Journal of Interdisciplinary Dentistry

ORIGINAL ARTICLE
Year
: 2020  |  Volume : 10  |  Issue : 3  |  Page : 105--110

Comparison of tensile and tear strength of three silicone materials for maxillofacial prosthesis in Indian climatic condition


Vibha Shetty, Anoop Sharma, Sweekriti Mishra 
 Department of Prosthodontics and Crown and Bridge, Faculty of Dental Sciences, Ramaiah University of Applied Sciences, Bengaluru, Karnataka, India

Correspondence Address:
Dr. Anoop Sharma
Department of Prosthodontics and Crown and Bridge, Faculty of Dental Sciences, Ramaiah University of Applied Sciences, Bengaluru - 560 054, Karnataka
India

Abstract

Aims: The aim of the study was to evaluate and compare physical properties such as tensile strength and tear strength of three commonly used silicone maxillofacial materials in Indian climatic condition. Settings and Design: An in vitro study was conducted to compare the tensile and tear strength using a universal testing machine with 100 N load at a speed of 20 mm/min. Specimens were prepared from three commercially available silicones, i.e., medical grade silicone (control group), prosthetic grade silicone (test Group 1), and a locally available medical grade silicone (test Group 2). One-way analysis of variance was carried out to test any significant difference between the mean values of tensile and tear strength between the tested materials. Post hoc test of Tukey was used to find which of the two groups' means shows a significant difference. Results: Prosthetic grade silicone showed the highest tensile strength with a P< 0.001* among the two tested materials. Tear strength and mean elongation was highest in control showing statistically significant results (<0.001*). Locally available medical grade silicone showed the least favorable properties among all the tested materials, though tensile strength was within acceptable range. Conclusions: Tensile strength of prosthetic silicone was highest followed by medical grade, whereas tear strength and elongation were maximum in medical grade silicone. Locally available medical grade silicone showed the least favorable properties.



How to cite this article:
Shetty V, Sharma A, Mishra S. Comparison of tensile and tear strength of three silicone materials for maxillofacial prosthesis in Indian climatic condition.J Interdiscip Dentistry 2020;10:105-110


How to cite this URL:
Shetty V, Sharma A, Mishra S. Comparison of tensile and tear strength of three silicone materials for maxillofacial prosthesis in Indian climatic condition. J Interdiscip Dentistry [serial online] 2020 [cited 2021 Jan 19 ];10:105-110
Available from: https://www.jidonline.com/text.asp?2020/10/3/105/304154


Full Text



 Clinical Relevance to Interdisciplinary Dentistry



Silicone being the mostly commonly used material in the field of maxillofacial prosthesis, orthopedics and cosmetics does require modification as per indian climatic conditions

 Introduction



The face is the identity of an individual; therefore, a facial deformity has an extreme psychosocial impact. Maxillofacial prostheses are the prosthesis used to replace part or all of any stomatognathic and/or craniofacial structures.[1] Mimicking facial structure is challenging in terms of comfort, color, and function. Earlier prostheses were made using wood, wax, and metal, but in 1901, Upham used vulcanite rubber to replace nasal and auricular defects. Later, latex rubber and glycerin-gelatin were introduced for the same.[2]

Since 25 years, room temperature vulcanized (RTV) silicones have taken a commonplace in the fabrication of maxillofacial prosthesis but have certain limitations.[3] Literature shows that frequent re-fabrication of prosthesis is required, especially in tropical climatic conditions. Therefore, this study was targeted at identifying changes in the material that could happen in tropical climates and serve as a guide for possible modifications.

Settings and Design

An in vitro study was conducted in the Department of Prosthodontics and Crown and Bridge, Faculty of Dental Sciences, Bangalore (Karnataka, India). Based on literature, a sample size of ten in each group was chosen to get a 95% power of the study.

The samples were fabricated from the most common three commercially available silicone elastomers being medical grade (Technovent limited, UK) – control group, prosthetic grade (Link Composites, India) – test Group 1 and locally available medical grade silicone (M P SAI enterprise Pvt Ltd., India) – test Group 2 [Figure 1]. The type of chemical reaction and processing conditions of each material is shown in [Table 1]. According to ASTM No. D4123 and D624 specifications, the dumbbell samples and unnicked 90° angle samples were made, and for standardization, metal mold was used for fabrication of the samples [Figure 2] and [Figure 3].{Figure 1}{Figure 2}{Figure 3}{Table 1}

