the tear strength was expressed in kilonewtons per meter (kN/m) and obtained by the machine software [Figure 4]. The output file was exported to excel format file for statistical analysis.
Testing tensile strength
The tensile strength of the control and test specimens was determined using the same UTM (GT-C01-1, Gester International Co., Quanzhou, China). The test specimens were clamped by the same way explained in testing of tear strength, whereas the program test was selected according to manufacturer’s instruction. At the point of breakage, the tensile strength was expressed in megapascals (MPa) and obtained using machine software.
Testing percentage of elongation
The percentage of elongation was determined on the same specimens that were used to test the tensile strength. The length of the test specimens was recorded initially and at the time of fracture, it is recorded using a digital vernier caliper (Model CD-6, Mitutoyo, Japan) to determine the percentage of elongation. At the point of breakage, the percentage of elongation was obtained using machine software.
Data were analyzed using IBM SPSS software version 21.0 (IBM Corp., Armonk, NY, USA). The mean and standard deviations (SDs) were described for quantitative data. The comparison between the independent variables was performed using the t-test for two variables and F-test (analysis of variance (ANOVA)) for more than two variables. ANOVA was done followed by a post hoc test for identification of level of significance between every two groups. For all evaluation, α < 0.05 was considered as statistically significant.
| Results|| |
The recorded data from the World Weather Online website and from the study of Mousa reported high temperatures ranging from 34°C to 41.1°C (with a mean ± SD of 39.1 ± 2.56). Humidity showed a low percentage ranging from 13% to 27% (17.7 ± 4.45), whereas the UV scale was high in all months of the test showing 8–9 scales, except for October when it was moderate (6 scales) (8.2 ± 1.16)., In hot, dry, and high UV weather, all test specimens showed significant changes in their mechanical properties at the time of examination.
Concerning the tear resistance, as shown in [Table 2], the t-test showed a significant decrease, within the same group, in the tear resistance in all weathered specimens when compared with the non-weathered specimens (P < 0.05), except for T, which expressed no significance between the weathered and non-weathered specimens (P > 0.05). Comparing the non-weathered groups, ANOVA revealed that T showed the least tear resistance (P < 0.05), whereas B showed the highest tear resistance among all control and test specimens when comparing the weathered and non-weathered specimens. However, there were no significant differences between the weathered specimens other than B (P > 0.05).
|Table 2: Comparison between tear resistance (kN/m) in the weathered and non-weathered test materials|
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[Table 3] shows the mean and standard deviations of the tensile strength and the comparison of different test specimens. The t-test revealed that both B and C showed a significant reduction in their tensile strength when comparing the weathered with the non-weathered specimens (P < 0.05), whereas A and T showed no significance. Comparing the groups, ANOVA revealed that C showed the least tensile strength, whereas T showed the highest tensile strength specimens when comparing the groups in weathered and non-weathered specimens (P < 0.05).
|Table 3: Comparison between tensile strength (kg/cm2) in the weathered and non-weathered test materials|
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Regarding the percentage of elongation, [Table 4] shows the mean and standard deviations of different types of test specimens. When comparing the groups, both B and T showed a significant decrease in the percentage of elongation, whereas A and C showed an insignificant difference between the weathered and non-weathered specimens. When comparing the groups, T showed the highest percentage of elongation (941.40 ± 8.50 and 888.20 ± 13.83) in weathered and non-weathered specimens, respectively (P < 0.05).
|Table 4: Comparison between percentage of elongation in the weathered and non-weathered test materials|
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| Discussion|| |
Exposure to UV radiation can cause significant degradation to medical silicone elastomers, leading to the need for replacement every 3–12 months mainly due to color degradation and physical properties. UV radiation was found to cause photooxidative degradation in elastomeric materials in the form of elaboration of free radicals. The free radicals react with oxygen, producing oxy- and peroxy-radicals. The oxy- and peroxy-radicals break the chains in the polymer and are linked with each other. This process leads to a reduction in the molecular weight because of the production of volatile degradation products, leading to changes in physical and mechanical characteristics of the material., These changes depend on whether the dominant mechanism inside the polymer is cross-link or breaking the chain. When the cross-linking is the dominant mechanism, the link between the existing chain segments becomes harder, and, consequently, the density of the structural network increases. Finally, the material becomes harder. When the breaking of the chain is the dominant mechanism, the link between the chains becomes weak, and the material becomes softer. In the last 10 years’ published literature, few studies focussed on the mechanical properties of the silicone elastomeric materials,, whereas most of the researches focus on color degradation.,,
Saudi Arabia has two main seasons: summer and winter. The climate of Saudi Arabia is characterized by high temperatures during the day and low temperatures at night. The country follows the pattern of the desert climate, except for the southwest, which features a semi-arid climate. The weather of the north area of Saudi Arabia is characterized by high temperature, ranging from 37oC to 41oC in summer, which decreases to become 20–35oC in winter; the humidity levels are very low (very dry), ranging from 13% to 18%, with moderate to high pressure and high to extreme UV index radiation (8+). Jouf Province was selected as the location of the outdoor weathering tests as representative of the environmental conditions of the north area of Saudi Arabia, as the climate in the Kingdom is highly variable between areas. The most significant impact of weather on the mechanical behavior of elastomeric materials is due to UV radiation, which is high during summer. For this, the authors chose to conduct outdoor weathering in the months that are usually associated with high UV radiation.
