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ePaper created 2016-10-04, 09:33:57 | version 1.38.0
| literature review CAD/CAM material and systems 12 CAD/CAM 3 2016 stepisusuallyassociatedwitha0.2percentshrinkage accounted for the designing software.19 Nowadays, blocks of lithium disilicate are available for both in-officeandin-laboratoryfabricationofall-ceramic restorations; monolithic blocks require layering or staining to achieve good aesthetic results.8 Different invitrostudiesthatevaluatethemarginalaccuracyof milledlithiumdisilicaterevealthattheserestorations could be as accurate as 56 to 63 microns.20 Accordingtothemanufacturerspecifications,thede- signing principles for lithium disilicate are produced bydefaultinthedesigningsoftware,butinfullall-ce- ramic crowns structures the minimum thickness must be applied in the preparation design (Table I). Duringthecrystallisationprocess,theceramiciscon- verted from a lithium metasilicate crystal phase to lithiumdisilicate.Somecommercialtypesofceramics are Empress CAD (Ivoclar 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 introduced 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 approximately 1.5 µm and 70 percent crystal volume incorporated in a glass matrix.20 In 2014, Vident released Suprinity; the first ceramic reinforcedwithzirconia(10percentweight);thisma- terial is a zirconia reinforced lithium silicate ceramic (ZLS)availableinaprecrystallisedorfullycrystallised (Suprinity FC) state indicated for all kind of single all-ceramic restorations. Zirconia Zirconia has been used in dentistry as a biomaterial for crown and bridge fabrications since 2004; it has been useful in the most posterior areas of the mouth where high occlusal forces are applied and there is limited interocclusal space.22 Zirconia isapolymorphicmaterialthat can havethree differentformsdependingonthetemperature:mono- clinicatroomtemperature,tetragonalabove1,170°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 au- thors state that when zirconia is heated to a tempera- ture between 1,470 °C and 2,010 °C and cooled, a vol- umeshrinkageof25to35percentcanoccurthatcould affectmarginalfitorpassivenessoftherestorations.22 This feature limited the use of pure zirconia until 1970 whenRiethandGuptadevelopedtheyttria-tetragonal zirconia polycrystal (Y-TZP) containing 2 to 3 percent mol-yttria in order to minimise this effect.10 One of the most interesting properties of zirconia 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 structure, 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 ofcracksintherestoration;thereisalocalisedexpan- sion 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 sintered zirco- niumoxideblanks(green-state).Proponentofmilling fully sintered zirconia claim that fitness of resto- rations is better because it avoid volumetric changes duringthefabricationprocess.Ontheotherhand,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 re- quiresmoretimeandintensivemillingprocesses;this micro defects or surface flaws can affect the final strengthofthefinalrestorationandcouldpotentially chip the marginal 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 modification 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 attractive for restorative den- tistry; some of these properties are: high mechanical strength,fracturetoughness,radiopacityformarginal integrityevaluation,andrelativelyhighaesthetics.13–14 Different manufacturers are using zirconia as one of their main materials such as: Ceramill Zolid (Amann Table I: Recommended dimensions for E-max CAD by Ivoclar Vivadent. Material thickness Anterior Premolar Molar Veneers Staining technique 1.2 1.5 1.5 0.6 Cut-back technique 1.2 1.5 1.5 0.6 Layering technique 0.8 0.8 – – Values are expressed in millimetres 32016 Staining technique 1.21.51.50.6 Cut-back technique 1.21.51.50.6 Layering technique 0.80.8 – –