الفهرس 
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Abstract
In this study the effect of Mandibular telescopic overdenture retained by zirconia copings and BioHPP copings on the stresses induced around the abutments was evaluated using strain gauge analysis.
According to the material used for constructing the primary and secondary copings of telescopic retainers; two experimental complete mandibular telescopic overdentures were constructed:
Experimental denture 1: Mandibular telescopic overdenture retained by zirconia copings.
Experimental denture 2: Mandibular telescopic overdenture retained by BioHPP copings.
A lower acrylic educational model with removable teeth was used in this study. All teeth were removed. All sockets other than the canines were blocked out, the lingual, buccal and labial slopes of the ridge were reduced (modified) to obtain a U shaped ridge with parallel sides and flat top The two right and left canines were prepared to a height of 5 mm and 10-12 degrees of occlusal taper approximately to support the primary copings of the telescopic crowns.
The modified edentulous lower model and the prepared mandibular telescopic abutments were scanned using S600 ARTI scanner Zirkonzahn, Basic modeling software Zirkonzahn modellier (Fully automated optical structured-light scanner with 2 high-resolution USB 3.0 cameras and a standard transformation language “STL” files was generated. In the STL file virtual superimposition of the right and left canines into their sockets was done leaving, between the socket inner surface and the printed tooth root surface 0.25 mm space simulating the periodontal ligaments. Two grooves 1 mm mesially and distally to each socket were designed parallel to the long axis of the sockets for the installation of the strain gauge. The STL file modifications were done by 360 DPS software. The STL file was sent to the additive manufacturing device; Printer Formlabs to print the model and the abutments utilizing the projection of ultra violet light.
The primary and secondary copings were milled from Zirconia (Bruxzir) for the retainers of experimental denture 1 and from BioHPP for the retainers of experimental denture 2 (breCAM BioHPP, Bredent, Germany) by the milling machine (CNC milling machine, VHF S1 AG,Germany). The milled copings were finished and polished. Primary copings were cemented permanently to the prepared telescopic abutments and two mandibular overdentures were constructed and were placed in their position on the primary copings. Auto-polymerizing acrylic resin was used to pick up the secondary copings on the fitting surface of the dentures.
Four channel strain meter was used to asses and record the strains induced at the distal and mesial aspects of the telescopic abutments. A loading device was used to produce a standardized static load within the physiologic limits of 60 N at the distal aspect of second premolar and central fossae of first molar unilaterally and bilaterally.
Based on the data obtained from this experimental in vitro study, it can be concluded that:
• The use of BioHPP lead to favorable stress distribution and decreases the stresses induced around the abutment teeth supporting and retaining mandibular telescopic overdenture compared to Zirconia.
• Unilateral loading was more traumatic to the abutments compared to bilateral loading as bilateral loading provides wide distribution of stresses.
• Stresses induced at the distal aspect are more than the mesial aspect in both unilateral and bilateral loading.
Recommendation
Thereby using BioHPP coping in telescopic overdentures is recommended when the prognosis of the abutments is questionable. However, further clinical studies are required to confirm this recommendation.