Purpose: The aim of the study was to evaluate the effect of a prosthetic cylinder and casting on the misfit and loosening torque of screw-retained multiple-unit implant-supported dental prostheses under masticatory simulation.
Materials and methods: Screw-retained, three-unit fixed dental prostheses (FDP) and screw-retained full-arch FDP frameworks were waxed using calcinable (plastic cylinders) or overcasted (premachined cast-on cylinders) on the dental implant abutments. The cylinders were cast in Co-Cr alloy to obtain four groups according to cylinder type and prosthesis type (n = 10). The screws were tightened with 20 N/cm (abutment) and 10 N/cm (prosthetic) torque according to the manufacturer's recommendation. After 24 hours, the initial loosening torque was analyzed. The initial misfit measurements were performed according to the Schiffleger test. The screws were retightened, and the specimens were submitted to 10(6) mechanical cycles (2 Hz/280 N). Loosening torque and misfit were reevaluated (final measurements), and data were submitted to ANOVA, Tukey's HSD, and Pearson's correlation tests (α = 0.05).
Results: The calcinable three-unit FDP demonstrated greater misfit (initial: 107.53 ± 40.36 μm; final: 99.00 ± 40.85 μm) than did the overcasted three-unit FDP frameworks (initial: 51.50 ± 22.98 μm; final: 44.33 ± 14.14 μm) (initial: p = 0.0005; final: p = 0.0007). No difference was noted between the calcinable and overcasted full-arch FDP frameworks (p > 0.05). Masticatory simulation did not affect the misfit (p > 0.05). The overcasted full-arch FDP presented a lower abutment screw loosening torque (12.05 ± 1.80 N/cm) than did the calcinable ones (14.75 ± 1.72 N/cm) in the final measurement (p = 0.0024). The calcinable groups presented a lower prosthetic screw loosening torque than did the overcasted groups in the final evaluation (p < 0.05). After masticatory simulation, the prosthetic screw loosening torque of the calcinable three-unit FDP decreased (initial: 5.49 ± 1.07 N/cm; final: 3.73 ± 1.15 N/cm; p = 0.0044). Correlation between misfit and loosening was observed only for the prosthetic screws (p < 0.05).
Conclusions: The overcasted components provided a better fit in three-unit FDPs but did not influence the fit of full-arch FDPs. Prosthetic screws of overcasted frameworks presented higher stability, whereas masticatory simulation did not influence misfit but did reduce the prosthetic screw loosening torque of calcinable three-unit FDP frameworks.
Keywords: Marginal misfit; castable; cyclic loading; dental implants; screw loosening.
© 2015 by the American College of Prosthodontists.