Sample preparation

Control group – Using a 300 g weighing machine (Tangent KP 104), base and catalyst were mixed in a ratio of 10:1 according to manufacturer's instructions on a ceramic tile with stainless steel spatula, in a sweeping motion until no air bubble was seen. Subsequently, it was packed into the dumbbell and unnicked 90° stainless steel mold, load of 1 kg was applied to remove any excess material, and the molds were kept for 1 h at 100°C for polymerizationTest Group 1 – Similar to the fabrication of a previous sample, prosthetic grade silicone was mixed and packed into the mold, which was kept at room temperature for 24 h for polymerization of the materialTest Group 2 – As it is a premixed material which is supplied in a tube, the material was squeezed out of the tube and spread on a ceramic tile with stainless steel spatula. After packing, the excess material was removed from the stainless steel mold and was kept at room temperature for 24 h for complete polymerization.

Parameters measured

Tensile strength – According to ASTM, specimens of dumbbell shape with dimension 100 mm × 80 mm and 2 mm and thickness 1.8 mm was used for evaluating tensile strengthTear strength was evaluated with 90° angle-shaped specimens free of nicks with a thickness of 1.8 mm, which is a modification of the test specimen described by ASTM D624 (standard test method for rubber property–tear resistance)Elongation – above specimens were also used to check stretchability in tensile and tear stress.

All the thirty samples were subjected to Universal Testing Machine with a loading cell of 100 N at a speed of 20 mm/min for the evaluation of the above parameters [Figure 4].{Figure 4}

 Results



Tensile strength

Among all the materials tested, the test Group 1 showed the highest tensile strength of 55.06Mpa, whereas test Group 2 had the least tensile strength of 12.06Mpa as shown in [Figure 5]. Tukey's HSD post hoc test showed a significant (P ≤ 0.001*), the mean difference in tensile strength between three groups. The control group had a mean difference of −17.50 Mpa when compared to test group 1, whereas when the control group was compared with test Group 2, the mean difference 25.50 Mpa was noticed. Mean elongation in tensile strength of the control group was the highest (102.7 mm) followed by test Group 2 (80.02 mm), as shown in [Figure 6], which was statistically significant (P ≤ 0.001*).{Figure 5}{Figure 6}

Tear strength

Group 1 showed the largest mean difference of 13.80 Mpa and mean elongation of 155.64 Mpa as compared to the entire tested group [Figure 7] and [Figure 8]. Tukey's HSD post hoc test revealed that mean difference among the control group and Group 1 was 114.65 Mpa, whereas in control group and Group 2 was 108.22 Mpa. This mean difference was found to be statically significant (P ≤ 0.001*).{Figure 7}{Figure 8}

 Discussion



The main objective of maxillofacial prosthesis is to reduce morbidity and suffering of the patient by improving his quality of life.[4] The prosthetic substitute must replace the lost anatomy that looks normal, both topographically and optically and should become a part of the adjacent tissue. Considerable skill and experience are required to create such a prosthesis. To achieve all these requirements, choice of material for the fabrication of prosthesis plays a critical role. Barnhart in 1960 demonstrated the use of silicone elastomers for facial prostheses.[5] Literature quotes many advances in the field of maxillofacial material such as polymethylmethacrylate, polyvinyl chloride and copolymers, polyurethane elastomers, silicone elastomers, silicone block copolymers, and polyphosphazene.[3] Unfortunately, all these maxillofacial prosthetic materials come with some or the other disadvantages, till now no material has been found that can fulfill all the ideal requirements.[6] This is further exaggerated in the tropical climatic conditions. Little is known about the physical properties of the materials under the above circumstances. A systematic review and meta-analysis done by Rahman AM et al. in 2018 concluded that the scientific evidence on the physical properties of maxillofacial material in Asian countries is very limited, where climatic conditions are very different than the climatic condition where these materials are manufactured.[7]

This study evaluated conventionally used medical grade silicone, prosthetic grade, and locally available medical grade silicones. Medical grade silicone (Technovent M51) is imported from the United Kingdom was used as a control group as it is widely used maxillofacial material in India.[8] The climatic condition between India and the UK differs, the average climatic condition in a temperate control environment like the United Kingdom is 18°C–20°C with a humidity level of 73%, whereas in India, the average temperature is 32°C–40°C with a humidity of 65%–70%. Therefore, patients show a failed MFP due to loss of color matching and tear along the edges. This tear along the edges could be due to repeated insertion and removal of the prosthesis causing fatigue or it could be due to change in the properties of the material due to the weather condition. This not only requires a frequent refabrication of the prosthesis but also adds as a financial burden for the patient. Thus, it is imperative for the maxillofacial material to not only fulfill the physical and mechanical requirements suited to our climate but also be economical so that more patients can be rehabilitated.

