Sorption and solubility of denture base acrylic : a comparative surface treatment study

Abstract

Background: Heat-cured PMMA is one of the most frequently used materials in the manufacture of removable dentures. Dentures fabricated with heat-cured material are soluble and able to absorb and release substances. These factors all directly affect the longevity and performance of the prosthesis. Due to laboratory circumstances and time constraints, the polishing procedure may be neglected by technicians. Dentures may also be adjusted by dentists during the fitting procedure while the patient is in the chair to ensure the optimum fit and comfort of the prosthesis. A lack of knowledge, time or equipment may result in the altered surface not being re-polished. Furthermore, dentures may also lose their polished layer as a result of masticatory erosion associated with prolonged use, or because of the actions of patients who alter their prosthesis themselves. These factors increase the surface roughness of the denture and may result in an increase in the sorption and solubility of the denture base material. Aim: The aim of this study was to compare the sorption and solubility rates of surface-treated, heat-cured acrylic specimens against those of untreated acrylic specimens, soaked in distilled water and artificial saliva. Methods: Altogether 90 specimens were prepared according to the ISO Standard 20795-1: 2013 (E) to test for sorption and solubility of Type 1, Class 1 denture base material. The specimens underwent a surface treatment procedure in the form of mechanical polishing, or the application of Optiglaze™ light-cured varnish and were soaked in grade two distilled water or an artificial saliva solution. The specimens were conditioned to a constant mass, after which the volume of each specimen was calculated. The specimens were soaked for a seven-day period, after which they were reconditioned to a constant mass. The sorption and solubility of the specimens were calculated as recommended by ISO Standard 20795-1: 2013 (E). Results: The data was analysed using the One- and Two-Way analysis of variance, with the Tukey-Kramer multiple comparison test being used to indicate significant differences among the means of the different sample groups. Mechanical polishing proved to be the most effective surface treatment for reducing solubility, with the specimens recording a mean solubility value of 0.0909 μg/mm3. The application of Optiglaze™ light cure varnish proved to be the most effective surface treatment for reducing sorption, with the specimens recording a mean sorption value of 21,5355 μg/mm3. The results also indicated that the composition of the medium affects the sorption and solubility of Vertex™ Rapid Simplified heat-cured acrylic: the specimens soaked in the artificial saliva solution recorded lower mean sorption and solubility values than the specimens soaked in distilled water. Conclusion: The results indicated that both surface treatments, and the composition of the medium in which the specimens were submersed, were successful in reducing either their sorption or solubility levels. The analysis of the results suggests that overall, mechanical polishing was the most effective surface treatment procedure and that artificial saliva was the medium in which the specimens recorded the lowest sorption and solubility values. The results from this study and a review of comparable literature support the suggestion that dentures should be polished by a trained professional at calculated intervals. The application of a light-cured varnish to denture base material may be considered as an alternative to mechanical polishing or used in conjunction to produce optimum results. The submersion of the specimens in an artificial saliva solution imitated the clinical situation of a polished denture in the oral cavity and suggests that the molecular structure of the liquid affects the rate of sorption and solubility of heat-cured denture base material.