Why has my root-treated tooth fractured?

It is a well-known fact that teeth which have a root canal treatment are weaker.

These teeth have a reduced capacity to resist forces during chewing and biting.

Whenever you undergo a root canal treatment, the dentist may tell you the tooth will also need a crown also known as extra-coronal restoration.

There are two clinical reasons why root treated tooth are weaker and more prone to fracture.

1.  Generally a root canal treatment is carried out when there is already extensive caries which has now involved the dental pulp. These teeth have therefore already undergone extensive coronal and radicular tooth tissue lost due to the pathology.  However this does not fully explain why a tooth which has had a root canal treatment is still weaker than an equivalent restored vital tooth. 

2.  The second reason is that the dentine of a non vital tooth undergoes desiccation due to loss of the pulp.  In order to understand this, we need to appreciate that tooth dentine is 30% mineral in content and 70% organic which includes water.  The water content of the organic component is about 10%. It varies because younger  teeth have a higher moisture content.  Also due to the formation of tertiary dentine, water content of the dentine is reduced because tertiary dentine  esp reparative dentine is more dense and has a higher mineral content.

This water content of dentine is held in the dentinal tubules produced by odontoblasts which are filled with water at particular hydrostatic pressure. 

 This theory of dentine moisture dehydration was first reported by G.V. Black. It was later explored in more detail by Helfer et al who reported that non-vital teeth have 9% less moisture than normal healthy teeth. However, Papa et al and other studies  reported insignificant difference in the moisture content between endodontically treated teeth and teeth with vital pulps.

It is important for a dentist to take precautionary steps when carrying out a root canal treatment on a tooth for a patient.

Firstly, as well as pre-warning the patient that the tooth will become weak and will require some sort of extra-coronal restoration. The access cavity procedure must try to avoid taking away too much sound tooth enamel/dentine structure. But many inexperienced will try to be over-cautious in drilling too small an access cavity that they end up compromising in the ability to locate all the canals and being able to fully clean the root canal system which causes the root canal to fail. It is better to over prepare the access and this allows better vision and better access hence the chances of a more successful root canal.

For many years dentists believed that placing a post inside the root would reinforce the remaining tooth and make it less prone to fracture. However this is not true and a post can actually make the tooth even weaker as what is important is the remaining volume of dentine remaining. It is better to have more remaining dentine than trying to drill a post losing more tooth and cause weakening. Even when a post is required due to lack of sufficient coronal tissue for retention, the current concepts all mention using a ferule around the crown preparation which fully encompasses the post.