Double silicone technique: a method of taper measurement for clinical crown preparations
Previous methods used to measure taper of clinical preparations for full coverage crowns had shortcomings, which affected the values obtained and the deductions based on these. To measure taper of teeth prepared to receive crowns using a new method. Two special tube trays of slightly different dimensions were made from a sheet of aluminium. Five stone dies obtained from impressions of five molar teeth prepared in the mouth to receive crowns were used. The smaller tray was used to record the impressions of the dies using a vinyl-polysiloxane putty impression material. Silicone dies were made from the impressions by placing the tray within the larger one and adding a vinyl-polysiloxane light body impression material of a different colour from the putty. Extruding the die with the impression gave five specimen blocks. Each specimen block was then sectioned with a Stanley blade mesiodistally at the mid-proximal, buccal and lingual thirds of the proximal surface. This gave three different cut surfaces from each die. Photographs of these surfaces were taken, and magnified and lines were plotted on them for taper analysis. Fifteen measurements were carried out in all using a protractor. The values obtained were plotted on a table. These values were then compared with those obtained from five similar photographs of the mid-proximal sections using a different magnification. There was no significant difference between the two corresponding sets of taper values obtained. This method presents an accurate way of measuring taper. It eliminates the error that usually arises from taking photographs of a 3-dimensional object and does not depend on the angle of the photograph or the distance of the object from the lens. It also provides the added advantage of giving taper values at different sections of a preparation and this gives an idea of the true retention of the crown. This was not possible with previous methods.