Acrylic denture based resins are frequently used in daily dental practice, as they are able to provide the essential properties and have necessary characteristics for their use in diverse functions. Acrylic resin bases of removable partial or complete dentures and tooth-supported or implant-retained overdentures are used to replace the lost tissues and transfer masticatory forces from the dentures to the residual ridges.
Despite various methods of processing to initiate the polymerization reaction, the conversion of monomer to polymer is not complete and some unreacted monomer called residual monomer will be left.2 This residual methyl methacrylate (MMA) monomer affects physical properties such as water sorption, hardness, dimensional stability, tensile strength, flexural strength and biocompatibility.3
Allergic reactions to residual MMA monomer from acrylic resins can show local and systemic manifestations such as palatal erythema delineating the contact area, erosion of the oral mucosa, burning sensation on the mucosa and the tongue, difficulty in swallowing, chronic urticaria.
Irritation of the oral mucosa beneath or adjacent to poly methyl methacrylate (PMMA) dentures is certainly the most severe local clinical adverse effect.4,5,6
In an effort to achieve reduced residual monomer content in acrylic denture bases, many investigators introduced various methods of curing with domestic pressure cooker7, microwave oven8,9 apart from the modifications in conventional water bath curing with varying time and temperature.
1 It was also reported that the effect of water bath and microwave post polymerization treatments even reduced the residual monomer content of acrylic denture bases.10,11
Over the years, several methods12 have been reported in literature for estimation of residual monomer content of polymerized acrylic resins which include chemical analysis, infrared spectroscopy, ultraviolet visible light spectrophotometry, gas chromatography and high performance liquid chromatography (HPLC). Amongst these, HPLC is a well established method for determining the residual MMA monomer content in denture acrylic resins, because all the procedures will be carried out in liquid at room temperature, thus eliminating heat and yielding more accurate results.3
Several studies1,8,9,12 on lesser residual monomer content in acrylic resins, have been done using any one specific method of polymerization such as water bath, microwave, autoclave etc. Till date very few studies10,11 have been done to evaluate the residual monomer content in acrylic resins using different polymerization methods with and without post polymerization treatments. The present in vitro study will be conducted to evaluate the residual monomer content of heat cure acrylic denture base resin using short curing cycle with terminal boil and autoclave methods of polymerization with and without post polymerization treatments.
The results that will be obtained from this study will be helpful in utilizing feasible and simple polymerization methods, which saves time and money by delivering more biocompatible dentures to the patients in day-to-day clinical practice.
Authors conducted a study using high performance liquid chromatography to investigate residual monomer content of heat cured acrylic resin with post polymerization water bath (55°C for 60 min) and microwave (550W/3 min) treatments and evaluated reduction in residual monomer content using post polymerization treatments with improved mechanical properties and biocompatibility of denture base materials.11
Authors conducted a study using ultraviolet spectrophotometry method to compare residual monomer content for 3 different groups of heat cure acrylic resins. Disk shaped 5 specimens (20 mm in diameter 2 mm thick) were used in each of test groups respectively. 1st group using autoclave polymerization methods for 60°C/30 min followed by 130°C/10 min. 2nd group using autoclave polymerization method for 60°C/30 min followed by 130°C/20 min. 3rd group using conventional water bath method 100°C for 30 min. They concluded decreased amount of residual monomer content with increased hardness was seen using autoclave polymerization method.12
Authors compared residual MMA content using high performance liquid chromatography after different polymerization methods (heat, auto, microwave energy), different curing process (heat and auto) and length of storage of polymerized 120 specimens. The lowest residual MMA content was obtained for heat cured resin after long term terminal boil and storing in distilled water for atleast 1 day. For auto cured resin by additional curing in water at 60°C and storing in distilled water for atleast 1 day. For microwave cured resin with storing in distilled water for atleast 1 month. The results showed that different polymerization methods and curing processes have different effects on residual MMA content. Authors concluded that storing a dental acrylic resin specimen in distilled water at 37°C is a simple but effective method of reducing its residual MMA content.3
Authors conducted a study using a high performance liquid chromatography method to analyze the amount of monomer released from heat cured, chemical cured and light cured resin materials during storage at 37°C in water. The results indicated that the amount of released methyl methacrylate was related to processing condition and composition of the materials. Light cured acrylic resin released considerably less methyl methacrylate (MMA) than heat cured and self cured resin. They concluded that for minimizing monomer release, heat cured and chemical cured resin dental appliances should be immersed for 1 day in water before wearing.2
Authors conducted a study using infrared spectrophotometer for heat cured acrylic resin to analyze the residual monomer content, processed in a domestic pressure cooker. They concluded that curing for 30 minutes under steam pressure resulted in minimal residual monomer content and is even lower than the residual monomer content found with conventional rapid curing cycle.7
Is there any significant difference in the residual MMA monomer content between short curing cycle with terminal boil and autoclave polymerization methods with/without post polymerization microwave and water bath treatments?
