منیره نیلی احمدآبادی

Short implants are used in the posterior mandible where the bone is insufficient. In these cases, the clinical crown is often made to match the level of the occlusal. As a result, the length of the lever arm and the stress of the implant complex, as well as the surrounding bone, are increased leading to the biomechanical problems. The purpose of this study was to evaluate dynamically the effect of increasing crown height space on the maximum stress induced in the abutment screw and the surrounding bone of the short dental implants in the posterior mandible using finite element analysis. Mandibular bone resorption in the posterior region, second premolar with three-crown lengths of 8.8, 11.2, and 13.6, two implants of 4 and 8 mm, two abutments, and two abutment screws were modeled using Solidworks software in this laboratory experimental study. In addition, Abacus software was used to the dynamic reconstruction of screw tightening and external load at an angle of 75.8 degrees with the occlusal plane. The stress values were calculated for the screw, abutment, fixture, and bone. In both 4- and 8-mm implants, the increased vertical height of the crown decreased the stress on the abutment screw and increased the stress on the abutment and fixture. For a 4-mm implant, the stress to the abutment screw at all three heights was less than 8 mm. At all heights, the stress values to the abutment and the fixture were more than 8mm for the 4-mm implant. The increased vertical height of the crown resulted in an increase in compressive and tensile stress in the surrounding bone for both 4-mm and 8-mm implants. The magnitude of these stresses in the 4-mm implant was more than 8 mm.   Increased vertical height of the crown and crown-to-implant ratio reduced the stress on abutment screws as the weakest member of the implant. However, it probably increased the failure due to fatigue in the abutment and fixture as well as bone resorption.

  Considering the increasing rate of dental implant use, many general dental practitioners ask the question which system has the highest frequency of use and satisfaction rate so that they can decide on the use of one superior system, using other dentists' experience. This study evaluated dentists' experience in relation to the various implant systems. The present descriptive-analytical/cross-sectional study was carried out with the use of a questionnaire with 40 questions on different aspects of implant treatment. The questionnaire was handed in to 100 dental practitioners working in Isfahan, consisting of 51 general dentists, 21 prosthodontists and 28 oral and maxillofacial surgeons and periodontists. Data were analyzed with Chi-squared and Kruskal-Wallis tests (α = 0.05). There was no significant difference between general dentists and specialists in the selection of different system (p value = 0.417) and DIO system was the most frequently used system. Prosthodontists were significantly more successful in satisfying patients compared to surgeons, and surgeon were more successful than general dentists (p value = 0.02). There was no significant difference between dentists in relation to the most common failure reason (p value = 0.817); 88% of dentists believed that financial problem was the main reason in implant treatment rejection by the patients. Most of the dentists (65%) believed that patient satisfaction rate was moderate. DIO was the dominant implant system used due to its more favorable cost, quality and services

The most common impression techniques used in fixed prosthodontics are one and two step with spacer. Sometimes impression details are not registered and as a result, use of second wash would be necessary. The aim of this study was to evaluate the effect of second wash of speedex impression material on dimensional accuracy of stone model in one and two stage impression techniques. In this experimental study, two metallic dies (with and without undercut) were used. Impressions were made using four techniques (one step, one step with second wash, two steps, and two steps with second wash). For each technique the impression taking was repeated 15 times and totally 60 impressions were provided. Impressions were poured with Vel-mix stone. Height and diameter for each die as well as distances between the dies were measured by sterio microscope and data were analyzed using ANOVA and T-tests.This study showed that in all four techniques height of dies with undercut was increased, but in the two step technique, the difference was not statistically significant (p= 0.282). In one step and two step with second wash technique, heights of dies without undercut were increased, while in the other two techniques they were decreased. In two step technique, diameters of the two dies were increased, while in other techniques they were decreased. In the two step technique, the distance between the dies was decreased, mean while other techniques did not show any increase and the differences were not statistically significant (p<0.05).The two step technique was more accurate than the other techniques. The one step with second wash technique was more accurate than one step technique. The use of two step with second wash technique is not recommended in fixed partial dentures, but in single crowns it can be used only with special considerations.