الفهرس 
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Abstract
this experimental study was carried out to assess and compare the stress distribution induced by Cobalt chrome versus polyether ketone ketone materials (PEKK) used as extra coronal attachment of removable partial denture in lower Kennedy class II.
The use of computer-aided design and manufacturing (CAD/CAM) technology in fabrication of RPDs is an alternative for the conventional casting technique. Digital surveying of the scanned casts and subsequent designing and production of restorations by milling technology allow easy fabrication. Also CAD/CAM technology prevents operator variations and improves quality control in dental laboratories
An educational stone model of lower kenndy class II arch was selected in which the last abutment was the second premolar. Then was scanned using the desktop scanner. On the virtual cast, preparation of the first and second premolar were done to receive two segmented crowns, also rest seat preparation on the second premolar and first molar on the contra lateral side to receive double aker clasp and preparations for slots of strain gauges were done. The first one (slot 1) was prepared 1mm distal to the socket of the second premolar and the second slot (slot II) was prepared 1 cm away from the first one. A space for mucosa stimulating material was created on the virtual cast. A two typically acrylic models were 3D printed, mucosa simulating material was injected into the plastic index then the index was adapted to the acrylic cast.
Digital designing of two segmented crowns of the first and second premolar was done and selection of extra coronal attachment were done on the software. Crowns and attachments were milled out of PEKK and Cobalt chrome materials.
For each model, eight metallic frameworks were fabricated through a conventional lost wax technique.
Strain gauges were fixed in their planned position on the printed model. After that strain values were recorded distal to the last abutment and at the distal part of the ridge by the strain gauge device.
The load was applied unilateral and bilateral. The load applied was 100 N at constant rate 0.5mm/min. The data obtained were statistically analysed by using unpaired t test to study the difference between group mean values (P< 0.05). Microstrain readings for each group were recorded and statistically analysed.
The analysed data revealed a statistical significant difference of the micro strain values of the two studied groups both during unilateral and bilateral loading and the higher values were recorded for the cobalt chrome group.