FRACTURE RESISTANCE, FINITE ELEMENT ANALYSIS AND WEIBULL RISK OF FAILURE FOR ENDODONTICALLY TREATED MOLARS RESTORED WITH LITHIUM DISILICATE AND HYBRID CERAMIC ENDOCROWNS WITH TWO PREPARATION DESIGNS

Objective: The present study aimed at evaluating the fracture resistance of endodontically treated molars restored using IPS e.max CAD (e.max CAD) and Enamic endocrowns using two preparations designs after thermomechanical fatigue.
Materials and methods: 40 intact mandibular molars were selected and endodontically treated following a standardized procedure. Molars were decapitated 3.5mm coronal to the cervical line then embedded in epoxy resin blocks. Samples received standardized preparations for endocrowns including the preparation of the pulp chamber, then they were divided into two groups; group 1 where No-Ferrule was included in the preparation and group 2 with 2.5mm Ferrule extracoronally. Each group was further divided into two subgroups according to the material used for constructing endocrowns; e.max CAD endocrowns and Vita Enamic endocrowns. Endocrowns were constructed using Cerec CAD/CAM machine. Constructed endocrowns were adhesively bonded to their respective teeth then subjected to a thermomechanical fatigue procedure (49N, 1.6Hz, 120,000cycle, 5o-55o C) in a chewing simulator. The fracture resistance of each sample was determined by subjecting the samples to a static compressive load until failure. Failure loads were recorded as an indication of fracture resistance. Stresses at failure were analyzed using finite element analysis (FEA) and failure probability percentage was evaluated using Weibull risk of failure. Data were collected, tabulated and statistically analyzed.
Results: For fracture resistance, the highest fracture load values were recorded with samples of Ferrule with e.max CAD endocrowns, followed by samples of No-Ferrule with e.max CAD endocrowns and then samples of Ferrule with Enamic endocrowns. The lowest fracture load values were recorded with samples of No-Ferrule with Enamic endocrowns. The magnitude of stresses generated in different parts of the samples as reveled by FEA generally followed a different order than in the fracture resistance. Weibull Risk of failure showed that samples of No-Ferrule with Enamic endocrowns had the highest failure probability especially under high loads.
Conclusions: Ferrule design would improve the fracture resistance of endodontically treated teeth especially when restored with lithium disilicate based (e.max CAD) endocrowns. Under low magnitude of forces, both materials would work safely with either design.