Some clinical studies have shown that there is a weakness in the overlay materials that causes fractures at the marginal ridge areas. How much strength do you really need with zirconia if the overlay materials have less strength that can result in fractures?

The fractures are most likely caused by insufficient support of the overlay porcelain in the marginal ridge area. Porcelain is much stronger in compression that in is in tension and shear. If the marginal ridge area isn’t built out with sufficient zirconia, it will leave the porcelain in that area unsupported and vulnerable to tensile and shear forces. If the framework is designed in such a way as to provide support for porcelain at the marginal ridge, you won’t see as many fractures. Another cause of fracture is internal stress caused by the heating and cooling of the zirconia and porcelain during firing.