Abstract
Fixed implant-retained cantilevered restorations for edentulous patients have been successfully used in implant dentistry. In recent years, with the improvements in CAD/CAM technology, zirconia has been in use as a more affordable alternative to metal frameworks for fixed restorations due to the rising cost of noble metals. The dimensions of implant-retained fixed cantilevered dentures are important to prevent mechanical and biological complications. Information on the optimum thickness and cantilever length for improved strength of zirconia frameworks is limited in the literature. The aim of this study was to investigate the effect of cantilever length and occlusocervical thickness on the load-to-fracture and strain distribution of zirconia frameworks. Twenty-seven rectangular prism-shaped specimens (6 mm thick buccolingually) were fabricated using a CAD/CAM milling technique. Specimens were prepared in 9 groups (n=3) according to their vertical dimensions (6₉6 mm, 8₉6 mm and 10₉6 mm) and cantilever loading distance (7 mm, 10 mm and 17 mm). All specimens were heat-treated in a porcelain furnace and thermocycled for 20000 cycles before the tests. Each framework was secured using a clamp attached to the first 20 mm of the framework. Three-dimensional image correlation (3D-DIC) technique was used for a full-field measurement of strain during testing. A load-to-fracture test was used until the specimens fractured. Maximum force and the principal strain data were analyzed by ANOVA using the maximum likelihood estimation method. The frameworks tested in this study fractured at load values ranging from 1065 - 8870 N. The effect of occluso-cervical thickness and cantilever length was significant for the load-to-fracture (P< .001). No statistically significant interaction was observed between the two factors. The strain values for all specimens were similar. No statistically significant effects were found for occluso-cervical thickness and cantilever length or between them on the strain distribution. Increased occluso-cervical thickness allowed the cantilever to withstand higher loads. Decreased cantilever length allowed the frameworks to withstand higher loads. The occluso-cervical thicknesses and cantilever lengths of zirconia frameworks tested withstood the maximum bite force that has been reported in the literature. However, the properties of components in the dental implant-abutment-framework assembly should be taken into consideration when interpreting these results.