CAD/CAM - international magazine of digital dentistry

industry report _ lithium disilicate I Fig. 5_Pre-existing clinical condition of maxillary anterior teeth to be restored. Fig. 6_Veneer preparations for the anterior restoration. Fig. 7_E4D LabWorks system used for the scan, design, and milling of the veneer restorations. ration when the core material will be close to the restoration’s exterior surface. The other disadvantage is that although the high-strength material has great mechanical prop- erties,thelayeringceramicwithwhichitisveneered exhibits a much lower flexural strength and frac- ture toughness.2, 3 The zirconia core (with a 900 to 1,000 MPa flexural strength) is less than half of the cross-sectional width of a restoration; it must be completedwithaveneeringmaterialwithaflexural strength in the range of 80 to 110 MPa (depending on delivery method).4 The veneering material tends to chip or fracture during function. Also, such restorations depend significantly on the ability to create a strong bond interface between the dis- similar materials of oxide-ceramic and silica-based glass-ceramic,abondthatisnotdifficulttocreate.5 However, the quality of the bond interface can vary substantially because of cleanliness of the bond surface, furnace calibration, user experience and other issues. In today’s industry, monolithic glass-ceramic structures can provide exceptional aesthetics with- out requiring a veneering ceramic. Greater struc- tural integrity can be achieved by eliminating the veneered ceramic and its requisite bond interface.6 The relative strength of the available glass-ceramic material has traditionally been the disadvantage of these restorations. Owing to their flexural strength of 130 to 160 MPa, they are limited to single-tooth restorations, and adhesive bonding techniques are needed for load sharing with the underlying tooth.6 This has been resolved through the development of highly aesthetic lithium-disilicate glass-ceramic materials. The 70 % crystal phase of this unique glass- ceramic material refracts light very naturally, while also providing improved flexural strength (360 to 400 MPa).7 This gives more indications for use and the ability to place restorations using traditional cementationtechniques,whilealsohavingstrength and aesthetics. With a monolithic technique (Figs. 1 & 2), most restorations built from lithium-disilicate materials can be completely fabricated. This approach pro- vides high strength and aesthetics but requires surface colourants for the final shade. When in- depth colour effects are needed, a partial layering technique may be employed. Although no longer a purely monolithic structure (Figs. 3 & 4) because the restoration maintains a large volume of the core material, the resulting restoration should rea- sonably maintain its high strength. However, no evidence supports this. _Aesthetic options If covering or masking underlying tooth struc- ture is part of the treatment plan, the restorative team can imagine doing so in an aesthetic way. The ceramist can make that vision a reality with IPS e.max (Ivoclar Vivadent) by using a very high opacityingot.IngotopacitiesavailableforIPSe.max include high opacity (HO), medium opacity (MO), low translucency (LT) and high translucency (HT).7 TheMOingotcanbeusedasananatomicframework material if restorations must be fully layered. LT in- got can be employed with stain and glaze methods Fig. 6 Fig. 7 Fig. 5 I 27CAD/CAM 1_2012