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Objectives: The objectives of this study were to evaluate the effects of different veneering methods and the effects of different sizes of abutment on the fracture resistance of the crowns, and to assess the mode of failure of these crowns.
Materials and methods: A hundred and eight implant abutments (Straight 3.5/4.0 TiDesignTM, AstraTech Dental) were fabricated into 3 groups with different sizes of abutments [Ø4.5 (s), Ø5.5 (m), and Ø6.5 (l)]. Each group of implant abutments contained 3 subgroups of 12 specimens each (n=12). Zirconia frameworks were fabricated on all implant abutments. Various veneering materials were then applied and processed on the zirconia frameworks. Fluorapatite veneering ceramics were used as the control group (ZAC). Lithium disilicate crowns were fabricated as the veneering layer on the zirconia frameworks with different procedures: group A bonded via fired Crystal/Connect glass ceramic (FCC) and group B bonded via resin cement (BRC). Resin cement was used for cementation. All specimens were placed in a thermocycling unit and tested with a universal testing machine. Statistical analysis was performed using two-way ANOVA and Tukey B test.
Results: The mean of fracture resistance in the ZAC group was at the highest value (1787-3295N) of cohesive failure. The mean fracture resistance of the FCC group (1714-2809N) was higher than that of the BRC group (1565-1809N). The mean fracture resistance of the abutment diameter 5.5 mm (m) was at the highest value. The largest size of abutment (l) had a mean fracture resistance higher than the smallest size of abutment (s). The two main factors, veneering method and abutment size, had individual effects on fracture resistance. There were significant differences of fracture resistance in all groups with different veneering methods and different sizes of abutment. Adhesive failure was found in the BRC group. Meanwhile the FRC group was found to have both adhesive and combination failure.
Conclusion: The mean fracture resistance of crowns fused with Crystal/Connect was significantly higher than that of crowns bonded with resin cement, but all the crowns had adequate fracture resistance to be used as implant supported restorations in the posterior region.
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