Fracture strength, failure type and Weibull characteristics of lithium disilicate and multiphase resin composite endocrowns under axial and lateral forces

Objective. Multiphase resin composite materials have been advocated as an alternative to reinforced ceramics but limited information is available to date on their stability. This in vitro study evaluated the effect of axial and lateral forces on the strength of endocrowns made of Li2Si2O5 and multiphase resin composite. Methods. Sound human molars (N = 60, n =10 per group) were randomly divided into 6 groups: Group C: Control, no preparation or restoration; Group LI: Endocrown made of Li2Si2O5 (IPS e.max CAD) and Group LA: Endocrown made of multiphase resin composite material (Lava Ultimate). After decapitation and endodontic preparation, immediate dentin sealing was performed. Following CAD/CAM fabrication, their cementation surfaces were silica coated (CoJet System) and silanized (ESPE-Sil). Endocrowns were then adhesively cemented (Variolink II). All specimens were thermocycled (x10,000 cycles). While half of the specimens in each group were subjected to axial (C-A, LIA, LA(A)), the other half was subjected to lateral static (C-L, LIL, LA(L)) loading (1 mm/min). Failure type and location after debonding/fracture were classified. Data were analyzed using ANOVA and Tukey's post hoc test (alpha = 0.05). Two-parameter Weibull distribution values including the Weibull modulus, scale (m) and shape ((0)), values were calculated. Results. Under axial loading, mean fracture strength (N) did not show significant difference between groups: LA(A) (2675 +/- 588)(a), LIA (2428 +/- 566)(a), (C)A (2151 +/- 672)(a) (p > 0.05) and under lateral loading, LA(L) (838 +/- 169)(A) presented significantly lower mean values than those of other groups: C-L (1499 +/- 418)(B), LIL (1118 +/- 173)(B) (p < 0.05). Both endocrown materials and the control group were more vulnerable to lateral loading than axial loading. Under axial loading, Weibull distribution presented higher shape ((0)) for Groups LIA (5.35) and LA(A) (5.08) than that of the control (3.97) and under lateral loading LIL (7.5) showed higher shape ((0)) than those of other groups (4.69-6.46). After axial loading, failure types were mainly cohesive in the material and after lateral loading primarily adhesive between the material and dentin for both LI and LA, most of which were repairable. Significance. Under axial loading, molars restored with endocrowns performed similar with both Li2Si2O5 and multiphase resin composite but the latter was less durable under lateral loading. (C) 2016 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.