Influence of cyclic loading on the fracture toughness and load bearing capacities of all-ceramic crowns

The purpose of this study was to investigate how cyclic loading influenced the fracture toughness of hot-press lithium disilicate and zirconia core materials and whether there was an increase in the propensity for crown failure. Two types of all-ceramic crowns including the IPS e.max Press system ( n =24) and the Lava zirconia system ( n =24), were selected. Sectioned specimens were subjected to cyclic loading with the maximum magnitude of 200 N ( R =0.1) until two million cycles. The material properties including Young's modulus ( E ) and hardness ( H ) and the fracture toughness ( K IC ) of the core materials were evaluated using indentation methods ( n =12 each). The load-bearing capacities of the specimens were examined by means of monotonic load to fracture ( n =12 each). It was found that the material properties, including E , H and K IC , of the two types of dental ceramics, were reduced. Statistical analysis indicated that there were no significant influences of fatigue loading on material properties E and H for both types of dental ceramics or K IC for zirconia, while for the IPS e.max Press core, K IC , which was parallel to the direction of the lithium disilicate crystals, was significantly reduced ( P =0.001). A conclusion was drawn that zirconia possesses high mechanical reliability and sustainable capacity to resist fatigue loading, while fatigue loading remarkably degraded the anisotropic mechanical behaviour of hot-press lithium disilicate ceramics. All-ceramic crowns: strength in zirconia Zirconia-based ceramic crowns are strong, mechanically reliable and resist loading fatigue over time, according to research from China. Crowns made entirely from ceramics are frequently used as tooth replacements, but repeated stresses can cause cracks to propagate and sometimes lead to total failure of the crown. Dong-Sheng Zhang at Shanghai University and co-workers compared the impact of cyclic loading on two leading types of ceramic crown: one is made from hot-press lithium disilicate and the other from zirconia core materials. The researchers subjected the crowns to two million loading cycles of different magnitudes and measured the changes in the crowns' elasticity, hardness and fracture toughness over time. They found that cyclic loading had no significant influence on zirconia crowns, but the lithium-based crowns had a markedly lower load-bearing capacity.