[Evaluation of the accuracy of three common methods in determining hinge axis points]

Determining the true hinge axis point in order to transfer the orientation relation to articulator is of great importance in reducing occlusal errors. Because of the high cost and time consuming nature of kinematic method in determining hinge axis point, different arbitrary methods have been introduced which rendered controversial results throughout literature. The aim of this study was to evaluate some arbitrary methods for determining hinge axis point and to introduce a precise arbitrary method which has the least distance from kinematic point. This cross-sectional study was performed sequentially on 40 subjects without any TM-disorders with an age range of 21-37. At first true hinge axis point was determined by DENAR hinge axis locator and marked bilateraly on the skin. Then 3 arbitrary points [Beyron, Gysi, Tetruck and Laundeen] was also determined by a flexible ruler and marked on the skin. The distance between the arbitrary points and kinematic point was measured and the data were charted and subjected to Repeated ANOVA and Paired sample test. According to the results of the study, Beyron method was significantly more accurate than the other two methods. 88.88% of Beyron points were within 3 mm distance from kinematic point, but the other points didn't show any significant difference with each other. It is concluded that Beyron method has a high accuracy as an arbitrary method and because of its proximity to the true point, application of this method is recommended

[Die dimension variation in three impression taking methods]

Dimensional stability of die plays very important role in fixed prosthodontic treatments. Different impression taking techniques may be the main source of die dimensional changes. However, in this regards, contradictory results have been reported in the literature. This study was designed to assess the dimensional accuracy of master casts prepared by means of three impression techniques using Speedex silicone impression material. This quasi-experimental study was carried out using laboratory stainless steel master models that contained three complete crown abutment preparations. The one-step, two-step with and without relief methods were the three putty-wash impression techniques used in this investigation. Fifteen impressions were taken for each technique and velmix casting material was used to prepare stone models. Profile Projector and micrometer were used to measure the final die's height, diameter and different dimensions in order to compare them with laboratory model. ANOVA and paired samples t-test were used for data analyses. The two step technique with spacer was the most accurate method with zero LC distance [distance between left and middle abutment]. However, in the other two techniques, the mean difference was 0.22 [ +/- 0.01], [P<0.05]. Regarding the distances between abutments, there was no significant difference when comparing one-stage and two-stage technique without spacer. Although, the height of abutment was more accurate in two-stage technique without spacer than one-stage. The two-step technique with spacer was the most accurate technique when using Speedex impression material