COMPARATIVE EVALUATION ON THE INFLUENCE OF DIFFERENT CURING CYCLES ON THE MECHANICAL PROPERTIES OF THREE COMMERCIALLY AVAILABLE DENTURE BASE RESINS: AN IN-VITRO STUDY

Aim: The aim of the study was to evaluate and compare the influence of different curing cycles on the mechanical properties of three commercially available heat cured acrylic denture base resins. Methodology: A total of 120 samples were fabricated with the help of a customized 3 piece aluminium mould. Three brands of heat-cured acrylic denture base resins (DPI, TREVALON, and VERACRIL) were selected for the study. These materials were polymerized using 2 different curing cycles – Short Curing Cycle (74 0 C for 2 hours followed by 100 0 C for 1 hour) and Long Curing Cycle (74 0 C for 8 hours followed by 100 0 C for 1 hour). The cured samples were then stored in water for one month to ensure full saturation. Once the immersion time had elapsed, each group of samples are subjected to a 3 Point Bending Test and Pendulum Impact Test. The data obtained were subjected to statistical analysis. Results: The mean flexural strength and impact strength values of all 3 brands of commercially available denture base resins were higher in the long curing group when compared to the short curing group (p < 0.05). Trevalon heat-cured acrylic denture base resin was found to have superior flexural and impact strength values as compared to other two denture base resins when cured using long and short curing cycles. Conclusion: The type of polymerization cycle played an important role in influencing the flexural and impact strength of heat cured acrylic denture base resin. Introduction The loss of teeth and associated structures by trauma or disease had plagued mankind throughout the ages. In order to re-establish a degree of function and appearance, it has been necessary, always to fabricate prosthesis by adapting to the contemporary materials that were available at that period of history. Before eighteenth-century materials like wood, bone and ivory were used for denture fabrication. Dentures were carved out of a single piece of wood and natural teeth were held to it with the help of screws. These materials posed aesthetic and hygienic concerns. Polymethyl methacrylate was introduced in 1937 by Dr. Walter Wright and since then it has been successfully used for the fabrication of denture bases and artificial teeth. In addition, it has many other applications in the field of aesthetic dentistry. It has excellent aesthetic properties, adequate strength, low water sorption, low solubility, lack of toxicity and can INFLUENCE OF DIFFERENT CURING CYCLES ON THE MECHANICAL PROPERTIES OF DENTURE BASE RESINS 2(3);2016 24 Journal Of Applied Dental and Medical Sciences 2(3);2016 Figure 1: Customized 3 piece Aluminium mould be constructed by simple moulding and processing technique. Mechanical properties including flexural strength and impact strength are affected by the processing technique. Flexural strength is especially important since acrylic resin prosthesis is susceptible to fracture over long periods of clinical use. The two variables, time and temperature are important in the curing of heat cured acrylic denture base resins. Curing cycles that last more than six hours, with or without a terminal boil are found to have lower residual monomer leading to its improved mechanical properties. Polymerization using longer curing cycles with a terminal boiling point resulted in almost complete conversion of monomer which significantly improved the flexural and impact resistance. Shorter curing cycle of about 74C for 2 hours with a terminal boiling was introduced to shorten the polymerization process as it saved considerable laboratory time. Porosity and high residual monomer content were found to be a frequent problem with this curing cycle. Residual methyl methacrylate concentration is found to be high, if the polymerization process is incomplete or if the terminal boiling is avoided in the polymerization process. This residual methyl methacrylate leaches out from the denture base resin and is the cause for adverse effect on the oral mucosa. Polymerisation cycle that includes a terminal Figure 2: Samples obtained after Long and Short Curing Cycles boiling period is preferred, as almost complete conversion of monomer is achieved. Long curing cycle of 8 hours at 74C with a terminal boiling for 1 hour and a short curing cycle for 2 hours at 74C with a terminal boiling for 1 