An evaluation of acrylic complete dentures using the discrimination of elastic bodies or viscous fluids.

The discrimination test on elastic bodies and viscous fluids was carried out for 16 young dentists and 13 denture patients, because it is considered that dentures affect the ability to discriminate food. The materials used in this test were six thermoplastic polyurethane elastomers with compressive elastic moduli from 11.8 to 62.0 MPa and six silicone impression pastes with coefficients of viscosity from 1.77 x 10(-2) to 23.0 x 10(-2) MPa x s. The subjects (dentists and patients) discriminated the materials based on differences in elastic moduli or coefficients of viscosity by chewing the materials for 10 s. From these experiments, it became clear that the ability to discriminate between elastic bodies or viscous fluids upon chewing, of subjects who use complete-dentures in combination with their own teeth and either an upper or lower complete-denture (S/D) or who have a set of complete-dentures in both jaws (D/D) decreased considerably, compared with that of subjects who have their own teeth in upper and lower jaws (S/S). The ratios of S/D and D/D against S/S were 0.5 and 0.3, respectively. This indicates that with an increase in the area occupied by dentures in the oral cavity, the value of the differential threshold increased markedly and made food discrimination more difficult. In addition, discriminating viscous fluids was 2.4 to 3.1 times more difficult compared with the discrimination of elastic bodies, independent of denture placement.

Consistency of alginate impression materials and their evaluation.

The apparent viscosity of 17 commercially available alginate impression materials was examined using a rheometer. Each material was mixed for 15 s at a temperature of 23 +/- 0.5 degrees C, using a specially designed alginate mixing instrument. The water powder ratio was determined according to the manufacturer's instruction. The mixed alginate paste was immediately transferred to the sample stage of the rheometer or to a commercial perforated metal tray. Over the same time scale adjusted according to the rheometer test, a discrimination test (subjective test) was performed by 16 young dentists (each with more than 4 years experience). The apparent viscosity of all materials rose as a function of time after mixing and most of the material had properties similar to pseudoplastic fluids. The relationship between the results of the subjective test and the experimental values (objective test) with the rheometer was expressed in a quadratic equation. The maximum value of the apparent viscosity derived from this result was 1.52 x 10(3) Pa.s.