الفهرس 
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Abstract
This in-vitro study was performed on 3D printed models simulating a lower edentulous mandible with two intra-foraminal implants splinted by milled bars. The effects of CAD/CAM milled zirconia and Pekkton bar attachments on the retention of implant-assisted mandibular overdentures and the fracture resistance of the bar attachments were compared.
An educational edentulous mandibular stone model was scanned for virtualization of the model and creation of a digital model. Space for the mucosa simulating material and creation of two implant beds with 20 mm distance between them were designed using the software.
Sixteen models and trays were printed using photopolymer resin material in a liquid crystal display (LCD) printer. Mucosa simulation was done by an addition silicon rubber base utilizing the printed tray to mimic the viscoelasticity of the mucoperiostium.
Two implant analogues were inserted in the corresponding implant beds to which Ti-bases were screwed. Scanning was done to transfer the three-dimensional implant position to the CAD software.
A bar joint design was selected from the software library. STL files from the CAD software were transferred to the CAM software and milling was done using a 5-axis milling machine. Eight bars were milled from zirconia blank and eight bars from Pekkton Ivory blank. The bars were cemented to the Ti-bases using resin cement following sandblasting and application of primers.
To create a duplicate model for the laboratory steps open top impressions of the models were made by single step impression technique. Sixteen overdentures were constructed, and two bar clips for each were picked up.
The overdentures were classified into two equal groups according to the retaining bar material. group 1 overdentures were retained with zirconia bars and group 2 with Pekkton bars.
For simulating the clinical conditions of six and twelve months of overdenture use, Chewing Simulator was used to apply the calculated dynamic cyclic loading to each implant-assisted mandibular overdenture for 125,000 and 250,000 cycles. Also assuming four daily removals and insertions each overdenture was pulled out manually for 720 and 1440 cycles.
Universal Testing Machine was used to apply vertical dislodging forces at the center of the overdentures to assess their retentive force. The tests were carried out before and after a simulated six and twelve months intervals. Data were collected, tabulated and statistically analyzed.
The Universal Testing Machine was also used to apply vertical compressive load at the center of the retaining bars using load applicator to assess the fracture resistance of each bar material. Data were collected, tabulated and statistically analyzed.
A primary higher but insignificant retentive force of Pekkton bars was evident compared to zirconia bars.
Although a decrease in the retentive force of both types of bars was evident by time. However, a significant less decrease of the retentive force of Pekkton bars was evident compared to zirconia bars.
The results also revealed a statistically significant higher fracture resistance of the milled zirconia bars compared to the milled Pekkton bars.