It is well documented in the literature that restored teeth that are endodontically treated are at a higher risk of failure than the restoration of vital teeth. So, why do endodontically treated teeth have a higher risk of failure, and how do they tend to fail?
Post loosening: Although you may think that the most common way endodontically treated teeth fail after being restored is root fracture, the literature shows that loosening of the post inside the canal is the number one mode of failure in these types of patients. For example, a patient comes into your office complaining about a loose crown. Sure enough, when you look in their mouth you notice that the crown is loose, but doesn't easily come off. It wiggles enough to grab the awareness of the patient, but doesn't fall out. If you keep wiggling it and apply enough pressure, you will eventually get it off. And when you do, you typically find that the crown and post core come out of the canal in one piece.
When evaluating the remaining tooth structure after removing the crown, you will notice that although the cavosurface margins of the prep are intact, there is a considerable amount of carious dentin present in the canal. Removing the caries and cleaning up the area will drastically thin out the remaining tooth structure, and yet when trying the restoration back in, it will still fit, albeit loosely. Given the lack of retention, your first instinct is to bond the post and crown with resin cement. Although this will keep the restoration in place, it will inevitably lead to the second most common mode of failure of endodontically treated teeth: fracture of the root.
Root fracture: It's unknown how long it will take for a root fracture to occur since it varies from patient to patient. However, once the root has fractured the tooth will need to be extracted and replaced with an implant.
It is important to note that whether the post comes loose or the root fractures, they can both be thought of as catastrophic modes of failure. The whole scenario seems very similar to a "pay me now of pay me later" kind of situation; either way you look at it the tooth will eventually need to be replaced with a single tooth implant.
The key to predictably restoring teeth that have been endodontically treated essentially comes down to the amount of remaining tooth structure. The more tooth structure that you have to help support the restoration, the more predicable the restoration will be.