What are some reasons why porcelain crowns develop cracks?

Cracks in porcelain can be caused by many things, but frequently are caused by stresses that are set up while firing/cooling. With metal-ceramic restorations, the coefficients of thermal expansion of the metal and porcelain must be very similar, or when the restoration cools, the metal and porcelain will cool at different rates, causing small fractures within the porcelain. These weak areas can be accentuated with heavy grinding of the surface, which can cause cracks to propagate, or grow. 

Another reason for cracking is if a thick area of porcelain is unsupported by the metal substructure. In the case of metal-ceramic restorations, the feldspathic porcelain that is baked to the metal substructure must have support by metal to maintain its strength. If porcelain is carried out to an interproximal area, for example, that is more than 2 mm beyond the support of the metal, it is prone to fracture. Porcelain is very strong in compression, but shear or tensile forces can fracture the restoration. You definitely should talk to your lab to make sure they are using a compatible metal and porcelain system that heats and cools at the same rate, and that they are paying attention to the design of the metal substructure that they are building.

One way to repair a craze line, one that could eventually crack in the patient’s mouth, that you see prior to cementation, is to re-fire the restoration. Craze lines can repair themselves if the porcelain is allowed to re-vitrefy.

All-ceramic restorations crack for different reasons. If you are using lithium disilicate, zirconia or another ceramic that is a “monoblock” type, make sure that the restoration is thick enough. Pay attention to the manufacturer’s instructions for minimal thickness. Fractures can also occur in zirconia restorations where zirconia is the core and leucite-reinforced ceramic is baked onto the zirconia core. These fractures occur for similar reasons to the metal-ceramic restorations.