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ePaper created 2017-09-29, 09:04:12 | version 1.38.0
literature review _ CAD/CAM material and systems 850 °C for 20 to 30 minutes to precipitate the final phase. This crystallization step is usually as- sociated with a 0.2 percent shrinkage accounted for the designing software.19 Nowadays, blocks of lithium disilicate are available for both in-of- fice and in-laboratory fabrication of all-ceramic restorations; monolithic blocks require layering or staining to achieve good aesthetic results.8 Different in vitro studies that evaluate the mar- ginal accuracy of milled lithium disilicate reveal that these restorations could be as accurate as 56 to 63 microns.20 According to the manufacturer specifica- tions, the designing principles for lithium disil- icate are produced by default in the designing software, but in full all-ceramic crowns struc- tures the minimum thickness must be applied in the preparation design (Table I). Material thickness Anterior Premolar Molar Veneers Staining technique Cut-back technique Layering technique 1.2 1.2 0.8 1.5 1.5 0.8 1.5 1.5 – 0.6 0.6 – Values are expressed in millimetres Table I_Recommended dimensions for E-max CAD by Ivoclar Vivadent. 18 2_2017 During the crystallisation process, the ce- ramic is converted from a lithium metasilicate crystal phase to lithium disilicate. Some com- mercial types of ceramics are Empress CAD (Ivo- clar Vivadent) and IPS E-max. The first one is a leucite based glass ceramic with a composition similar to Empress ceramic. IPS E-max was in- troduced in 2006 as a material with a flexural strength of 360 to 400 MPa (two to three times stronger than glass ceramics); the blocks are blue in the partially crystallised state but it achieves the final shade after it is submitted to the firing process in a porcelain oven for 20 to 25 minutes to complete the crystallisation; the final result is a glass-ceramic with a fine grain size of approx- imately 1.5 μm and 70 percent crystal volume incorporated in a glass matrix.20 In 2014, Vident released Suprinity; the first ce- ramic reinforced with zirconia (10 percent weight); this material is a zirconia reinforced lithium sili- cate ceramic (ZLS) available in a precrystallised or fully crystallised (Suprinity FC) state indicated for all kind of single all-ceramic restorations. _Zirconia Zirconia has been used in dentistry as a bio- material for crown and bridge fabrications since 2004; it has been useful in the most posterior ar- eas of the mouth where high occlusal forces are applied and there is limited interocclusal space.22 Zirconia is a polymorphic material that can have three different forms depending on the temperature: monoclinic at room temperature, tetragonal above 1,170 °C, and cubic beyond 2,370 °C. According to Piconi (1999) ‘the phase transitions are reversible and free crystals are associated with volume expansion’. Different authors state that when zirconia is heated to a temperature between 1,470 °C and 2,010 °C and cooled, a volume shrinkage of 25 to 35 percent can occur that could affect marginal fit or pas- siveness of the restorations.22 This feature limit- ed the use of pure zirconia until 1970 when Rieth and Gupta developed the yttria-tetragonal zir- conia polycrystal (Y-TZP) containing 2 to 3 per- cent mol-yttria in order to minimise this effect.10 One of the most interesting properties of zir- conia is transformation toughening; Kelly (2008) describes it as: ‘A phenomenon that happens when a fracture takes place by the extension of an already existing defect in the material struc- ture, with the tetragonal grain size and stabilizer, the stress concentration at the tip of the crack constitutes an energy source able to trigger the transformation of tetragonal lattice into the monoclinic phase’. This process dissipates part of the elastic energy that promotes progression of cracks in the restoration; there is a localised expansion of around 3.5 percent that increases the energy that opposes the crack propagation.4 Zirconia restorations can be fabricated from fully sintered zirconium oxide or partially sin- tered zirconium oxide blanks (green-state). Pro- ponent of milling fully sintered zirconia claim that fitness of restorations is better because it avoid volumetric changes during the fabrica- tion process. On the other hand, the partially sintered zirconia (Fig. 4) is easier and faster to mill and proponents of milling partially sintered blanks claim that micro cracks can be induced to the restoration during the milling process and it also requires more time and intensive milling processes; this micro defects or surface flaws can affect the final strength of the final restoration and could potentially chip the mar- ginal areas; however further research is needed about this topic.10 One of the first systems that used zirconia was In-Ceram Zirconia (Vident), which is a mod- ification of the In-Ceram Alumina but with the addition of partially stabilised zirconia oxide to the composition. Recently many companies have integrated zirconia into their CAD/CAM workflow due to its mechanical properties, which are at- tractive for restorative dentistry; some of these