Effect of Ultrasonic Bath Immersion on Physical Properties of an MMA-based Orthodontic Base-plate Material

Abstract

Introduction: Self-cured Methyl Methacrylate (MMA)-based acrylic resin is commonly used for fabricating removable orthodontic appliances. However, residual monomer can leach out and cause mucosal irritation. Furthermore, it can reduce the physical properties of the appliance. Using ultrasonic immersion with ethanol solution is an effective method to reduce the level of residual monomer. However, its effect on the physical properties has not been reported. Aim: To investigate the effect of ultrasonic bath immersion in water or ethanol solutions on the physical properties of an MMA-based orthodontic base-plate material. Materials and Methods: Eighty rectangular and 80 disc-shaped specimens of an MMA-based orthodontic base-plate material (Orthoplast, Vertex-Dental) were prepared in stainless steel moulds as per the manufacturer’s instructions and randomly divided into eight groups: Group I, untreated control; Groups II and III, immersed in 25°C water for 24 and 72 hours respectively; Groups IV-VIII, immersed 10 minutes in an ultrasonic bath at 50°C of water, 10%, 20%, 30% and 40% ethanol, respectively. The flexural strength and flexural modulus were measured with a universal testing machine and microhardness was measured with a microhardness indenter. Water sorption and solubility were measured per ISO 20795-2. The data were analysed by one-way ANOVA, followed by Tukey’s HSD test (p<0.05). Results: The flexural strengths of Groups IV and VIII were significantly higher compared with Group I, while the others were not significantly different from Group I. For flexural modulus, Groups II, IV, VI, VII, VIII were significantly higher than Group I, however, Groups III and V were not significantly different. There were no significant differences in microhardness between the groups. The water sorption of Groups VI, VII, and VIII were significantly lower compared with Group I while that of the others were not significantly different. The water solubility of the experimental Groups were not significantly different compared with that of Group I, except for that of Group IV, which was higher. Conclusion: An ultrasonic bath immersion with either 50°C water or 50°C ethanol (10%, 20%, 30% and 40%) resulted in improved or unaltered physical properties of an MMA-based orthodontic base-plate material.

Keywords

• acrylic resin
• flexural strength
• microhardness
• water solubility
• water sorption