The effect of weather on elastomeric materials can be investigated either by outdoor weathering or virtually through accelerated aging. Accelerated aging is used to estimate the performance of the medical silicone elastomers after virtual weathering and can also estimate the lifetime of the materials. However, the main drawback of accelerated aging is that it can lead to an inaccurate estimation of the life time of silicone materials because it cannot provide a real environment for outdoor weathering., Thus, data on the performance of silicone materials after outdoor weathering are more preferable, compared with artificial aging, to predict their lifetime under regional service conditions.
In this study, varying weather conditions led to significant changes in tear resistance of all test materials except for T, and so the null hypothesis was rejected. For tensile strength, the null hypothesis was confirmed for B and T, whereas for percentage of elongation, the null hypothesis was rejected for B and T.
The tear strength of a material is the resistance of the material to the force of tearing. From a clinical point of view, the tear strength is considered the most critical property of maxillofacial prosthetic materials, particularly when used in thin sections such as eyelid and nasal prostheses, due to susceptibility to tearing upon frequent removal., Although the weathered B specimens showed significant decrease in the tear strength when compared with the non-weathered, it had the highest tear strength when compared with the other weathered specimens. This increase in tear strength of B specimens can be attributed to the continuous polymerization of silicone associated with exposure to UV rays which result in more strong association between polymer chains (the cross-linking is the dominant mechanism). The results from the previous studies regarding the behavior of silicone materials due to weathering showed contradictory results. One results showed no significant differences in mechanical strength between elastomeric materials, which have A and C in between. These findings were in contradictory with other study which examined the influence of weather on three different silicone materials with A and T in between, whereas T specimens showed highest tear, tensile strength, and percentage of elongation when compared with A and MED-4210. The differences in the results among the studies can be attributed to the differences in testing environments and protocols. According to these results, B elastomeric material is advised for managing the deficiency involving thin section such as the eyelid and nose.
Tensile strength is the ability of a material to withstand a pulling force without fracture. It specifies the point when a material goes from elastic to plastic deformation, whereas the percentage of elongation is a measure of the amount of ductility of a material. The tensile strength and percentage of elongation are important in terms of the elasticity of the elastomers. The tensile strength of different elastomers ranged from 21 to 70 kg/cm2. A material with a high percentage of elongation and low hardness is preferred when restoring areas of the head and neck that need stretching or movement during functioning., T had the highest tensile strength and percentage of elongation values. This agrees with a study that compared the effect of weathering on the mechanical properties and color changes of three maxillofacial materials: T, MED-4210, and A silicone elastomers. There was no way to compare the present study with other on the same basis due to the diversity of the test specimens, additive materials to the specimens, and the variables that have been tested.,,, These results can be interpreted as the main dominant reaction between the hydroxyl-groups on the filler and chains of the polydimethylsiloxane in TechSil S-25 (T) materials is the cross-linking. This makes the T material strong enough to resist the rupturing under the applied force. This result makes both B and T materials preferable for the management of different thin maxillofacial defects.
This study will assist prosthodontist in making adequate decisions when selecting the type of maxillofacial materials in the restoration of maxillofacial defects. According to the results of the present study, A-2000 TechSil S-25 showed that the most appropriate materials can be used for the restoration of most of the area in the face and head, particularly the area of part defects that can be moved or stretched, as they showed favorable tensile strength and percentage of elongation. These results can be applied, with some limitation, to areas in the world with hot and dry weather, such as most of the Gulf areas, Northern Australia, and southern India.
In this study, only four maxillofacial silicone materials were examined to identify the influence of outdoor weathering on the performance of these materials. This can be considered a limitation of the present study. Moreover, human factors are not considered in the outcome of the prostheses. Future studies are required to examine the effect of these factors on the same materials.
| Conclusion|| |
Within limitations of the present study, the hot and dry weather adversely affects the mechanical properties of the maxillofacial silicone materials. After weathering, A-2000 recorded the highest values for tear strength when compared with other specimens. TechSil S-25 achieved the highest values for tensile strength and percentage of elongation. With some modification and improvement of physical properties, TechSil S-25 can prove to be a promising material for the managing of maxillofacial defects.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
All the authors of Jouf University have a major contribution in the work. M. A. M.: study conception, personal funding, data collection, data acquisition, and manuscript writing. B. K. A., M. G. S., Sh. S., M. O. H.: study conception, data collection, funding, and final approval of the manuscript. N. B. J.: statistical analysis, final approval of the manuscript. E. L.: manuscript language revision, final approval of the manuscript.
Ethical policy and Institutional Review board statement
Patient declaration of consent
Data availability statement
The data are available with the corresponding author.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4]
[Table 1], [Table 2], [Table 3], [Table 4]