Dennis Ouye, CDT addresses the importance of proper design and structural support.
Fractures in ceramic restorations have always been a challenge for technicians. Traditionally, they were caused by the lack of strength of feldspathic ceramic and insufficient bonding to the tooth.
While newer, stronger ceramics and improved bonding agents help prevent fractures, it's still important to remember that proper substructure design that provides adequate porcelain support is crucial to the success of a case, and this is as true for zirconia restorations as it is for PFMs. Even though zirconia is an incredibly strong material (approximately 1,345Mpa) and is stronger than most alloys, the veneering porcelain is still the weak link because it has a strength in the range of only 70Mpa. As with PFM technology, a zirconia understructure that provides adequate porcelain support is the best way to prevent fractures. Consider these four tips:
Design zirconia substructures so that any extension of porcelain beyond 1.5mm of porcelain is adequately supported. Copings designed by early CAD systems were generally made as uniform shells covering the prep and lacked the ideal design for support. Recognizing this problem, many manufacturers of milling systems have improved their software to create a substructure design that provides a uniform 1.5mm of space for building the ceramic veneer layer. I personally recommend extending support of the zirconia substructure to contact the adjacent teeth to prevent chipping at marginal ridges. Porcelain can be added occlusally beyond 1.5 mm (I've added up to 3mm) without risking fracture, but it cannot extend horizontally over the edge of the coping beyond 1.5mm.
The risk of fatigue is greatly increased at the marginal ridges, incisal edges and cusp tips of crowns where occlusal adjustments are made, so design the coping with adequate framework support in these functional areas. Ceramic needs support wherever it comes in contact with opposing occlusal cusps.
Sharp corners on the body of the preparation and/or shallow chamfer/shoulder margin designs also contribute to fracturing. Inadequate reduction of the lingual margin frequently occurs and doesn't provide a positive stop at the margin. A shallow chamfer margin won't prevent the coping from sliding past the margin during strong occlusal forces, and this increases the likelihood of the coping flexing and creating a crack. CAD software allows the coping to be designed with necessary space relief, a virtual die spacer so to speak, so it only truly contacts the tooth at the margins. Therefore, adequate depth in the margin design and/or 90° shoulders provide a more stable foundation and resistance to vertical occlusal forces and a more positive seating of the coping.
Unlike metal, zirconia absorbs heat more slowly than ceramic, so it's important to slow down the heat rate to allow the zirconia to catch up with the overlying ceramic to facilitate a stronger bond to the core and a stronger ceramic surface. For example, if the rate you presently use is 45°C/minute, drop the rate to 27°C/minute. Of course, this significant drop in the heat rate may mean you need to lower the high temperature bake by about 10°C, as the veneering porcelain will likely mature at a lower temperature due to the increased time in the muffle with the lower heat rate.