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Two of the most important qualities of dental restorations are esthetics and strength.
Today I want to talk about zirconia crowns which are known for their high strength. With a flexural strength that can reach 1400 Mpa, zirconia is the strongest restorative material available today. Unfortunately, this strength comes at an esthetic cost as zirconia is a very opaque material.
There are many types of zirconia available on the market, some of which are labeled as high translucency. The high translucency zirconia goes a long way to increase the esthetics, but it also has different physical properties than the low translucent zirconia which make it only half as strong. For this reason, we only use the high strength zirconia at O’Brien Dental Lab.
Layering ceramic on top of a zirconia coping is another way to increase the esthetic outcome and in most cases is a requirement for esthetic acceptance of anterior restorations. Unfortunately, zirconia veneering ceramic is substantially weaker than the core material with a flexural strength between 60 and 100 Mpa.
A solution that gives us the strength of the zirconia and the esthetics of the veneering ceramic is to selectively layer the ceramic. We call this technique micro layered zirconia or MLZ™ for short. Layering ceramic allows us to meet expectations of custom shading, intrinsic characterization and varying degrees of translucency.
We begin with a full contour design, followed by a minimal cutback of the facial surface. After the zirconia is milled and sintered, ceramic is custom layered to the facial surface by one of our ceramists. The exposed zirconia is then hand polished for a smooth and strong surface which is important to prevent wear to the opposing dentition.
MLZs are ideal for maxillary single units as well as splinted restorations but are typically contraindicated in the mandibular arch. The reason for this is that we want to protect all functional areas of the crown which we can do in the maxillary without having exposure of the zirconia in the esthetic zone. In the lower arch the zirconia would need to come up over the incisal edge or cusp for full protection which would create a transition line between the two materials in the esthetic zone.
