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Background : The retention of double-crown-retained prostheses differs according to different crown tapers, heights, used materials and the friction between the axial walls of the inner and outer crowns. The retention might decrease over time due to the wear between the materials. Gold alloys has been regarded as the most acceptable choice in constructing double-crown restorations. Researchers are seeking less expensive, aesthetic and bio-compatible alternatives with similar accuracy and long-term retention. Aim/Hypothesis : To evaluate the effect of thermo-mechanical dynamic loading on the retention of telescopic mandibular overdentures with different metallic and NO-metallic material combinations. Materials and Methods : Four groups (each 8) were tested: ZP (zirconia abutments/PEEK framework), PP (PEEK abutments/PEEK framework), TP (titanium abutments/PEEK framework), and TG (titanium abutments/gold copings). Four implants arranged in an arch over PVC-cylindrical bases. Injection technique was used to manufacture the PEEK frameworks, while the gold copings were electroformed. The abutment retention height was 4 mm with 1° convergence angle. Thermo-mechanical dynamic loading was carried out in a chewing simulator for 1 200 000 loading cycles corresponding to 5-year clinical fatigue. A 60 N vertical load was applied at a speed of 30 mm/sec and thermo-cycling was applied with temperature ranged between 5-55°C. The retentive force was measured using the universal testing maCHINA 10 times before and after the chewing test with a speed of 8 mm/sec. Paired T-test was used to compare the retention force before and after the dynamic loading. Statistical significance level was established at P below 0.05. Results : The mean retentive force increased significantly from 13.2 N (±4.6) to 16.4 N (±6.1) in group ZP, P = 0.002, while in group TP, the mean retentive force decreased significantly from 4.9 N (±2.1) to 3.3 N (±1.7), P = 0.046. There was no statistically significant change in the retentive force for group PP as well as group TG. Conclusions and Clinical Implications : A durable retention was achieved with the use of different metallic and NO-metallic combinations in the construction of telescopic mandibular overdentures, which might be suitable for clinical use.

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