Dr. Ed McLaren, DDS, MDC, Director of the UCLA Centers for Esthetic Dentistry and Esthetic Design, is at the forefront of research on zirconia and 3M™ ESPE™ Lava™ restorations. He talks about his experience with porcelain-to-zirconia restorations and how to minimize chipping.
At the UCLA School of Dentistry, we've been involved with zirconia since 2003, and have placed more than 1,200 Lava restorations that we continue to follow. Around 2005, we began seeing some chips and delaminations, at a rate of about 3% per year on our early samples.
We talked with ceramic engineers about the thermal behavior of zirconia and the significant differences between porcelain and zirconia. With porcelain fused to zirconia, the coefficient of thermal expansion (CTE) is a critical issue just as it is with porcelain fused to metal. However, zirconia presents a new challenge due to its slower heating and cooling rates (i.e., its thermal diffusivity).
Research shows that the conventional firing process with a one- or two-minute slow cool can cause stress to build at the porcelain-to-zirconia interface, as the zirconia takes much longer to give off its heat. If a zirconia restoration is fired this way, the porcelain can microscopically separate from the zirconia and micro-cracks can develop in the porcelain. These spread throughout the porcelain due to normal oral stresses. This is what we believe is the main cause of the chipping and cracking we've observed.
Four years ago we developed the following protocol that's significantly minimized the cracking issue. It includes a considerably slower cooling process and emphasizes proper framework design, which is even more critical because of the thermal problems inherent in the larger volume of the zirconia framework.
- If any grinding is necessary once the coping is sintered, use a new fine diamond under copious amounts of water. Gently sandblast ground areas using 50 μm aluminous oxide at 20psi.
- If there's significant grinding, a thermal transformation firing should be done with the zirconia framework at 1050°C and hold for five minutes in air.
- Next, apply a dentin wash porcelain in a thin layer and fire to a minimum of 50°C higher than the manufacturer's firing recommendation. This creates a wetting or bonding layer.
- Build the porcelain and then fire at a slow heat rise of 40°C per minute on each bake.
- Hold the bake for one minute, and then slow cool for three minutes on the body bakes.
- The most important bake to slow cool is the last bake. On the last firing cycle (i.e. the glaze cycle), cool from the high temperature to 500°C for six minutes, cooling at 50°C per minute.
- Leave the framework on the muffle until the temperature is under 200°C. This takes 10 minutes, so the total slow cool cycle on the last bake takes approximately 16 minutes.