**5.2 Dental ceramics**

Dental ceramics are in-organic, non-metallic, and brittle restorative materials producing dental prosthesis that are used to replace missing or damaged dental structures which has high compressive strength and low tensile strength. FEM provides a mathematic analysis to predict strength values without the potential for errors in dental ceramics [18].

Tensile stresses tend to be more critical than compressive stresses for ceramic materials. The strength of ceramic restorations is significantly affected by the presence of flaws or other microscopic defects. Tensile stress concentration at cementation surface of the ceramic layer suggested as the predominant factor controlling ceramic failure [6].

Belli et al. in 2005 evaluated the effect of hybrid layer on distribution and amount of stress formed under occlusal loading in a premolar tooth restored with composite or ceramic inlay. They concluded that the hybrid layer has an effect on stress distribution under loading in restored premolar tooth model with composite or ceramic inlay [19].

Rezaei et al*.,* in 2011 determined the effect of buccolingual increase of the connector width on the stress distribution in posterior FPDs made of IPS Empress. Three models of three-unit bridges replacing the first molar were prepared with the buccolingual connector width varied from 3.0 to 5.0 mm. They were loaded vertically with 600 N at one point on the central fossa of the pontic and a load of 225 N at 45° angle from the lingual side. They concluded that, increasing the connector width decreases the failure probability when a vertical or angled load is applied [20].

Thompson et al*.,* in 2011 compared the inlay supported all-ceramic bridge with that of traditional full crown supported all-ceramic bridge. They demonstrated peak stresses in the inlay bridge around 20% higher than in the full crown supported bridge. They suggested the use of an ideal inlay preparation form and an optimized bridge design emphasizing on broadening of the gingival embrasure, so that the forces derived from mastication can be distributed adequately to a level that are within the fracture strength [21].

Matson et al*.,* in 2012 compared the stress distribution generated in a veneer restoration of an upper central incisor to intact teeth by applying a 10 N lingual buccal load at the incisal edge. Veneers used in restorative rehabilitations for anterior teeth are retained by the adhesive systems and resin cements. These restorations are mechanically not strong, because they are made of brittle materials, but they have good retention due to the resin-dentine bonding. They recommended the use of veneers to replace enamel for rehabilitation [22].
