Retracted: Stress Distribution in Three-Implant- Retained Mandibular Overdentures Using Finite Element Analysis

Background and Aim: Demand for implant-supported overdentures has increased due to the problems of conventional dentures. Despite the high success rate of implants, implant failure remains a major challenge. Implant overload can cause cortical bone loss and im-plant failure. Using finite element analysis (FEA), this study aimed to find the best design and type of attachments causing minimum stress in the alveolar bone. Materials and Methods: The geometrical model of the mandible was produced using computed tomography (CT) data and three ITI implants were placed in the midline and the location of the first premolar teeth. All conditions were simulated using finite element software. Three bar-ball, bar and ball attachments were considered to support the overdenture. Maximum von Mises stress was calculated in the supporting bone in differ-ent overdenture designs. Results: The greatest amount of stress in bone was around the upper thread and the neck of the implant. The ball and the bar-ball attachments applied the most and the least amount of stress to the peri-implant bone, respectively. Maximum stress was applied to the ball attachment in the bar-ball design. The maximum amount of movement was in bar-ball attachment. Conclusion: The bar-design decreased the stability of overdenture, as well as the stress in the peri-implant bone. Ball design increased concentration of stress in bone around the implant and increased the stability of overdenture

Comparison of the Effect of Feldspathic Porcelain and Zirconia on Natural Tooth Wear

Background and Aim : Enamel wear is among the main disadvantages of ceramic restorations. Recently, use of full zirconia crowns without dental porcelain has been suggested. The aim of this study was to compare the effect of feldspathic porcelain and zirconia on the wear of natural teeth .   Materials and Methods : In this experimental study, 22 zirconia specimens were fabricated out of which, 11 specimens were polished and chosen as zirconia specimens while the remaining 11 were used to fabricate porcelain specimens. A total of 22 natural human teeth were also obtained. The natural teeth were photographed by a stereomicroscope in a fixed position and the distance from the cusp tip to a reference point was measured. Next, 11 teeth opposed zirconia and the remaining 11 opposed porcelain specimens in a chewing simulator and subjected to 120,000 masticatory cycles. The teeth were photographed again and the greatest difference between the before and after values ​​ was recorded .   Results : The mean (± standard deviation) wear rate was 153.8±95.68 and 306.3±127.74, in the zirconia and porcelain groups, respectively and the two groups had a statistically significant difference in this respect (p=0.007 ).   Conclusion : The mean wear was significantly lower in teeth opposing zirconia than in those opposing feldspathic porcelain