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Previously,the choice of prosthetic implant-retained overdentures has depended on data from previous studies about the retention-fatigue strength of the attachment system selected.Little or no data have been available on the correlation between the attachment system selected and the overdenture support configuration.The purpose of the present study was to evaluate the retention force and fatigue resistance of three attachment systems and four support designs of overdenture prosthesis.Four lower edentulous acrylic models were prepared and eight combinations of attachments groups were investigated in the study.These included:O-Rings with mini-dental implants (MDIs),Dalbo elliptic with Dalbo Rotex and fabricated flexible acrylic attachments with both MDI and Dalbo Rotex.The study was divided into four test groups:groups A and B,controls,and groups C and D,experimental groups.Control group A contained three overdenture supports:two free standing MDIs in the canine region and at the midline,and one simulated tooth root with Dalbo Rotex screwed in.Control group B contained four overdenture support foundations:two free standing MDIs in the right canine region and the first premolar region,and two simulated tooth roots with Dalbo Rotex screwed in at the same MDI position,but on the left side of the model.Experimental group C contained three overdenture support foundations:two free standing MDIs in the canine region and at the midline,and one simulated tooth root with MDI screwed in.Experimental group D contained four overdenture support foundations:two free standing MDIs in the right canine region and the first premolar region,and two simulated tooth roots with MDIs screwed in at the same MDI position,but on the left side of the model.Each group was further divided into two subgroups according to attachment type used.Five samples were prepared for each group.Retention force (N) values were recorded initially (0 cycles) and after 360,720,1440 and 2880 insertion and removal cycles.During the tensile test a cross-head speed of 10 mm/min was applied.Values of absolute force (AF) and relative force (RF) were statistically analyzed by two-way ANOVA and multiple comparison Tukey's tests between groups and cycles periods.The results of fatigue tests showed a 50％ reduction in retention force in the subgroups with flexible attachments.A triangular design of overdenture support foundations with O-Ring attachments revealed the lowest value of AF and a relatively high reduction in RF.The four overdenture support designs with flexible acrylic attachments improved the retention force and reduced the fatigue retention.Furthermore,the results of the investigation demonstrate that flexible acrylic attachments for both teeth and implant-supported overdentures offer a wide range of retention forces.