A Comparison of Dimensional Accuracy of Addition Silicone of Different Consistencies with Two Different Spacer Designs - In-vitro Study.
INTRODUCTION
Dimensional accuracy of impression materials is crucial for the production of working casts in Fixed Prosthodontics. The accurate replication of tooth preparations and their arch position requires impression materials that exhibit limited distortion.


METHODS
This study was conducted to comparatively evaluate the dimensional accuracy of additional silicones by comparing two different techniques and spacer designs, by measuring the linear changes in interpreparation distance. The impressions were made from a stainless steel master die simulating a three unit bridge. A total 80 die stone (type IV, Ultrarock) models were obtained from the impressions made using two different parameters. The two different parameters are Multimix and Monophasic technique and different spacer designs.


RESULT
The interpreparation distance of the abutments in the casts was measured using a travelling microscope. Each sample was measured thrice and the mean value was calculated. The results obtained were statistically analysed and the values fall within the clinically acceptable range.


CONCLUSION
The most accurate combination is multimix technique with spacer design which uses less bulk of impression material.
Comparative Evaluation of the Dimensional Accuracy of Different Putty-Wash Techniques Using Additional Silicon Impression Material In Vitro Study
Purpose: The aim of the present study was to evaluate and compare the dimensional accuracy of stone models made by addition silicone impression material using three putty- wash impression techniques: one step putty wash (dual viscosity), two step and modified two step putty wash technique. Material and Methods: A stainless steel master model was constructed simulating a three-unit fixed prosthesis. Fifteen impressions were make of this master model with each impression technique. The first technique, one step dual viscosity was made with putty in the tray and light body on brass dies and impression was make. The second technique, two step putty/wash, the first step using putty on the tray with acrylic spacer on the dies and impression was make then followed by loading wash material on the dies and reseating the first impression in the second step. The third technique, modified two step made using putty/light body as in the first technique then a hole was made at the edge of two abutments in the polymerized impression, then light body was loaded into the tray and over the model and the tray is reseated. Total of forty five impressions were obtained and casts poured using type IV stone. The accuracy of these casts measured using a travelling microscope. Three dimensions diameter, height, and interabutment distances measurements were obtained and statistical analysis was done using paired T-test, One way ANOVA, and Least Significant Difference (LSD) tests. Result: Showed that a significantly larger dimensions of the resultant stone casts of all techniques in comparison to the master model (P
Mechanical behaviour and durability of high volume fly ash cementitious composites
The purpose of this research is to separate different morphological particles of ASTM class F fly ash, and study their effect on mechanical behaviour and durability of high volume cementitious mixtures. In this research wet separation of raw fly ash is carried out, which resulted in three layers of different morphological particles. The first layer of particles float, comprise of about 1-5% of fly ash, is identified as cenospheres or hollow spheres. The second layer of particles is measured to be 55-60% of raw fly ash and consisting of porous spherical and rounded particles rich in Si and Al. The third layer particles is measured to be about 35-40% of raw fly ash. High volume fly ash cementitious composites containing second or third layer particles are tested under compression and bending, highlighting a higher strength and ductility in comparison to cementitious ones containing raw fly ash particles. KEYWORDS. Cementitious matrix; Raw fly ash; Second layer particles; Third layer particles.