Clinical aspects
In the search for the ultimate aesthetic restorative material, many all-ceramic systems have been proposed. Dental research is nowadays directed towards metal-free prosthetic restorations in order to improve aesthetical outcome of FPD restorations. Natural look of soft tissue in contact with fixed partial dentures is influenced by two factors: mucosal thickness and typology of restorative material. Metal-free restorations allow to preserve soft tissue color more similar to the natural one than porcelain fused to metal restorations.27 Many ceramics, such as spinel, alumina, and ceramic reinforced with lithium disilicate, have been proposed for the construction of metal-free restorations.28 These materials have precise indications for fixed partial dentures (FPD); Luthy measured average load-bearing capacities of 518 N for alumina restorations, 282 N for lithium disilicate restorations, and 755 N for zirconia restorations.29 Raigrodski analyzed different all-ceramic systems and concluded that reinforced ceramics can only be used to replace anterior teeth with single crown restorations or maximum with three units FPDs. On the other hand, ZrO2 restorations have a wider application field. Other ceramic technologies only allow the construction of structures that are resistant to chewing stresses on anterior teeth. On the contrary, Zr-ceramic FPDs can also be used on molars.28 Tinschert compared lifetime of different metal-free core for FPD and reported that Zr-ceramic with alumina oxide had the highest initial and most favorable long-term strength.30 Connecting surface area of the FPD must be at least 6.25 mm 2.31 For this reason, ceramic FPDs should only be used when the distance between the interproximal papilla and the marginal ridge is close to 4mm. In a comparison between 3-, 4- and 5-unit zirconia fixed partial dentures and minimal connecting surface resulted, respectively, 2.7mm2, 4.0 mm2 and 4.9mm2.32 Height of abutment is fundamental to obtain ZrO2 frameworks with correct shape and dimension in order to ensure mechanical resistance of restoration. This aspect must be carefully considered when realizing a metal-free FPD. Zirconia restorations have found their indications for FPDs supported by teeth or implants.