الفهرس 
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Abstract
Since dental implants are in direct contact with the bone, they lack the buffering action of periodontal ligament found in natural teeth. So, all the occlusal forces will be transmitted through implant abutment crown complex and they are subjected to overloading. Overloading on dental implants may cause many biomechanical complications like looseness or fracture in one or more structures of dental implants and their superstructure.
In order to control the load on dental implants, many authors recommend the usage of a material with good damping properties to allow dissipation of the forces, and high mechanical properties to withstand the occlusal loads without fracture.
Polyetherketoneketone(PEKK) is a high-performance polymer that is claimed to combine good shock absorption characteristics together with high mechanical properties. However, it should be veneered in areas with high esthetic demands.
Three designs are available when fabricating implant superstructure: screw-retained, cement-retained or screwmentable design. The screwmentable design combines the advantages of both screw and cement-retained restoration. Nevertheless, the properties of a material do not necessarily reflect the actual material performance when tested in different conditions, designs and veneering materials.
So the aims of the present study were to evaluate screwmentable design hybrid abutments made of PEKK and veneered either with zirconia crowns or composite resin crowns in regard to strain generated in implant-bone surrounding marginal bone and the fracture resistance of those implant abutment crown complexes.
The implants were embedded in autopolymerised resin with a modulus of elasticity near the jaw bone. Ti-inserts were screwed to the implant then scanning was done using an extraoral scanner. Designing of the abutment and the crown was done according to the average size of maxillary premolar.
After milling all surfaces were treated according to the manufacturers’ recommendations. The hybrid abutments were cemented to their ti-bases using multilink hybrid abutment cement followed by cementation of the crown to PEKK abutment using dual-cured self-adhesive resin cement.
Strain gauges with small sizes were bonded to the buccal and lingual marginal bone. A load of 100 N were applied vertically on the central fossa. The stresses transmitted to the crestal bone were transferred to computer software. All specimens underwent thermocycling to stimulate the oral conditions then the fracture strength was recorded.
An optical microscope was used to identify the mode of failure. Representative samples were selected to be examined using a scanning electron microscope to detect the fracture origin and the direction of crack propagation.
The results showed that:
1- Composite-PEKK group showed significantly lower microstrain values than Zirconia-PEKK group.
2- Zirconia-PEKK group showed significantly higher fracture resistance than Composite-PEKK samples.
3- There is a strong correlation between the fracture resistance of a material and the amount of force it will transfer to the underlying bone.