Influence of cyclic loading and luting agents on the fracture load of two all-ceramic crown systems

Inherent mechanical properties, fabrication techniques, luting agents, and intraoral conditions are primary factors attributing to longevity of all-ceramic crowns. Before doing time-consuming and costly clinical studies, preclinical in vitro studies should be conducted to evaluate the durability of these crowns. This study investigated the influence of different luting agents and cyclic loading under wet conditions on the fracture load of CAD-CAM and pressable all-ceramic crowns. Ninety-six human premolars were prepared for all-ceramic crowns with the following preparation criteria: 6-degree axial taper, 1.5-mm shoulder finish line placed 0.5 mm occlusal to the CEJ, 2-mm occlusal reduction, and occluso-gingival height of 5 mm. Sixteen unprepared premolars served as controls. Forty-eight crowns were laboratory fabricated from a lithium disilicate glass-ceramic (IPS-Empress 2), while 48 CAD-CAM crowns were directly fabricated using a leucite-reinforced glass-ceramic (ProCAD) and the Cerec 3 CAD-CAM system. Three luting agents (Panavia F, Superbond C&B and ProTec CEM) were used for cementation (n = 16). After 1 week storage in water, half of the specimens of each subgroup (n = 8) were cyclically loaded and thermal cycled under wet conditions for 600,000 masticatory cycles and 3500 thermal cycles (58°C/4°C; dwell time 60 seconds) in a masticatory simulator. All specimens were loaded in a universal testing machine with the compressive load (N) applied along the long axis of the specimen at a crosshead speed of 1 mm/min until fracture. Fracture load was recorded for each specimen. The Kruskal-Wallis test was used first to detect overall significance, followed by Mann-Whitney U tests to identify which pairs of groups had significant differences (P=.05). Cyclic loading significantly decreased the median fracture load of ProCAD crowns luted using Superbond from 987.2 to 786.0 N (P=.014) and those luted using ProTec CEM from 914.4 to 630.7 N (P=.007). Also, the median fracture load of Empress 2 crowns luted using ProTec CEM decreased significantly from 977.3 to 622.9 N (P=.013). However, cyclic loading did not decrease the median fracture load of crowns luted using Panavia F, Empress 2 (P=.431) and ProCAD (P=.128). With the same loading conditions and luting agents, there were no significant differences in the fracture loads of ProCAD and Empress 2 crowns (P>.05). Cyclic loading reduced the fracture load of ProCAD crowns luted with Superbond C&B and ProTec CEM and of Empress 2 crowns luted with ProTec CEM.