Assessment of the interaction of Portland cement-based materials with blood and tissue fluids using an animal model.

Portland cement used in the construction industry improves its properties when wet. Since most dental materials are used in a moist environment, Portland cement has been developed for use in dentistry. The first generation material is mineral trioxide aggregate (MTA), used in surgical procedures, thus in contact with blood. The aim of this study was to compare the setting of MTA in vitro and in vivo in contact with blood by subcutaneous implantation in rats. The tissue reaction to the material was also investigated. ProRoot MTA (Dentsply) was implanted in the subcutaneous tissues of Sprague-Dawley rats in opposite flanks and left in situ for 3 months. Furthermore the material was also stored in physiological solution in vitro. At the end of the incubation time, tissue histology and material characterization were performed. Surface assessment showed the formation of calcium carbonate for both environments. The bismuth was evident in the tissues thus showing heavy element contamination of the animal specimen. The tissue histology showed a chronic inflammatory cell infiltrate associated with the MTA. MTA interacts with the host tissues and causes a chronic inflammatory reaction when implanted subcutaneously. Hydration in vivo proceeds similarly to the in vitro model with some differences particularly in the bismuth oxide leaching patterns.

Methyl methacrylate monomer-polymer equilibrium in solid polymer.

OBJECTIVE: Poly(methyl methacrylate) (PMMA) is commonly processed in dentistry by thermally initiating the free-radical polymerization of methyl methacrylate (MMA). Residual MMA, a tissue irritant, is a concern. The concentration of MMA ([MMA]) versus time and temperature was studied to identify optimum processing conditions. MATERIALS AND METHODS: One hundred milligram portions of plain and dental PMMA powders were incubated (10-170 degrees C, 1-384 h), with and without 6.0 microL MMA added. After incubation, [MMA] was determined by GC. RESULTS: For plain PMMA alone, equilibrium was attained in about 100 h. The equilibrium data for log[MMA] versus 1/T was better fitted by a quadratic than a straight line, and formed an upper bound to the values of [MMA] when PMMA was incubated with MMA at temperatures > approximately 120 degrees C. The response surface for [MMA] versus log(time) and reciprocal temperature was fitted. An 'overshoot' in the equilibration process was identified, and postulated to be due to a rapidly formed intermediate of unknown chemistry. SIGNIFICANCE: Minimization of the residual MMA in acrylic denture bases prepared by processing a mixture of PMMA and MMA is important for reasons of mechanical properties and irritancy. The response surface mapped here allows direct identification of the optimum processing conditions.

Minimization of the inevitable residual monomer in denture base acrylic.

OBJECTIVES: Residual monomer ([MMA](R)) in denture base acrylic continues to be of concern. The response surface of concentration vs. time and temperature for the equilibration of methyl methacrylate (MMA) and its polymer (PMMA) allows a prediction of the time to the minimum at any temperature for a closed system. It was the purpose here to determine whether this prediction applies to normal denture base processing, and whether optimum conditions could be identified. METHODS: Denture bases were processed following normal laboratory procedures, including pre-cure for 3 h at 70 degrees C for all tests. Commercial powder and liquid were used at either 95 or 100 degrees C, or a plain PMMA powder and the same liquid at 95 degrees C, for times ranging from 5 to 192 h. Residual MMA was determined by gas chromatography. RESULTS: [MMA](R) decreased steadily from approximately 0.25% to as low as approximately 0.07% with increasing time at temperature, but did not approach equilibrium. The rate of diffusive loss of MMA appears to exceed the rate of depolymerization. SIGNIFICANCE: Residual monomer is inevitable for all PMMA-based products no matter what the curing conditions are. However, extended time at high temperature can allow low values to be attained, and the time allowed can compensate for processing temperatures somewhat lower than the ordinarily recommended 100 degrees C. It is suggested that overnight processing at 95 degrees C should be adopted to minimize [MMA](R) and save energy. This result is of importance for work at high altitude.