Fracture Behavior of In-Ceram® Zirconia CAD/CAM Substructures Under Cyclic Fatigue

OBJECTIVE: The aim of this study was to test In-Ceram Zirconia CAD/CAM (computer assisted design/computer assisted machining) posterior three unit fixed partial denture substructures under cyclic fatigue in water environment.

METHODS: A dental model was used to make full crown chamfer preparations on a lower second bicuspid and a lower second molar adjacent to a missing first molar. Epoxy resin dies (Scotch-Weld Epoxy Adhesive DP100, 3M) replicating the praparations were made after impression with adition silicone (Express™, 3M ESPE). The preparations were scanned and thirty-six anatomically identical substructures were machined on a CEREC (ceramic reconstruction) machine. The substructures were cemented with a resin cement (RelyX Unicem, 3M ESPE) to epoxy resin dies and submerged in water for testing under cyclic fatigue with a constant load of 400 N. The residual strength of the non-fractured specimens was tested after different number of cycles (initial – 3 samples, 5,000 – 4 samples, 15,000 – 5 samples, 25,000 – 5 samples, 50,000 – 4 samples, and 85,000 – 3 samples) through a three point bending test.

RESULTS: Twelve substructures fractured under fatigue. The mean residual strength of the groups was .878 Kn, .914, .772, .751, .724, and .624. The non parametric test of Kruskall-Wallis was used to verify significance in residual strength between the groups considering p < 0,05. Within the limitations of this study, it was concluded that there is a decrease in strength as the number of mastication cycles increase, and that it is more pronounced after 50,000 cycles.

CONCLUSIONS: Fracture resistance of the samples decreased as the number of cycles increased, specially after 50,000 cycles. The samples showed a significant dispersion regarding lifetime under fatigue. The mode of fracture most frequently encoutered was through the connector with the smallest height:width ratio.