Influence of Weather on Hardness and Surface Roughness of Maxillofacial Elastomeric Materials

INTRODUCTION

Diversity of materials can be used in maxillofacial prosthetic rehabilitation. Examples of these materials include polymethyl methacrylate, polyurethanes, polyvinyl chloride, chlorinated polyethylene, and maxillofacial silicones elastomers materials (MFSEM). Maxillofacial silicones elastomers materials is the most commonly used material to construct maxillofacial prostheses that are used to replace the congenital and acquired facial missing parts due to its texture, biocompatibility, strength, durability, simplicity in handling and fabrication, coloring, tear and tensile strength, and comfort of the patients.^1–5

According to the vulcanization reaction, the silicone elastomers can be divided into two distinct types; high-temperature vulcanizing (HTV) and room-temperature vulcanizing (RTV) silicones.⁶ Different types of silicone elastomers have a different range of surface characteristics and mechanical properties. It was reported that HTV silicones have superior advantages regarding the physical properties, thermal stability, and color stability comparing to RTV.⁷ In disagreement with that, one study have reported superior properties of the RTV silicones compared to other different materials including HTV regarding color stability.⁸ One study found the results of evaluation of mechanical properties of TechSil S25 as a HTV silicone after exposure to weather and solar radiation is a promising,⁹ while other study had a contradictory report.¹⁰

However, there is agreement that both types of the MFSEM are subjected to degradation in the mechanical and physical properties and changes in the color and surface smoothness of the materials.^11–13 This mainly happened due to exposure to solar radiation, humidity, air pollution, and prostheses maintenance materials.^11,14–17 This degradation can be faster or less depending on geographic area, climate changes, and the materials used in maintenance of the prosthesis itself.^18–20

The roughness of a material is a measure of the fine irregularities of the surface texture in the materials. Surface roughness average (R_a) is the deviation of the surface valleys and peaks in microinches or micrometers; the surface is considered rough surface when the deviations are great; if the deviation was minor, the surface is considered smooth. The surface roughness of a material can be measured by different methods such as optical techniques, surface profilometer, and scanning electron microscopy.^20–22

Hardness, as a mechanical property, is defined as resistance to penetration or permanent deformation. Deformation or penetration of materials occurred as a result of many causes such as scratching, indentation, cutting, mechanical wear, and bending. There are many factors that should be considered when measuring the hardness of such material, such as, the type, size and shape, surface condition, degree of flatness, homogeneity, and work environment. Hardness is one of the most widely measured properties used to characterize rubber-base materials. There are two different ways to measure the hardness of rubber materials; International Rubber Hardness Degree (IRHD) scale and shore A scale.^23,24

Selection of proper material depends on the sustainability of this material regarding surface characters in different weathering effects. Various studies have been conducted to investigate the influence of weather on the physical and mechanical properties of MFSEM.^18,19,25 However, to the best of the author’s knowledge, no studies were conducted to evaluate the hardness and surface roughness of MFSEM in the hot and dry weather with high UV scales. The present study aimed to compare the influence of weather of Jouf Province, Saudi Arabia, which characterized hot, dry, and high solar ultraviolet (UV) scales on the hardness and surface roughness of four types of MFSEM, including two room-temperature vulcanized (RTV) (A-2186 and A-2000) and two heat-temperature vulcanized (HTV) (Cosmosil M-511 and TechSil S-25). It is assumed that the high UV scale, hot, and dry weather have no adverse effect on the hardness and surface roughness of MFSEM (the null hypothesis H₀). The results of the present study (H₁) were compared with that of the null hypothesis (H₀).

MATERIALS AND METHODS

RESULTS

The climate of Jouf Province is a hot (average 40.8/26.4) and dry (average humidity 17.6%) weather with very low rainfall (average 3.2 mm/6 months) in summer with a high UV scale (average 8.9 indexes) as shown in Table 2.

The comparison between the hardness of the nonweathered and weathered test specimens is shown in Table 3 and Figure 1. Within group, all the test specimens showed a significant increase in the IRHD scale due to hot and dry weather (p < 0.05) except Cosmesil M-511 which showed a significant decrease in the IRHD scale (21.00 ± 0.79 for control and 16.88 ± 0.95 for test specimens, p < 0.05). Cosmesil M-511 also showed the least hard material among groups (16.88 ± 0.95) while A-2000 showed the hardest one among the groups after weathering (37.34 ± 2.36, p < 0.05).

