الفهرس 
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Abstract
Completely edentulous mandibular cast was prototyped using 3D printing technology.
This was done via scanning an educational model and designing an STL file including all the details required; implants beds, mucosal space for mucosa simulation and vertical slots for strain gauges which were 1mm distal and mesial to each implant.
Mucosa simulation was planned to mimic the viscoelasticity of the fibrous mucoperiostium by prototyping a mucosa simulator over the scanned model after removing 2 mm space over the crest of the ridge and making interlocking grooves for stabilization of the removable mucosa.
Printing was ordered and mucosa simulation was done by an addition silicon rubber base utilizing the printed mucosa simulator. Two implants were placed in their beds, Ti inserts and scan bodies were seated over them. Scanning was done to transfer the three-dimensional implant position to the CAD software.
A bar joint design was selected from the software library. After finishing and confirmation of the proper design, STL files from the CAD software were transferred to the CAM software and milling was done using a 5-axis milling machine.
Six bars were milled from BioHPP blank and six bars from Co-Cr blank. In this study two evaluation methods were done: 1-Passivity of fit 2- Micro strain analysis.
For passivity of fit evaluation, cementation of one side of the bar was done and the other side was inspected under a stereomicroscope at five equidistant points at the buccal and distal sides. Cementation of the other side was done after the evaluation.
An overdenture was constructed, and two bar clips were picked up via an auto polymerizing acrylic resin.
A universal testing machine was used for applying vertical loads (100 N) using a T-shaped load applicator to fit on the denture teeth bilaterally between the distal aspect of lower second premolar and mesial aspect of lower first molar for five times on each bar.
After fifteen minutes the same load was applied unilaterally for five times on the right side to represent the working side using, I bar shaped load applicator and the mean micro strain values of the five load applications in the loaded and unloaded sides were taken. The point of load application was selected at the site of the central occlusal fossa of the first molar and notched in the acrylic resin surface of the record block with a diamond bur.
Results showed that there was significant difference between the two bar materials in the amount of vertical gap as BioHPP showed less amount than Co-Cr but both results were within the accepted range.
Regarding bilateral stresses, there was no significant difference between mesial and distal micro strains in BioHPP bars while there was a significant difference between mesial and distal micro strains in Co-Cr bars. Both groups showed no significant difference when compared to each other’s.
Upon applying unilateral loads, there was no significant difference between both groups in micro strains in the loaded and in the unloaded side.
Conclusion
Within the limitations of this in-vitro study, it could be concluded that:
1- BioHPP and Cobalt-Chromium hybrid milled bars had an acceptable passive fit, however BioHPP bar showed better passive fit than Cobalt-Chromium.
2- No significant statistical differences were found between BioHPP and Cobalt-Chromium milled bars regarding micro strains developed in the supporting structures either upon bilateral or unilateral loading.