RESEARCH HYPOTHESIS (H1): There will be difference in the residual MMA monomer content between short curing cycle with terminal boil and autoclave polymerization methods with/without post polymerization microwave and water bath treatments.
NULL HYPOTHESIS (H0): There will be no difference in the residual MMA monomer content between short curing cycle with terminal boil and autoclave polymerization methods with/without post polymerization microwave and water bath treatments.
All the materials will be collected or purchased through scientific chemical or dental suppliers.
The study specimens will be evaluated for residual monomer content at Azyme Biosciences Private Limited, Biotechnology Company, Bengaluru, Karnataka, India.
Considering the primary end point as residual MMA monomer content for calculation of sample size, using following formula13,
Calculated sample size is 18. Three in each group will be considered for this study.
Cured acrylic specimens with porosities.14
For specimen preparation:
Disk shaped specimens of heat cured acrylic denture base material (n=18) with dimensions of 50mm diameter and 3mm thickness (ISO 1998)14 will be prepared by using circular stainless steel mold. First, wax specimens will be made, invested using type 3 dental stone (Asian chemicals, India) within curing flasks and dewaxed using conventional method. The recommended polymer/monomer (3:1 by volume) mix at dough stage will be packed under hydraulic pressure into the mold space and will be subjected to different curing methods as follows-
The flasks will be bench cooled for 30 min in air followed by 15 min in running water. After deflasking, finishing and polishing will be done by removing 0.5mm from the surface of each side of the cured specimens using finer grades of silicon carbide paper of 80, 600 and 1200 grits respectively under light pressure with copious water for lubrication and cooling.10 The specimens will then be immersed in 37°C distilled water for 24 hours stored in an amber glass screw-neck bottle.2
Specimens will then be divided into following groups:
Heat cure acrylic denture base resin using short curing cycle with terminal boil polymerization method. Heat cure acrylic denture base resin using short curing cycle with terminal boil polymerization method and post polymerization microwave treatment at 550W for 3 min. Heat cure acrylic denture base resin using short curing cycle with terminal boil polymerization method and post polymerization water bath treatment at 55°C for 60 min.
Heat cure acrylic denture base resin using autoclave polymerization method. Heat cure acrylic denture base resin using autoclave polymerization method and post polymerization microwave treatment at 550W for 3 min. Heat cure acrylic denture base resin using autoclave polymerization method and post polymerization water bath treatment at 55°C for 60 min.
Each cured polished specimens will be broken into small pieces approximately of 2mm square dimensions using a hammer mill, which is later weighed using digital weighing machine (Mettler Toledo, Precision Weighing Balances, England) to obtain three samples of 100mg each (Total=18) that will be frozen stored. Each 100mg of frozen sample will then be introduced into one-mark 25ml glass flask containing 2ml Methanol (extraction solvent), magnetic stirring (Remi 1mlh Magnetic Stirrer, Remi Sales and Engineering Ltd, India) will be done for 2 hours in an ice bath with a clean PTFE (Polytetrafluoroethylene) coated magnetic stirrer, then transferred to centrifuge tubes for centrifugation (Remi Cooling Centrifuge, Sigma Scientific Products, India) at 10,000rpm for 15 min. 0.2ml of upper clear portion is pipetted from centrifuge tubes to glass tubes containing 0.2ml aliquot of acetonitrile-water solution. From this 100L will be injected from each sample into the HPLC system.
Chromatographic conditions: C-18 column (5m particle size 100°A pore size 0.46cm i.d x 15cm length) using acetonitrile/water 55/45 (v/v) plus 0.01% triethylamine for specimens, at a flow rate of 0.8mL/min. The residual MMA monomer components will be detected by their UV absorbance at 230nm.
The determination of the monomer content (g/mL) of three samples from each (18) specimen will be performed and hence a mean RM value for that specimen will be calculated as percentage of RM in the specimen according to the equation:
Where RM is the percentage of RM, CRM the concentration of RM (g/L), Ci the initial concentration of the samples in extraction procedure (g/L), Vp the volume to be pipetted of centrifuge solution (L), and Vt is the total volume after addition of 200L of solution (L).
In the present study, for quantitative analysis, Kruskal-Wallis one-way ANOVA test will be used as a test of significance.