Table 4 and Figure 2 show the comparison between the surface roughness (R_a) of the test specimens within and among each test specimen. Within the same group, A-2000 was the only test specimen which shows a significant decline in the surface roughness (for control 0.23 ± 0.08, and 0.09 ± 0.05 for test materials, p < 0.05) while others showed insignificant changes in their surface roughness (p %3E; 0.05). Among groups, A-2000 also was the specimen with the least rough surface comparing the other test specimens (p < 0.05), while Cosmesil M-511 test specimen was the roughest materials (p < 0.05).

DISCUSSION

The surface roughness as a surface property is a good indicator of mechanical performance of elastomeric materials. An irregularity in the surface might produce nucleation sites for cracks, corrosion, and bacterial contamination. The surface roughness was evaluated using a novel optical noncontact technique for 3D characterization using a combination of a light sectioning microscope and a computer vision system. This optical approach technique simplifies the assessment of roughness compared to electron microscope and profilometer approaches in terms of being noncontact, fast, and cheap.^22,29,30 All test specimens, except TechSil S-25, were affected by the weather, either by increasing or decreasing value, and the null hypothesis was rejected except for TechSil S-25. A-2000 became smoother and finer in test specimens after weathering. This can be explained by the fact of continuous polymerization which promotes further arrangement and supplement of polymer causing the surface to become smoother and finer. This makes this material more suitable for facial defects including missing parts of the oral cavity to decrease the plaque deposition upon using. On the contrary of A-2000, Cosmesil M-511 became rougher after weathering. This can be explained by an increase of the fillers added to material during manufacturing. Comparing this study, up to the author’s knowledge, there was no conducted study in the effect of natural weathering on the surface roughness on test specimens used of this study to compare with. According to this study, A-2000 considered better option regard to increasing hardness and surface smoothness, so it can be used for larger defects and smooth areas of the facial surface. The clinician needs to decide which suitable materials he needs according to each case requirement and type of his/her country weather.

The hardness of elastomeric materials can be measured through shore A hardness test and International Rubber Hardness Degree (IRHD) scale. Using touch automated machine provides advantages of accuracy, reproducibility, and consistency results with easy access to the samples and less need for operator hands technique for measurement of the hardness compared with the operator hand technique that used in case of shore A hardness test.³² Maxillofacial silicone elastomers should have a suitable hardness ranging from 25 to 35 to provide good resemblance of skin tissue in the facial defective side.^25,33 All the silicone elastomer specimens examined in the present study became harder after subjecting to hot and dry weather and the null hypothesis (H₀), accordingly, was rejected. A-2000 had the highest IRHD hardness scale. This makes this material more suitable for replacement of ear and nose defects. Cosmesil M-511 also showed the least hard material among groups (16.88 ± 0.95) which make it soft and easily adaptable materials which make the material more appropriate for replacement of small facial defect with undercut area, to be easily inserted and removed. The result regarding the A-2000 was consistent with the result done on different MFEM, including A-2000 after weathering in Indian and Mediterranean climate.¹⁸ The result regard to Cosmesil M-511 was comparable to that study conducted on the exploration of the solar effect of different kinds of elastomeric materials, including Cosmesil M-511.^10,25,34 The increase in the hardness is explained by continuous polymerization, as a function of the aging process, and evaporation of the ingredients in the polymer.

The results obtained from this study would be beneficial to the maxillofacial prosthodontist to make adequate decision in the selection of proper silicone regarding the effect of environment. It helps the maxillofacial prosthodontists to fabricate a long-lasting stable maxillofacial prosthesis, avoiding remaking of the prostheses, wasting of the material, and time of fabrication.

In the present study, the effect of weather of hot, dry, and high UV was studied. No single factors of them were identified exactly if it was the cause of changes in surface roughness and hardness of test materials. This can be considered a limitation. Other more limitation of this study is the human factors, which was ignored also in the sustainability of the materials. Future studies are suggested to involve the effect of these factors over the same and longer period of time may be needed

CONCLUSION

Within the limitations of this in vitro study, it can be concluded:

• A-2000 maxillofacial elastomeric material shows an increase in the hardness and smoothest after weathering indicating adequacy of these materials for replacement of small facial defects, such as, nose and ear.
• Cosmosil M-511 is soft and easily adaptable materials after weathering compared with other test materials recommending its use for replacement of facial defects with deep undercuts.

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