Quality of Heat-activated Acrylic Denture Base Resins Cured using Improvised Techniques in Private Dental Laboratories in Nairobi, Kenya

Abstract

Background: Heat-activated acrylic resin based complete and removable partial dentures polymerized using improvised curing techniques are common in private dental laboratories in Nairobi, Kenya. There is no information regarding whether these improvised techniques have control for time and temperature which has a great impact on the quality of the final product obtained. Objective: To evaluate the quality of heat-activated acrylic resins polymerized using improvised curing techniques in private dental laboratories in Nairobi. Study Design: The pilot study was a descriptive cross-sectional study of the improvised curing techniques followed by an exploratory quasi-experimental laboratory study. Materials and methods: A pilot study using a self-administered questionnaire was conducted on all the 41 registered private dental laboratories to identify techniques used to process heat-activated acrylic denture base resins (HAADBR). A large number, 31 (75.6%) of the dental laboratories used improvised methods and equipment with no form of time and temperature regulation. Three of the improvised techniques were selected (ICT 1, ICT 2, ICT 3) based on frequency of application and ease of replication. Using each of the three ICT and a conventional curing cycle as control ( 1.5 hours at 73 °C and 0.5hrs at 100°C), four sets of twenty specimens were fabricated and evaluated for residual methyl methacrylate monomer content (MMA), indentation hardness, water sorption and solubility and examined for porosities. A single batch of a heat-activated acrylic resin (Lucitone 199®) was used and was manipulated according to manufacturer’s instructions by the investigator. The specimens for the evaluation of residual MMA and water sorption and solubility were fabricated by investing rectangular stainless steel moulds in type II gypsum (Kalabhai, Kaldent) in conventional denture flasks. The stainless steel mould had a lathe-cut circular depression at the center with a diameter of 50mm and a depth of 3mm and a flat stainless steel cover. Indentation hardness and evaluation of porosities specimens were processed by investing rectangular stainless steel blocks (65mmx40mmx3mm) in type II gypsum (Kalabhai, Kaldent) to produce moulds of the same dimensions. Specimens for residual MMA evaluation were refrigerated at 3°C in dark sealed polythene bags to minimize monomer loss for four (4) days. Residual monomer levels were evaluated using the High Performance Liquid Chromatography method (HPLC). Indentation hardness was evaluated using a Vickers hardness tester. Water sorption and solubility was evaluated after desiccation followed by storage of the specimens in distilled water in an oven maintained at 37°C±1 for seven days. Evaluation of presence of porosities was determined after sectioning each specimen lengthwise into three portions and visual examining for voids in the acrylic plates under natural lighting. Data were analyzed using SPSS v.20 (IBM, Corp.) and presented in tables and figures. Statistical analysis of the properties evaluated was completed using One-Way analysis of Variance (ANOVA) and differences between means identified with Fisher’s Least Significant Difference (LSD) Post Hoc test at a level of p≤0.05. Results: The mean residual MMA content observed for the three ICTs was higher than that of the control group 0.04%wt ( ± 0.01, SD) while ICT 2 exhibited the highest percentage 0.19%wt ( ±0.02, SD). Mean residual monomer in HAADBR cured with the other techniques were ICT1 0.13%wt (± 0.04, SD) and ICT 3 0.12%wt (±0.01 SD). One-way ANOVA test revealed a statistically significant difference (p=0.001) in the means of residual MMA content in the specimens. Fisher’s Least Significant Difference (LSD) Post Hoc Analysis Test for the ANOVA results revealed a statistically significant mean difference of the residual MMA between the control specimens and those processed using the three ICT. The mean Vickers’ Hardness Number of the specimens processed using ICT 2 19.37 (±0.55 SD), was higher than that of the control cycle and this was statistically significant (p< 0.001). The specimens processed by the control cycle showed higher mean water sorption levels (18.29 μg/mm3±0.8, SD) than all the three ICTs. There was minimal mean solubility observed in the specimens processed using the ICT 3 (0.0017μg/mm3). The levels of water sorption for the three ICT’s and the control specimens differed marginally, ICT 1 17.18 μg/mm3 (± 0.18 SD), ICT 2 18.00 μg/mm3 (± 1.35SD), ICT3 17.87 μg/mm3 (±0.37, SD) and control 18.29 μg/mm3 (±0.81 SD).There was no statistically significant difference between the mean water sorption of all the specimens processed by each of the ICTs and the control. There were no porosities observed in all the specimens processed using the improvised curing techniques and the control cycle. Conclusion: Majority of private dental laboratories in Nairobi used improvised curing techniques. Within the limits of this study, acrylic denture base resins processed with the improvised curing techniques studied are within recommended ISO standard values with regard to hardness, water sorption and solubility and porosity.