Dr. Christensen: We can only theorize causes, but we think there are ceramic formulation problems as well as handling problems in the lab. To match the CTE of zirconia, the veneer ceramics contain very low to no leucite. These are essentially glass ceramics. They are not as robust as the leucite containing ceramics used with metal frameworks. They are also less forgiving when deviations in techniques occur.

One of the handling problems we identified was lack of furnace calibration and, therefore, possibly lower-than-needed firing temperatures. Because the porcelain-to-zirconia combination is less forgiving, the porcelain furnace should be carefully and frequently calibrated. We also noted that the framework designs did not always properly support the veneer ceramics. Cusp tips, marginal ridges, and sometimes facial surfaces had thick ceramic present that broke after a few months’ service. Strict adherence to company directions for firing techniques may also be a problem. The firing techniques used with zirconia are different than those used with metal. With zirconia, we need to ramp-up slower and cool-down slower.

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