This study evaluated the effect of post-polymerization heat-treatments on degree of conversion (DC), residual methyl methacrylate concentration (MMA r ) and in vitro cytotoxicity of autopolymerizing acrylic repair resin.
A total of 336 specimens were prepared by bench- and hydroflask-curing and subjected to post-polymerization heat-treatments: a) water immersion at 60 °C for 30 min, b) microwaving at 500 W for 3 min, c) combined use of water immersion and microwaving d) no treatment (as control). Specimens were eluted in cell culture medium for 1, 2, 5 and 7 days. DC and MMA r in eluates were measured by FTIR spectrometry and HPLC, respectively. In vitro cytotoxicity of eluates on L-929 fibroblasts was determined by XTT assay. Data were statistically analyzed with Dunn’s multiple comparison and Pearson correlation tests ( p ≤ 0.05).
DC was highest (99.9%) in bench- and hydroflask-cured groups which were subjected to water immersion. At all elution periods, MMA r was detected in eluates of all treatment groups and were higher in bench-cured groups than hydro-flask cured groups. Cell proliferation values indicated slightly cytotoxic effect throughout 7 days; regardless of the curing method or post-polymerization treatment. The correlation between MMA r and cell proliferation was negative after elution of 1, 2, 5 days and was only statistically significant ( p < 0.05) at 5 days. At elution of 7 days, the correlation was positive with no significance.
Post-polymerization heat-treatment of autopolymerizing acrylic repair resin by immersion in water at 60 °C for 30 min is clinically recommended to improve the DC while reducing the leaching residual MMA.
Autopolymerizing acrylic repair resin is still one of the most commonly employed denture repair materials in daily prosthodontic practice so that denture repairs can be easily completed at room temperature in a short period of time without the use of any additional equipment .
During polymerization reaction of acrylic resins, all the monomer is not converted into polymer and therefore some unreacted monomers called residual monomers are left . Residual monomer is left in the polymer or might leach into water as well as human or artificial saliva . In a recent study investigating the identification and quantification of the leaching monomer from autopolymerizing resin specimens, it was shown that the majority of the leaching compounds was MMA.
The high levels of residual monomer in autopolymerizing acrylic resin are the result of the low degree of conversion (DC) achieved by the use of the chemical activator . To improve the DC, and to reduce the level residual compounds in the autopolymerizing resins, various alternative post-polymerization heat-treatments, such as heating in a drying box , microwave irradiation or immersion in water at elevated temperatures have been suggested.
Leaching of the residual monomer may influence the biocompatibility of the denture material . Case reports indicating symptoms, such as local chemical irritation, hypersensitivity, mucosal inflammation or ulceration, systemic allergic reactions due to acrylic resin have postulated that residual monomer in the denture base might have clinically effected the surrounding tissues. In addition to these symptoms, many studies have indicated different levels of in vitro cytotoxicity for autopolymerizing denture base resins. The common conclusion driven for the responsible cytotoxic effect was the leaching residual monomer. The effect of post-polymerization procedures on in vitro cytotoxicity of denture base materials has also been the topic of interest . However, the findings of these studies have indicated that post-polymerization heat-treatments, such as immersion in water at 55 °C or microwaving did not markedly influence in vitro cytotoxicity.
Although too many studies have been published for in vitro cytotoxicity of denture materials, there exists no available cell culture study, which in parallel identified or quantified the leaching components from the tested acrylic resin denture materials. Therefore, it seems reasonable that as a main component of residual compounds , the quantification of the residual MMA, which can leach out from the autopolymerizing acrylic repair resin into the cell culture medium and the establishment of a correlation between the leaching concentrations and in vitro cytotoxicity may provide additional scientific data to the existing knowledge. On the other hand, in routine dental practice, dentures are delivered to the patient as soon as the denture repair is completed. From a biocompatibility point of view, high levels of residual monomer and its leaching mechanism might have an effect on the surrounding tissues when the autopolymerizing acrylic resin is used as the denture repair material.
The aim of this study was to evaluate the effect of post-polymerization heat-treatments on 1) DC, 2) the leaching residual MMA concentration (MMA r ), 3) in vitro cytotoxicity of autopolymerizing repair resin, and to investigate a correlation between (MMA r ) and in vitro cytotoxicity.
The hypothesis was that post-polymerization heat-treatments would increase DC and reduce MMA r while improving biocompatibility of the autopolymerizing acrylic repair resin.
Materials and methods
Stainless steel discs (1 mm thick × 10 mm diameter) were conventionally molded into Type II dental stone (Moldano, Heraus Kulzer, Germany) with a powder/liquid ratio of 100 g/30 mL under aseptic conditions. Flasks were kept under hydraulic pressure (Kavo Elektrotechnisches Werk GmBH, Germany) of 2 atm for 45 min.
Autopolymerizing PMMA based denture repair resin without cadmium (Meliodent Rapid Repair, Self cure Denture Base Material, Heraus Kulzer GmBH&Co. Hanau, Germany) was mixed in accordance with manufacturers’ recommendations, with a powder/liquid ratio of 5/3.5 by weight for 30 s at room temperature (23 ± 2 °C). After 5 min of doughing time, acrylic resin was packed into molds and polymerized by 2 curing methods:
Bench-curing (B): under hydraulic pressure (Kavo Elektrotechnisches Werk GmBH, Germany) of 2.5 atm, at room temperature (23 ± 2 °C) for 15 min.
Hydroflask-curing (H): in a hydroflask (Kavo Elektrotechnisches Werk GmBH, Germany) that contains water at 40 °C with pressure of 2.5 atm for 15 min ( Table 1 ).