hour was proposed for this study. Thus, the aim of this study was to evaluate and compare the effect of two curing cycles on the mechanical properties of three commercially available brands of heat cured acrylic denture base materials. MATERIALS AND METHODS This comparative in-vitro study involved a total of 120 samples consisting of 12 groups, each with 10 samples. Three brands of heat-cured acrylic denture base resins (DPI, TREVALON, and VERACRIL) were selected for this study. These materials were polymerized using 2 different curing cycles – Short Curing Cycle (74 0 C for 2 hours followed by 100 0 C for 1 hour) and Long Curing Cycle (74C for 8 hours followed by 100C for 1 hour). A customized 3 piece aluminium mould of dimensions 64 mm long, 10 mm width and 3.3 mm height (BS EN ISO 1567:2001) was fabricated [Figure 1]. Two screws present at the corners of the mould helped in assembling the 3 pieces. This customized mould helped in the fabrication of polyvinyl siloxane strips. Polyvinyl siloxane putty material was adapted to the mould space and fastened by the screws. The material INFLUENCE OF DIFFERENT CURING CYCLES ON THE MECHANICAL PROPERTIES OF DENTURE BASE RESINS 2(3);2016 25 Journal Of Applied Dental and Medical Sciences 2(3);2016 Figure 3: Mean Flexural Strengths (MPa) values of test materials after Long Curing Cycle and Short Curing Cycle was allowed to set and excess material was cut off. The dimensions were cross-checked using a vernier calliper. Type II dental plaster was mixed according to manufacturer’s instructions and the polyvinyl siloxane strips were then placed on the partially set plaster in the base of the flask. The body of the flask was placed into position on the base and care was taken to see that no plaster remained on the rim as this could prevent complete seating of the two halves of the flask. Two coats of the cold mould seal were painted on the plaster surface of the base of the flask and were then allowed to dry. With the body in position, the flask assembly was then kept on a vibrator. Type II dental plaster was then mixed and poured to fill the body and the lid were placed and tapped to ensure that the flask was completely filled. After 30 minutes the flask was opened and the polyvinyl siloxane strips were removed, and two coats of the cold mould seal were applied into this mould space. The polymer and monomer were mixed in a ratio of 3:1 by volume in a ceramic mixing jar. When the dough stage was reached, it was kneaded thoroughly and packed into the mould space. The flask was then closed and kept under bench press for one hour. Then it was transferred by a conventional clamp and immersed in an acrylizer. The same procedure was Figure 4: Mean Impact Strengths (KJ/mm 2 ) values of test materials after Long Curing Cycle and Short Curing Cycle followed for all the 3 brands of heat polymerizing acrylic denture base resins respectively. The long curing group includes 60 samples and the curing procedure involves, keeping the samples at 74C for 8 hours followed by a terminal boil for 1 hour in the water bath. 60 samples were included in the short curing group and these were polymerized at 74C for 2 hours followed by a terminal boil for 1 hour. After curing, it was allowed to bench cool slowly from water bath temperature to room temperature for three hours. The flask was opened and the samples were retrieved and hand finished using a 320 grit silicone paper to a final dimension of 65 ± 0.2 mm in length, 10 ± 0.2 mm in width and 3.3 ± 0.2 mm in height [Figure 2]. The finished samples were then stored in water for one month to ensure full saturation. Once the immersion time had elapsed, each group of samples are subjected to a 3 Point Bending Test (MPa – Mega Pascal) and Pendulum Impact Test (KJ/mm – Kilojoule per square millimeter). The data obtained from the test results were tabulated and subjected to statistical analysis using Independent-t-test and ANOVA (Analysis of Variance) after adjusted for multiple comparisons using Bonferroni Correction. Statistical tests were carried out using SPSS 21.0 (Statistical Package for Social Science; IBM Statistics, 2012) for which the level of significance was fixed at 5%. INFLUENCE OF DIFFERENT CURING CYCLES ON THE MECHANICAL PROPERTIES OF DENTURE BASE RESINS 2(3);2016 26 Journal Of Applied Dental and Medical Sciences 2(3);2016 Table 1: Comparison of Flexural strengths (MPa) between DPI, Trevalon and Veracril after Long and Short Curing Cycle