الفهرس 
IntroductionOnly 14 pages are availabe for public view
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Abstract
In Vitro study (Part one)This in vitro study was conducted to assess the accuracy of the marginal fit of two-implant supported overdenture bars fabricated with selective laser melting technology, utilizing polyvinyl siloxane and vinyl siloxane ether impression materials and different impression techniques.Two implants with multi-unit abutments were inserted in a 3D-printed mandibular model and used as a reference model, then duplicated into a stone cast to fabricate custom trays for obtaining impressions with polyvinyl siloxane and vinyl siloxane ether impression materials, using both open and closed-tray techniques. This resulted in a total of 4 groups, each with 8 specimens.The impressions were poured, scanned, and 32 cobalt-chromium bars were fabricated using selective laser melting technology. Each bar was screwed onto one abutment of the reference model, and a standardized digital periapical radiograph of the opposite unscrewed side was taken, this process was repeated to the other abutment.The vertical misfit was measured at three predetermined positions on the unscrewed side, and six values per bar were recorded. The mean gap distance was measured and the data were subjected to statistical analysis.The present study found that the open-tray technique with VSXE impression material may offer improved accuracy for obtaining impressions for two implant-supported bars. Additionally, the open tray technique groups showed potentially better marginal fit than the closed tray groups, however, further clinical research is required to confirm these findings.In Vivo study (Part two)
The study aimed to investigate the influence of different bar materials, namely cobalt-chromium (Co-Cr) and polyetheretherketone (PEEK), on marginal bone loss around implants retaining mandibular overdentures. This randomized clinical trial adhered strictly to the Declaration of Helsinki and CONSORT guidelines and obtained ethical approval from the Ethical Committee of the Faculty of Dentistry at Minia University. The study included twenty completely edentulous patients who met specific inclusion criteria.Patients were randomly allocated into two equal groups to receive maxillary complete dentures and bar-retained mandibular implant overdentures as follows:
The Co-Cr group (n=10): Received mandibular implant-assisted overdentures retained by cobalt-chromium bars.
The PEEK group (n=10): Received mandibular implant-assisted overdentures retained by PEEK bars.Conventional complete dentures were constructed and delivered to the patients following a comprehensive assessment that included a full medical history, extraoral and intraoral examinations, and pre-operative CBCT scans. Implant surgeries were performed, with each patient receiving two implants in the lateral/canine area bilaterally.After 3 months, the submerged implants were uncovered, and multi-unit abutments were secured. Subsequently, Co-Cr and PEEK bars were fabricated and fastened to the multi-unit abutments according to the respective groups. The bar clips were picked within the fitting surface of the lower denture. Overdentures were then finished, polished and delivered to the patients, and the amount of marginal bone loss around the implants was measured radiographically at 6 and 12 months using a digital periapical parallel radiographic technique
The results of this study demonstrated that, after 6 months of prosthetic loading, the marginal bone loss around Co-Cr bars was slightly higher than that of PEEK bars. However, this difference was not statistically significant (P > 0.05). On the other hand, after 12 months of prosthetic loading, the Co-Cr bar group showed significantly higher bone loss compared to the PEEK group (P < 0.05)