Generating larger master apical diameters with excellent irrigation, prior to obturation, creates cleaner canals compared to those using smaller apical preparations.
But small and large are relative terms, and clinicians can vary with the size that they feel is appropriate to prepare molar canals.
I estimate that most clinicians prepare canals to an average size of 30mm with either a taper of 4mm or 6mm. The narrowest part of the apical foramen has a diameter of 28mm, and it is at this point that instrumenting usually finishes.
Therefore, preparation to a size of 25mm or 30mm is probably doing little to help shape the canal walls at the minor constricture.
Many traditional preparations for smaller master apical files can make it difficult to clean canals properly. This is because they are not large enough to irrigate properly and to ensure the complete removal of dentin.
I have been a private practice endodontist for 15 years, and devote approximately 75% of my work to the non-surgical retreatment of endodontic failures.
In my experience I have found that a key factor behind many of these failures is the fact that insufficient space has been allocated during apical preparations.
Endodontic literature also says that this as an important factor in determining successful treatments. Other important factors include the presence or absence of apical lesions, and the placement of a proper coronal seal after treatment. But it also makes intuitive sense that if the canal can be prepared to a larger diameter then it will be easier to keep clean. It can be clearly seen (Figures 1 and 2) that creation of a larger apical diameter means that – circumferentially – more dentin can be removed from the canal wall, compared to a smaller space.
Figures 3a to 3b show a clinical case treated to a larger master apical diameter. Seeing the larger prepared canal, it is obvious, that if such a preparation can be achieved safely and efficiently, it will provide the greatest canal cleanliness possible.
As well as being easier to clean, there are other benefits from creating larger apical canals. These include easier cone fit and more predictable, and efficient warm obturation using either the warm vertical compaction or SystemB technique. The chances of a canal blockage developing is also reduced, because of the improved irrigation potential. Additionally, once a canal reaches a critical threshold size, the chances of an iatrogenic effect taking place are lowered.
Traditionally, canal preparation has been carried out to approximately a 30mm tip size preparation and a 6mm or 4mm taper. Such preparations, (when using K files, Peezo reamers and Gates Glidden drills), run the risk of causing apical zips, tears, perforations and transportations of all types.
Master apical preparations created with rotary nickel titanium instruments run a risk of instrument fracture, irrespective of their size. Inherent in considering the preparation of larger master apical diameters, it is essential that such preparations are performed safely and without iatrogenic outcomes.
In part two of this series the exact means by which these larger master apical files sizes can be created will be addressed.
* Dr Mounce would like to thank LightSpeedUSA (San Antonio, TX) for the images in Figures 1 and 2.