13. August 2011 06:48
There are many reasons that crowns can break in a patients mouth. Once a crown is placed in the mouth, it probably will not last a lifetime. If the natural tooth cannot last that long, you should not expect a crown to last forever. Every patient and situation is different. Having said that lets review several of the reasons for failure. The first reason for failure is a cavity under a crown. The weak link is where the crown meets the tooth. There is a microscopic opening of several microns (very small) at this interface. One of the purposes of cement is to fill this gap. If the cement should break out, bacteria can enter and a cavity will form. This will result in crown failure.
The next reason for failure is stress on the tooth. If the amount of tooth structure under the crown is not sufficient, a heavy amount of sideways force can actually break the remaining tooth underneath. In that case the entire crown will break off the root portion of the tooth. There are certain parameters that are used to decide how much tooth should be needed prior to placing a crown. This can be different for different types of materials and crowns.
Reason three for failure, would be on a porcelain crown. The porcelain can break or chip on the crown. This could be from a traumatic event, such as eating the wrong foods or blunt trauma. If the tooth is all ceramic and the porcelain breaks, tooth would be exposed under it which could lead to sensitivity or cavities. If there is a substructure of ceramic or metal, the tooth may be OK, but the fractured area may affect the bite or cause gum irritation and necessitate replacement. Porcelain is a glass. It is very strong with direct pressure with something under it, but excessive sideways pressure will break porcelain every time.
Another critical reason for a crown to break is from a bad bite. The forces of a patient's bite can cause direct failure of the porcelain from chipping to fracture or can rock a crown over time causing the cement seal to fail as mentioned above. This bite issue is a very common occurrence and should be considered and evaluated prior to placement of a crown. As mentioned above, there are many reasons for failure and different types of crowns may be used in different situations which are very patient specific.