Analysis of the properties of commercially available silicone elastomers for maxillofacial prostheses Zubeda Begum, 1 Mohammed Zaheer Kola, 2 Paran Joshi 3 ABSTRACT Aim - Maxillofacial prosthetic materials are used to replace facial parts lost through disease or trauma. Silicone rubbers are the materials of choice. The aim of this study was to assess the properties of three new silicone rubber maxillofacial materials. Methods- Specimens of three commercially available maxillofacial materials were prepared in dental flasks according to manufacturers instructions. Tear strength, tensile strength, percentage elongation, hardness and water absorption were determined for each material. Result- Cosmesil high compliance had significantly higher tear strength than other materials and tensile strength was also higher than the other materials but not statistically significant. Hardness and percentage elongation were almost same for all the materials. Prestige elastomer had higher water absorption than the other two materials, but it was not statistically significant. Conclusion- None of the commercially available silicone rubber materials possessed ideal properties for use as a maxillofacial prosthetic material but within the limitations of present study, Cosmesil high compliance shows more favourable properties among the three materials. Key words- silicone rubber, maxillofacial prosthesis, mechanical properties,tear strength, hardness, water absorption. results of the prosthetic treatment are influenced by the properties of the prosthetic material. The most common reason for refabrication of facial prosthesis is degradation of colour and physical properties. Silicone elastomers were first used for external prosthesis by Barnhart in 1960. 3 The preference for silicone especially the room temperature vulcanizing (RTV) have been overwhelming. 4 Scientific investigations have demonstrated the superiority of high temperature vulcanizing (HTV) silicones which are generally stronger, tougher, and stiffer than RTV materials. 5-7 The major limitation of HTV silicone is in the fabrication. The material requires a milling machine and fabrication of metal moulds, although stone moulds are being used. 8 There are two major types of RTV silicones according to curing mechanism: addition and condensation silicones. Cosmesil (Principality Medical), a condensation silicone, was introduced in 1982 in the United Kingdom. The system contains the silicone elastomer and RTV sealant, colouring agent, and accessories needed for preparing facial and body prostheses. 9 A high compliance version was developed in 1993. 10,11,12 Introduction Silicone elastomers are the material of choice to replace missing facial parts which have been lost through disease or trauma. 1 The ideal material for external facial prosthesis has been well defined. 2 Some of the desirable properties are (1) Resistance to tear at the thin margins (2) Tensile strength and the resulting elongation gives an indication of flexibility of the prosthesis (3) Ease of moulding (4) Colour stability (5) Water absorption (6) Non toxicity and non-sensitivity to the host tissues. The primary goal of the maxillofacial prosthodontist is to restore the patient s appearance, improve their selfesteem and help them lead as normal a life as possible. Every human has the divine right to look human. The results of the prosthetic treatment are influenced by 1 Yu and Koran 13 studied permanent deterioration of four silicones before and after accelerated aging; however most of this materials are no longer in popular use. Yuet al 14 evaluated ultimate tensile strength, percentage elongation, shear strength, tear strength, and shore A hardness of four silicones after accelerated aging and found silastic 4-4210 adhesive (Dow corning corp, Midland, Mich.) was the best choice of materials. Lemon et-al 15 evaluated silastic 4-4210 and silastic medical adhesive type-a coloured with oil based pigments after natural weathering and accelerated aging. Artificial aging caused a greater change than natural weathering. Tariq aziz et al 1 studied five newer materials Cosmesil HC, Cosmesil standard, A-2186, Premium facial and body elastomer (prestige) and MED- 4290, and concluded that none of the commercially available materials possessed ideal properties for use as a maxillofacial prosthetic material. Factor II, however showed more favourable properties due to its high tear
strength, softness and ease of manipulation. However all studies evaluating the properties of prosthetic materials have used test specimens that have been die-cut from sheets of polymer. 16 The use of diecut specimens for tensile stress-strain testing has been criticized because of the introduction of small nicks,flaws etc that have been implicated in the growth of cracks and thus in the ultimate failure of the specimens. Moulded test specimens are probably more representative of the end product. Hence in this study moulded test specimens have been evaluated since the technique most closely approximates the laboratory fabrication of maxillofacial prosthesis. Materials and methods The study was conducted using three commercially available silicone elastomers used for the fabrication of maxillofacial prosthesis and type of chemical reaction and processing conditions are shown in Table I. Materials Premium facial and body elastomer (prestige) Cosmesil Cosmesil High Compliance Tensile strength Curing system 2 part addition 3 part condensation 3 part condensation Curing time 2 hrs at 100 0 C 24 hrs at room temperature 24 hrs at room temperature Table I- Materials used in the study Manufacturer Prestige Dental