الفهرس 
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Abstract
This study was designed to compare two CAD/CAM materials and two preparations designs for occlusal veneer restoration. The study investigated marginal adaptation and fracture resistance characteristics after thermocycling ageing as a way to give an idea about clinical performance of the two materials as occlusal veneers.
Natural human mandibular second molars were selected of average dimensions and the teeth were divided into two groups according to the preparation design; flat and anatomic. Each group was further subdivided into two sub-groups according to the material used for the fabrication of the occlusal veneer restoration; (lithium disilicate and flexible nanoceramic.Preparation of teeth was done for the anatomic group with an angle 150o between buccal and lingual cusps.
Occlusal veneers were designed with the aid of CAD software CEREC premium software 4.4.4. Millling was performed by the inLab MC XL milling unit. The restorations were checked and verified for the appropriate seating and thicknesses.
Surface treatment of both the restoration and the teeth was performed. Cementation of the occlusal veneer by self adhesive dual cured resin cement material was done.
Summary and conclusion
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Thermocycling of all specimens was done for 5000 thermal cycles between 5-55oc water bathes with dwell time 30 seconds and transfer time 10 seconds.
Marginal adaptation of all occlusal veneers was evaluated as 20 readings of marginal gap for each specimen and measured by means of a stereomicroscope. Image analysis was done by special software and data of marginal gaps were recorded.
Fracture resistance testing was done by loading samples in universal testing machine. Loads at which failure occurred were recorded as the fracture resistance of the specified occlusal veneer.
Two-way ANOVA test was to study the effect of different tested variables and their interaction on marginal adaptation (μm) and fracture load (N). Independent-t test was used for different inter group comparisons. The significance level was set at P ≤0.05 for all tests.
The results showed that marginal adaptation of the flat preparation design was better than anatomical one with no significant differences between the two tested materials. All data collected were within the clinically acceptable range of marginal gap values.
The results revealed that preparation design and material had no effect on the fracture resistance of occlusal veneer restoration when loaded vertically.