Background:Zirconium bar displays a good esthetical appearance, great mechanical strength and high biocompatibility; however,its retention compared to other prosthetic CAD/CAM materials is still unclear.
Materials and method:.Four dental implants were inserted in the canine and second premolar areas bilaterally in acrylic resin model. A screw retained zirconium bar was designed and milled using CAD/CAM machine. Three over-dentures were constructed; each over denture consisted of a perforated base frame work from one of three different materials:PEEK (group I),titanium (group II) and zirconium(group III)frameworks, with a ring at their geometric center in a higher level than the occlusal plane. A universal testing machine was used for measurement of retention values of the prosthesis by pulling the ring at a crosshead speed of 50 mm/min until the frameworks separated. Each prosthesis was inserted and removed a total of 3240 times to simulate 3 years of clinical usage. The maximum forces needed to separate the attachments were calculated in Newton's (N). One Way ANOVA was used to compare between tested groups followed by Tukey HSD for pairwise comparison.

Results: Retention force mean values were nearly: 52 N, 16 N and 27 N for groups I, II and II respectively at baseline. After simulated usage these values were nearly: 32 N, 19 N and 11 N for groups I, II and III.
Conclusion:It may be concluded that zirconium bar can provide significantly higher retention forces when used with PEEK frameworks, but it shows significant reduction of this force during 3 years of simulated overdenture use