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PubMed2002

Effect of a new tension system, used in acrylic resin flasking, on the dimensional stability of denture bases.

STATEMENT OF PROBLEM: The pressure of final closure may be released when the flask is removed from the mechanical or pneumatic press and placed in the spring clamp. This release in pressure may result in dimensional changes that distort the denture base. PURPOSE: The purpose of this study was to investigate differences between the dimensional stability of standardized simulated denture bases processed by traditional moist heat-polymerization and those processed by use of a new tension system. MATERIAL AND METHODS: A metal master die was fabricated to simulate an edentulous maxillary arch without irregularities in the alveolar ridge walls. A silicone mold of this metallic die was prepared, and 40 stone casts were formed from the mold with type III dental stone. The casts were randomly assigned to 4 test groups (A-D) of 10 specimens each. A uniform denture base pattern was made on each stone cast with a 1.5-mm thickness of base-plate wax, measured with a caliper. The patterns were invested for traditional hot water processing. A polymethyl methacrylate dough was prepared and packed for processing. The flasks in groups A and B were closed with the traditional pressure technique and placed in spring clamps after final closure. The flasks in groups C and D were pressed between the metallic plates of the new tension system after the final closure. The group A and C flasks were immediately immersed in the water processing unit at room temperature (25 degrees +/- 2 degrees C). The unit was programmed to raise the temperature to 74 degrees C over 1 hour, and then maintained the temperature at 74 degrees C for 8 hours. The group B and D flasks were bench stored at room temperature (25 degrees +/- 2 degrees C) for 6 hours and were then subjected to the same moist heat polymerization conditions as groups A and C. All processed dentures were bench cooled for 3 hours. After recovery from the flasks, the base-cast sets were transversally sectioned into 3 parts (corresponding to 3 zones): (1) distal of the canines, (2) mesial of the first molars, and (3) mesial of the posterior palate). These areas had been previously established and standardized by use of a pattern denture in the sawing device to determine the sections in each base-cast set. Base-cast gaps were measured at 5 predetermined points on each section with an optical micrometer that had a tolerance of 0.001 mm. Collected data were analyzed with analysis of variance and Tukey's test. RESULTS: Denture bases processed with the new tension system exhibited significantly better base adaptation than those processed with traditional acrylic resin packing. Immediately after polymerization (Groups A and C), mean dimensional change values were 0.213 +/- 0.055 mm for the traditional packing technique and 0.173 +/- 0.050 mm for new tension system. After delayed polymerization (Groups B and D), the values were 0.216 +/- 0.074 mm for the traditional packing technique and 0.164 +/- 0.032 mm for new tension system. With both techniques, dimensional changes in the posterior palatal zone were greater (conventional = 0.286 +/- 0.038 mm; new system = 0.214 +/- 0.024 mm) than those elsewhere on the base-cast set. CONCLUSION: Within the limitations of this study, the new tension packing system was associated with decreased dimensional changes in the simulated maxillary denture bases processed with heat-polymerization.

Acrylic Resins↗
PubMed1992

Determination of doughing and manipulation times of acrylic resin denture base material and a proposal for a specification for a packing plasticity test.

The doughing and manipulation times of a range of acrylic resin denture base polymers are presented in this study. The methods evaluated were: (i) probing following American Society for testing materials specification, F451-76; (ii) hole penetration following British Standard, 2487: 1989 and International Standard Organisation, 1567: 1988; (iii) probing using a penetrometer employing a 20 g load and 1 mm diameter needle; (iv) oscillating rheometer apparatus used at a fixed frequency of 1 Hz by sweeping through a programmed linear increase in strain. The results indicate that each of the methods is capable of evaluating the doughing and manipulation times of the unprocessed polymers. It is concluded that the Bohlin VOR Oscillating Rheometer is excellent for investigating the flow properties of denture base polymers but it cannot be recommended for a standards specification test because of the high cost. The penetrometer method has been shown to be superior to the current standards specification test and would be ideal for a packing plasticity standards test.

Denture Bases↗
PubMed1996

Fracture surface characteristics of damaged acrylic-resin-based dentures as analysed by SEM-replica technique.

The aim of this study was to determine some characteristics of the fracture surface of an acrylic resin denture. For this purpose, five complete upper dentures and two complete lower dentures that had fractured into two pieces were collected, the fracture surfaces were duplicated with the SEM-replica technique and duplicated surfaces were investigated with a scanning electron microscope. The reproducibility of the SEM-replica technique was tested by comparing one original sample surface and its replica surface of the same area. The SEM-photomicrographs revealed a series of fine striations in the fracture surfaces of both the upper and lower dentures perpendicularly orientated to the direction of the fracture propagation. A smooth area on the fracture surface was assumed to indicate a slow fatigue fracture process, and a rougher area fracturing with increased velocity.

Dental Restoration Failure↗