Bradford, UK Principality Medical, Newport UK Principality Medical, Newport UK The specimens for the evaluation of tensile strength were made in dumb-bell shape according to ASTM standards. A dumb-bell shape brass die was fabricated measuring 2mm in thickness, 80mm in length and 6mm (at periphery) and 3mm (at centre) in width. Gypsum mould was made using the die, and specimens were made using conventional flasking technique. The mould was changed for each type of material to avoid contamination. Five specimens from each material were made and subjected to tensile strength. The testing was carried out on Hounsfield testing machine (DEBEL, Bangalore), fitted with 1000N load cell linked to an IBM compatible computer. The speed maintained in the machine was 20mm/min. The tensile strength and percentage elongation were measured automatically by the software using equation: Stress (Nm -2 ) = Percentage Strain (%) = Tear strength Load ------------------------------------------- Initial cross sectional area Extension ----------------------- X 100 Original Length The same specimen shape was used as it was in tensile strength. Five samples of each material, total fifteen samples were subjected to tear strength testing. Testing was carried out using Hounsfield testing machine (DEBEL, Bangalore), fitted with 1000N load cell. Specimens were tested at a constant crosshead speed of 20mm/min at a gauge length of 25mm. On failure of the specimen the computer software automatically calculated the tear resistance by using following equation Where, Hardness Ts= F/t Ts = Tear resistance (N/mm) F = Load at failure (N) t = Thickness of the specimen (mm). The specimens were made rectangular in shape (40mm x 20mm x 2mm) with conventional dental flasking technique. Five specimens of each material were tested for hardness with Shore A Durometer (GTRE, HAL Bangalore). The results were then converted into international rubber hardness degrees (IRHD). Water Absorption Water absorption test specimens, were made disk shaped (40 mm diameter and 2mm in thickness) using mould obtained through brass die and conventional flasking technique. Five test specimens were fabricated for each material. The specimens were placed in a desiccator containing phosphorus pentoxide and calcium chloride until they achieved a constant weight to an accuracy of 0.0001g. The specimen were then placed in glass screw topped jars containing distilled water and maintained in an oven at 37 0 C. At recorded intervals specimens were removed, blotted to remove excess water and reweighed again to an accuracy of 0.0001g. In this study the weighing was done at interval of 6hrs, 12hrs, 24hrs, 36hrs, 48hrs and 72hrs. Percentage weight change was calculated by using the following equation: 2
Graph- I Tensile strength Graph- IV Percentage elongation Graph-II Tear Strength Graph- V Water Absorption Water Absorption = W2-W3 ------------------------ W1 Where, W1= Initial weight, Statistics W2= weight after absorption of water, W3= weight after desiccation of water. Graph- III Hardness One way analysis of variance (ANOVA) was used to test for any significant difference between the mean values of the materials tested. Post- test (Bonferroni method) was used to determine whether the mean value of any particular material differed significantly from another specified material, while considering all the data. 3
Results Within the limitation of the present study, following conclusion can be derived. No significant difference in tensile strength of materials was found, however Cosmesil high compliance showed better strength over other two materials (Graph I) Statistically significant difference was observed between the three groups with respect to mean tear strength (P<0.05). Higher mean tear strength (Kgf) was recorded in Cosmesil high compliance material followed by Prestige elastomer and Cosmesil standard respectively. (Graph II) Hardness and percentage elongation of all three materials were almost same, and no statistical difference was found. (Graph III, IV) Prestige facial elastomer showed higher water absorption rate than the other two materials, but it was not significant. (Graph V). Discussion Materials proposed for external maxillofacial applications should demonstrate reasonable tensile strength and yet be soft enough to respond adequately with facial movement. The physical properties of tensile strength, elongation, and tear strength were tested as potential indicators of overall strength and flexibility, durability, and marginal integrity in clinical service. Hardness reflects the tactile response of lifelike feel. 8 It has been reported that although the material must possess reasonable tensile strength tear strength is more important clinically in predicting the durability of material. Conroy et al 9 reported that tear strength must be interpreted carefully because in clinical use the elongation at break may have an overriding influence. They believed that a high percentage elongation and high tear strength produce most desirable combination. In the present study Cosmesil high compliance exhibited higher tear strength and the percentage elongation than Cosmesil standard and Prestige elastomer. The differences observed in the physical and mechanical properties of the commercial materials are due to different components used in their formulations. More specifically the variables may include different cross-linking systems (addition or condensation), differences in the molecular weight of the PDMS (Poly dimethyl siloxane), differences in cross linking density and differences in the grade and concentration of the silica filler used in the systems. 