Influence of different fillers on the properties of an experimental vinyl polysiloxane.
The aim of the study was to evaluate the effect of the incorporation of different fillers on an experimental vinyl polysiloxane (VPS) at two different concentrations, 20% and 40%. Different fillers were added to an experimental VPS. The study was developed in two stages: (i) incorporation of fillers in different concentrations: (a) 20 wt% fillers, and (b) 40 wt%. The fillers were added to experimental VPS and mixed with a speed mixer; (ii) characterization of experimental VPS; after the base paste and catalyst paste were mixed, the experimental VPS was used to make specimens specifically for each test, which were stored at 23°C for 24 hours. The tests were designed according to the specific standardization for the analysis of tensile strength, detail reproduction, Shore A hardness, and elastic recovery. For analysis of filler size pattern, scanning electron microscopy at 1500× magnification was used. The aerosil OX-50 40% (AE), and pure aluminum hydroxide 40% (PAH) groups presented the highest tensile strength and Shore A hardness values. However, those were the only groups that did not present continuous detail reproduction of an intersection of 20 μm line. The elastic recovery was not statistically significant. The undesirable characteristics of VPS (lowest Shore A hardness and tensile strength) were observed when it was added to the composition of acrylic polymer (AP) and fiberglass (FG) in both concentrations, 20% and 40%. In groups AE and PAH, agglomerates of nanofillers were shown in SEM micrography, while the other groups presented different shapes and fillers sizes.
Factors affecting the accuracy of elastometric impression materials.
OBJECTIVES
The purpose of this study was to evaluate the effects of (1) various impression materials, (2) different storage times and (3) the proportion of inorganic filler on the accuracy and stability of elastometric impression materials.


METHODS
The impression materials studied included three alginate impression materials (Algiace Z, CAVEX and Jeltrate), five commercial silicone impression materials (Aquasil, Exaflex regular type, Express, Coltex fine and Rapid liner) and two experimental silicone impression materials designed for this study (KE106A and KE106B). Impressions were made of 10 metal dies that mimicked prepared crowns. After an impression was taken, dental stone was immediately poured into the alginate impressions, while the silicone impressions was poured 30 min later and waited for 1 h for setting. The second and third stone dies were made 1 and 24 h later, respectively. The diameters of the occlusal surfaces of the metal dies and stone casts were determined using photographs of the surfaces taken with a Kodak DC 290 digital camera. The pictures were then measured using a photomicrograph digitized integration system to calculate any discrepancy. Because each impression was used to make three rounds of stone dies, two-factor mixed factorial ANOVA was used to evaluate the effect of materials and storage time on the accuracy of the stone casts. The simple effects analysis, combined with multiple comparisons considering the per family type I error rate, was performed following confirmation that an interaction between the two factors was significant.


RESULTS
The results showed that: (1) there was a significant interaction effect between materials and storage times on the accuracy of the impressions. (2) Two addition type silicone materials, Aquasil and Exaflex, had the greatest accuracy and stability. (3) The experimental material KE106A had the least accuracy in the first and second rounds and the alginate impression material CAVEX had the least accuracy in the third round. (4) The stabilities of CAVEX and Jeltrate were the least consistent of the 10 materials and decreased significantly with storage time. (5) When the experimental material had a low proportion of filler (KE106A), there was a significantly greater dimensional discrepancy compared to the same material with a higher proportion of filler (KE106B).


CONCLUSIONS
The accuracies varied among the 10 impression materials over three rounds. Of all the materials, the addition type silicone materials, Aquasil and Exaflex, had relatively greater accuracy and stability. The discrepancy of the alginate impression materials increased with storage time. The large loading of filler showed less discrepancy.
A new technique for decalcifying thin ground specimens of adult human enamel.
Abstract A new decalcification technique is described for the recovery of the organic matrix from thin ground specimens of adult human enamel. An aqueous solution of chromium sulphate which has both fixing and decalcifying properties is used. The results are compared with those obtained when an adjacent part of the same ground specimen was decalcified with the hydrochloric acid solution of Brain (1962). Dependent on slight differences in pH of the chromium solutions, two different appearances of the recovered matrix are obtained: above pH $ 3.2 a structural matrix which is positively birefringent at refractive index 1.55, whereas below this pH a matrix which is homogeneous and non-birefringent and almost similar to the matrix obtained after hydrochloric acid decalcification. The two types of matrices are discussed and the one favoured on morphological grounds, the chromium sulphate structural matrix, is related to present concepts of the structure of the native enamel matrix.