الفهرس 
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Abstract
The current study tried to compare in-vitro between the retention force of two retainers system for implant supported mandibular overdenture. An acrylic resin model was produced from a
commercially available mould and two identical implants were inserted at the canine area bilaterally. The implant abutments were tightened and two telescopic crowns were constructed from
Co-Cr alloy. The crowns fitted the implant abutments directly and were attached to a combined metallic and acrylic denture base by the aid of retention wings. The metallic part of the denture base had a u-shaped occlusal plate with a metallic ring for the attachment of the testing machine. A
universal testing machine was used to test the retention of the telescopic crowns system at the zero level (T0) and after 30 , 300 , 600, 1200, 1500, 1800, 2100 cycles of loading ( T1 to T7
respectively). The implant abutments were unscrewed and ball abutment were tightened. Ball retainer matrix was fitted over the ball abutments and were attached to the fitting surface of the overdenture by chemically initiated acrylic resin dough. The same testing procedures were repeated for the ball and socket retainers , data were
collected and statistically compared.
The results showed that the base level retention force was more than 45 Newton for ball and socket retainers and only about 16 N for telescopic retainers.
Great reduction of retention force was seen after 30 cycles of insertion and removal (more than 65% for telescopic retainers and more than 35% for ball and socket). The loss of retention
force continued throughout the study to reach nearly 15 N for ball and socket and 1.5 n for telescopic retainers at the end of the study (T7 after 2100 cycles of insertion and removal).