Bearing capacity of ceramic crowns before and after cyclic loading: An in vitro study

The objective of this study was to compare the resistance to failure of three types of all-ceramic crowns under static and cyclic contact loading. Three types of crown specimens were fabricated (N = 14 per group) including: i) sintered bi-layer crowns (SBC) made of IPS e.max CAD veneers fused on Lava zirconia core ceramics, ii) adhesive bi-layer crowns (ABC) made of IPS e.max CAD veneers bonded onto Lava zirconia core ceramics, and iii) monolith ceramics crown (MCC) made of IPS e.max CAD. All were bonded onto a compliant substrate of resin composite with commercial adhesive. Eight specimens were selected from each group for monotonic loading to failure via 6 mm diameter ball to obtain the initial load capacity (P_initial) of the crowns. The remaining six crown specimens were subjected to cyclic contact loading to 5 million cycles, and then loaded monotonically to fracture to obtain residual load capacity after fatigue (P_residual). A finite element analysis (FEA) was used to analyze the stress distributions within the three types of crowns and estimate the crack origins from the maximum stress. The initial load to failure (P_initial) for the SBC, ABC, and MCC groups were 1120 ± 170 N, 970 ± 150 N, and 950 ± 60 N, respectively, and significantly different (p = 0.027). The residual load to failure after fatigue (P_residual) for the crown types were 890 ± 240 N, 680 ± 240 N, and 1050 ± 120 N, respectively, and also significantly different (p = 0.012). For SBC and MCC, failure occurred predominantly by bulk fracture, whereas the ABC failed primarily by chipping. The FEA suggested that failure of SBC and ABC would originate in the cement layers, and MCC has a more reasonable stress allocation than that in bi-layer crowns (SBC and ABC), which agreed with the observed failure modes. The SBC crowns had superior load to failure, whereas the bi-layer systems (SBC and ABC) had inferior fatigue resistance compared with the monolith crowns (MCC). The cement interface was the weak-link of the bi-layer systems, especially for ABC. Considering their use in clinical practice, the ranking in resistance to failure is SBC>MCC>ABC.