Considerations for Lithium-Disilicate Crowns
Lithium disilicate is currently the most commonly used material for single-unit anterior crowns.Vsmile Ingots press block and Cad Block.The difference between the two materials is that the former is fabricated when pre-crystallized ingots are heat-pressed into a stone investment, and the CAD version is fabricated when partially crystallized blocks are carved into their final shape in a milling machine and then fully crystallized in a furnace. Many dentists are interested in knowing which of the two fabrication techniques leads to a superior crown. Laboratory research studies have reported that both fabrication techniques are capable of producing clinically acceptable marginal gaps smaller than 140 µm (with most studies reporting between 35 µm and 120 µm). Of the studies that have compared both fabrication techniques side by side, there is no consensus on which technique produces smaller marginal gaps. Three studies found slightly smaller marginal gaps with heat-pressing, and two studies found slightly smaller marginal gaps with milling. In our experience, however, pressed lithium-disilicate restorations more reliably produce closed margins. regarding strength, there is no apparent difference in the materials used for either fabrication technique.
These materials are also offered in two translucencies HT [high translucency, LT [low translucency], The translucency of the material does not to be altered by varying crystal composition or density and all translucencies of lithium disilicate offer a similar strength.
Lithium disilicate is translucent enough that it may be commonly used as a monolithic material. In our experience, however, a cut-back on the facial surface and/or incisal edge with a layer of porcelain is often needed to obtain the characterization necessary for single anterior crowns. A laboratory trial reported that a 2-mm-thick lithium-disilicate crown with a 0.7-mm cut-back demonstrated a 33% reduction in strength when loaded on the area of cut-back. The location of the cut-back, therefore, should be discussed with the laboratory.
Considerations for 4Y/5Y Zirconia Materials
As previously mentioned, there are now more translucent versions of zirconia available that can be used for monolithic anterior restorations. The increased translucency of zirconia is achieved by increasing the ratio of cubic to tetragonal phase within the zirconia. Cubic zirconia is more translucent than tetragonal zirconia because the arrangement of atoms in the cubic phase is optically isotropic, or symmetrical. This means that light can pass through in many different directions without being stopped or redirected. The amount of cubic zirconia within zirconia is determined by the amount of yttria stabilizer with the zirconia. At 3 mol% yttria (3Y), there is between 10% to 15% cubic phase; at 4 mol% yttria (4Y), there is between 25% to 37% cubic phase; and at 5 mol% yttria (5Y), there is between 50% to 58% cubic phase. As a result, 4Y zirconia is 13% more translucent than 3Y, and 5Y is 33% more translucent than 3Y. 5Y zirconia, however, is still less translucent than lithium disilicate. Although translucency is desirable to mimic natural enamel, different clinical situations will require varying degrees of translucency/opacity.
The increased translucency achieved from swapping tetragonal zirconia for cubic zirconia comes with a tradeoff in mechanical properties. Tetragonal zirconia is capable of undergoing transformation toughening, the ability to stop the growth of a crack within a material. 3Y zirconia is capable of transformation toughening. 4Y zirconia undergoes some transformation toughening. 5Y zirconia does not transformation-toughen. As a result, the strength and fracture toughness decrease from 3Y zirconia to 4Y zirconia to 5Y zirconia. However, 5Y still has a higher strength and fracture toughness than lithium disilicate.
Some clinical situations exist in which 4Y or 5Y zirconia offers clinical advantages over lithium disilicate. First, 4Y or 5Y zirconia may be an option for clinicians who desire to cement their crowns without bonding, particularly if the thickness of the crown is below the minimum recommended thickness for conventional cementation of lithium disilicate. This recommendation is based on a recent study that reported that although the strength of 5Y zirconia and lithium-disilicate crowns (with 0.8-mm to 1-mm thickness) were equivalent if the crowns were bonded with resin cement if the crowns were cemented with resin-modified glass-ionomer cement, 5Y zirconia outperformed lithium disilicate. This suggestion should only be followed in the absence of the zirconia manufacturer's recommendations, to which the restorative team should always adhere. The strength of the crown material also makes these materials more user-friendly prior to bonding, because their strength is not as dependent on bonding to the tooth structure as lithium-disilicate materials are. This strength is particularly advantageous to prevent fracturing a ceramic restoration during the process of trying it in.
Second, 4Y zirconia may offer strength advantages when the length of restoration exceeds the limit for which bonding to the tooth reinforces the material. The following picture shows 4Y zirconia anterior crowns, which were selected to reduce the likelihood of cohesive fracture of the long incisal edges.
Summary
The advantage of using a 4Y or 5Y zirconia is that it is less likely to fracture in try-in, which may be an issue with lithium disilicate depending on the skill level and experience of the clinician, and has the versatility to be cemented with various types of cement, not just bonded.
