Abstract
In the clinical practices of the Crown and Bridge Prosthetics, the developments and improvements achieved in the instruments have made the adoption of the indirect method a day-to-day affair and have enabled the making of fixed restorations with adequate accuracy. It has now become possible to easily make a bridge for the portion where 1 or 2 teeth are missing with the one-piece cast technique and this technique is now beginning to be applied also to the making of a bridge for the portion where 3 or more teeth are missing. However, in the cases where there are restrictions due to the abutment tooth contour or where there are many units, the assembly operation is done, even today when the indirect method is already well established, with the soldering method which is employed in parallel with the checking of fitness accuracy. Also, as in the cases where different kinds of metals are joined together or where the prosthetic techniques are employed using attachments or telescope system, there are many instances in which the reliance on the soldering method is either absolutely necessary or its adoption is preferable.
This soldering method not only uses many kinds of materials and requires complex assembling process but also necessitates a high level of technique. Hence, there are many factors that affect the fitness accuracy of the bridge assembled with the soldering method. For this reason, many a study has been made on the soldering method itself but, so far, we can hardly say as yet that a systematic soldering method has been established.
So, the author has taken up, from among the various factors that affect the fitness accuracy of the soldering method, the following two factors, viz., (1) soldering investment and (2) preheating procedure, and, with the purpose of clarifying the effects that these two factors give to the fitness accuracy, measured the three dimen sional distortions of the retainers. Following are the conclusions obtained as a result of the measurements and examinatios done by the author:
1. With respect to the linear distortion and rotational distortion, the effects resulting from the differences in the types of soldering investments and the preheating procedures used were particularly large in terms of the volume of mesiodistal distortion (ΔX).
2. As to the effects given to the fitness accuracy by the soldering investment and the preheating procedure used, the largest effect was given by the soldering inve stment, which was followed in the intensity of effects by the interacation of soldering investment and preheating procedure, and then by the preheating procedure.
3. It was found that, among the various kinds of soldering investments used at the time of the soldering, the Deguvest L (manufactured by the Degussa, Inc.) with the highest thermal expansion ratio produced good fitness accuracy regardless of the type of the preheating proce dures used.
4. It was found that, regardless of the type of the soldering investments used, the preheating at the tem perature of 150°C was a most important factor in obtain ing good fitness accuracy.
