Effect of Viscosity of Intermedium on the Retention of Double Crowns

Purpose: The aim of the study is to evaluate the effect of intermedium viscosity on the retention force of double crowns with different retention mechanisms and materials. Materials and Methods: Double crowns, which have a conus angle of 2° were fabricated from material couples of zirconia-electroformed gold (ZG), PEEK-PEEK (PP), PEEK-PEEK with an occlusal gap between the primary and secondary crown (PP-gap), zirconia-PEEK (ZP), zirconia-PEEK with an occlusal gap between the primary and secondary crown (ZP-gap). The intermedium between the primary crown and secondary crown was filled with fluids of various viscosities (1,50,100,350,1000 cSt) and the retention forces were evaluated by a universal testing machine with a crosshead speed of 20 mm/min after a seating force of 50 N was applied. Results: PP-gap group displayed the highest retention forces for all viscosities among other groups (35,73N for 1 cSt, 23,7N for 1000 cST). Viscosity variations did not cause any significant differences in retention except 1000cSt. 1000 cSt viscosity caused a significant decrease in retention force for PP-gap and ZP-gap groups. Conclusion: Viscosity of 1000 cSt caused a decrease in retention due to inhibited seating of secondary crown on primary crown especially for the groups having an occlusal gap. Smaller values of viscosity did not change the retention.