1 The tear strength of commercially available Cosmesil standard and Prestige elastomer was poor. These materials contain non-surface treated fillers with large surface area irrespective of the molecular weight of the polymer used in these systems. The interaction between the -OH group on the filler and PDMS is not strong enough to prevent the material from rupturing under an applied force. Cosmesil high compliance produced better acceptable tensile strength, hardness and tear strength when compared to the other two materials. The tear strength of the Cosmesil high compliance was significantly better than Cosmesil standard and Prestige elastomers. The interaction between the silica filler and polymer chain influences the mechanical strength of the polymer matrix. If the silica filler particles are surface modified with dimethyl silyl or tri methyl silyl groups, the resulting polymer matrix will withstand greater deformation without rupturing or tearing. The material with the highest percentage of elongation was Cosmesil high compliance but it was not statistically significant when compared to other materials. The hardness of commercially available material ranges from 16 to 45 IRHD. The difference in hardness readings may be due to differences in cross linking systems, cross linking density, molecular weight of the polymer and differences in the grade and concentration of silica filler. The condensation curing material like Cosmesil high compliance and Cosmesil standard were harder than additional curing material like Prestige elastomer, although the difference was very less and insignificant within the limitations of the present study. The water absorption studies on the commercial material allow us to determine which of these materials contain surface treated filler. Cosmesil standard and Prestige elastomers absorb more water when compared to Cosmesil high compliance. The rate of water absorption of material is continuous, which increases with time. Conclusion The materials evaluated in this study had reasonable mechanical properties for use as facial prosthetic material. However Cosmesil high compliance had significantly higher tear strength than other materials and tensile strength was also higher than the other materials but not statistically significant. Hardness and percentage elongation were almost same for all the materials. Prestige elastomer had higher water absorption than the other two materials, but it was not statistically significant. Within the limitations of present study, Cosmesil high compliance is a preferable material among the three materials. However other properties like flexibility, colour stability, biocompatibility and others have to be evaluated. References 1. Aziz T, Waters M, Jagger R. Analysis of the properties of silicone rubber maxillofacial and prosthetic materials. J Dent 2003; 31: 67-74. 4
2. Moore DJ, Glaser ZR, Tabacco MJ, Linebaugh MG. Evaluation of polymeric materials for maxillofacial prosthetics. J Prosthet Dent 1977; 38: 319-26. 3. Barnhart GW. A new material and technic in the art of somatoprosthesis. J Dent Res 1960; 39: 836-44. 4. Andres CJ, Haug SP, Brown DT, Bernal G. Effects of environmental factors on maxillofacial elastomers. Part II. Report of survey. J Prosthet Dent 1992; 68: 519-22. 5. Bell WT, Chalian VA, Moore BK. Polydimethyl siloxane materials in maxillofacial prosthetics: evaluation and comparision of physical properties. J Prosthet Dent 1985; 54: 404-10. 6. Lewis DH, Castleberry DJ. An assessment of recent advances in external maxillofacial materials. J Prosthet Dent 1980; 43: 426-32. 7. Lontz JF. State of the art of materials used for maxillofacial prosthetic reconstruction. Dent Clin North Am 1990; 34: 307-25. 8. Polyzois GL, Pettersen AH, Kullmann A. An assessment of the physical properties and biocompatibility of three silicone elastomers. J Prosthet Dent 1994; 71: 500-504. 9. Polyzois GL. Mechanical properties of two new addition-vulcanizing silicone prosthetic elastomers. Int J Prosthodont 1999; 12: 359-362. 10. Wolfaardt JF, Chandler HD, Smith BA. Mechanical properties of a new facial prosthetic material. J Prosthet Dent 1985; 53: 228-234. 11. Polyzois GL. Evaluation of a new silicone elastomer for maxillofacial prostheses. J Prosthodont 1995; 4: 38-41. 12. Polyzois GL, Andreopoulos AG. Some physical properties of an improved facial elastomer: A comparative study. J Prosthet Dent 1993; 70: 26-32. 13. Yu R, Koran A. Dimensional stability of elastomers for maxillofacial applications. J Dent Res 1979; 58: 1908-9. 14. Yu R, Koran A 3d, Craig RG. Physical properties of maxillofacial elastomers under conditions of accelerated aging. J Dent Res 1980; 59: 1041-7. 15. Lemon JC, Chambers MS, Jacobsen ML, Powers JM. Color stability of facial prostheses. J Prosthet Dent 1995; 74: 613-8. 16. Sweeney WT, Fischer TE, Castleberry DJ, Cowperthwaite GF. Evaluation of improved maxillofacial prosthetic materials. J Prosthet Dent 1972; 27: 297-305. About the Authors 1. Dr. Zubeda Begum MDS Professor, M R Ambedkar Dental College & Hospital, #1/36 Cline Road, Cooke town Bangalore-560005, Karnataka, India. 2. Dr. Mohammed Zaheer Kola Senior lecturer, Rajiv Gandhi college of Dental Sciences, Bangalore-560005, Karnataka, India 3. Dr. Paran Joshi Post-Graduate student, M R Ambedkar Dental College and Hospital, # 1/36 Cline Road, Cooke town, Bangalore- 560005, Karnataka, India. Mobile no- 7760755271 Email - paran.joshi@gmail.com Address for Correspondence Dr. Zubeda Begum MDS Professor, M R Ambedkar Dental College & Hospital, #1/36 Cline Road, Cooke town Bangalore-560005, Karnataka, India. Mobile no.-9845230390 Email: drzubedabegum@gmail.com 5