Prosthetic grade silicone (Platanisil 40tt) is a locally available RTV silicone which is used successfully for making limb prosthesis. Yet, very limited data are available on prosthetic grade silicone as a maxillofacial prosthesis material, so it was included as test Group 1.

Third material evaluated in this study was a locally available RTV medical grade silicone material. There are many case reports, in which this material is used as a maxillofacial material, but there is hardly any literature evaluating the properties of this material.[9],[10],[11] Therefore, this material was chosen as test Group 2.

The parameters chosen for evaluation were tensile strength, tear strength, and elongation. According to the ISO, standard tensile strength must be 2.0–7 MPa (300–1000 psi) and tear strength 53–175 N/cm (30–100 psi).[3] Studies have shown that a high percentage elongation and good tear strength is one of the most desirable properties for maxillofacial material.[12]

Tensile strength indicates stretch ability that a material can withstand without failure.[13] Clinically, it implies, its ability to withstand facial movement as well as strain incorporated during insertion and removal of the prosthesis.[14] The above property was assessed using dumbbell-shaped specimen fabricated according to ASTM guideline.[4] Another important property evaluated was tear strength, which is indicative of marginal integrity of feather edge of the prosthesis.[15] This property was evaluated using unnicked 90° angled specimen, which is a modification of the test specimen described by ASTM D624: (standard test method for rubber property).[6]

The findings of our study showed that test Group 1 had the best tensile strength. However, both control group and test Group 2 medical grade silicones showed tensile strength within the prescribed range and were good enough to be used as a maxillofacial material, though less than prosthetic grade silicone. The high tensile strength displayed by test Group 1 is a useful property for fabrication of finger prosthesis where more tensile loads are directed, and biocompatibility issues are not critical. With respect to tear strength, our study showed that the control group had the best tear strength, followed by test Group 1 and test Group 2, respectively. The average tear strength of the control samples was in the range of 130 N/cm, which is well within the required tear strength range for maxillofacial materials. Test Group 1 prosthetic grade silicone showed about 100 N/cm, which is within the stated range. The medical grade silicone made locally (test Group 2) showed quite low strength of 30 N/cm, which is below the recommended range. The elongation test showed best results with the control group over test Group 1. This is because the material was rigid and showed poorer ultimate tensile strength as compared to the control. Test Group 2 showed better elongation than test group 1 but less than the control. A high ultimate tensile strength is important for maxillofacial materials. Similarly, for tear strength, the ultimate tear strength was least for test Group 1, followed by test Group 2, and control showed the highest. This can be interpreted as locally available medical grade silicone had better ultimate tear strength but lower tear strength than test Group 1. However, in maxillofacial materials, tear strength is a critical factor as the edge of the prosthesis is kept very thin to be flush with the margins of the face. This study reveals that improvement in the tear strength of locally made medical grade material (test Group 2) is essential before it can be an acceptable maxillofacial material.

The findings of this study are in line with the findings of a study conducted by Mitra et al., in which RTV and HTV silicones were compared with the latter showing better tear strength over the former[3] and not in agreement with the study by Gregory L. Polyzois in 1994 where he stated that RTV silicone showed better physical properties over HTV silicone.[13]

Our study is also in agreement with that of Kheur et al. who evaluated the effect of tropical climatic conditions on M511 and Z004 (Principality Medical, Technovent, UK) maxillofacial material.[12] In his experiment, the samples within the group were prepared using HTV and RTV technique and authors concluded that, though HTV had superior properties over RTV, there was no statistically significant difference.[16] Our control group too showed no statistical significant difference in tensile and tear strength in a tropical climate as compared to accepted standards. However, further research is required for improving the properties of locally available medical grade material such that it can meet the economic needs of our patients. Additional research is also required on color stability of both experimental materials in simulated tropical climates.[17],[18],[19],[20] Prosthetic grade silicone needs to be assessed for biocompatibility with facial keratinized tissues as it's tensile and tear strength made it acceptable as a maxillofacial material. The limitations of our study included the climate in Bangalore, which is unlike some extreme weather conditions of India. Further color stability and shelf life of these can be compared, and in vivo study will aid in the better clinical assessment of material properties.

 Conclusions



Within the limitation of the study, the following conclusions can be drawn:

Tensile and tear strength of control group does not alter much in tropical climatesLocally available medical grade silicone has comparable tensile strength but inadequate tear strengthLocally made prosthetic grade silicone showed adequate tensile and tear strength for maxillofacial material and can be considered subject to its fulfilling color stability and biocompatibility criteria.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

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