Load Bearing Capacity of Zirconia Crowns Screwed to Multi-unit Abutment with and without Titanium-base: An In-vitro Pilot Study

Objectives: The aim of this study was to compare the static and dynamic load-bearing capacity as well as the failure mode of 3Y-TZP (zirconia) crowns screwed to multi-unit abutments (MUA) with and without Titanium-base (T-base).
Methods: Forty-eight CAD-CAM monolithic zirconia crowns were screwed to straight MUA which were torqued to laboratory analogs (6mm width) with a digital torque ratchet (30Ncm). The specimens were assigned to two groups (n=24): Group A- zirconia crowns were screwed directly to the MUA (screwed restoration); Group B- zirconia crowns were cemented to a titanium sleeve (T-base) and then screwed to the MUA (screwed-cemented restoration). All specimens were aged in 100% humidity (37°C) for one month, and then through thermocycling (20,000 cycles, 10sec dwell time, 5-55°C). After aging, the specimens were tested through static and dynamic loading, according to ISO 14801, until reaching a load in which 3 specimens survived 5,000,000 cycles. Failure mode was evaluated by stereomicroscope at 20x magnification.
Results: Force vs. cycles (log no. of cycles) Regression analysis revealed that neither the group nor the interaction group-cycles were significant (p≥0.233). Only the no. of cycles causing failure was significant (p<0.001). The load which the specimens survived 5,000,000 cycles was 250N in the two groups. However, there was a difference in the mode of failure between the two groups: Group A exhibited mainly failure within the metal (MUA/screw) and only at high loading force a zirconia failure was evident. Group B exhibited failure either within the metal (MUA/screw) or adhesive failure between zirconia and T-base.
Conclusions: Within the limits of this study, it is possible to assume that zirconia restoration screwed directly to a MUA is a viable procedure and the survival odds of this restoration are not inferior to that of a screwed - cemented restoration.