[Research on the mechanical differences of machinable lithium disilicate all-ceramic crowns]

Due to the superior pigment and high flexural strength, machinable lithium disilicate ceramics can be used as a monolithic crown or veneering porcelains on the zirconia core to form the all-ceramic crowns by sintering or bonding procedures. This paper reports the research on the differences in stress distributions amongst these three types of all-ceramic crowns under typical loading conditions. Three-dimensional numerical models of the restored crown based on the first mandibular molar were developed. The vertical concentrated load and 8-point uniformly distributed load were applied, respectively. The maximum stress and stress distribution were resulted from finite element evaluation. It was found that the maximum tensile stress in 3 types of restored crowns subjected to the concentrate load was less than the flexural strength of IPS e.max. The stress distributions in the sintered and bonded double layered crowns were basically identical, and different from the monolithic crown. The stress magnitude in veneer porcelain of the bonded crown was greater than that in the sintered crown. The use of IPS e.max computer aided design monolithic crown as molar restorations should be careful to avoid high stress as the cyclic stress is a concern of fatigue which may influence the longevity of the restored crown. The bonded double layer crowns bear greater risks of veneer chipping compared with the sintered crowns. The conclusions of this study provide helpful guidelines in clinical applications for preparation of computer aided design/computer aided manufacture lithium disilicate all-ceramic restorations.