Barry Musikant

FIGURE 1: A C-shaped canal that has yet to be widened.	FIGURE 2: A C-shaped canal that has been widened, demonstrating the removal of tooth structure from the inner wall.

The result is a curved canal that is uniformly wider.  In single-rooted teeth this feature might be considered advantageous.  However, in a molar protecting the tooth structure between the inner wall and the furcation is very important in order to avoid strip perforations particularly in MB roots of maxillary and mandibular molars.  NiTi can do no differently because it lacks the hardness and torsional strength to allow it to selectively cut dentin away from the furca to straighten the coronal portion of the canal.  When a post is to be placed, drills are finally used that will straighten the coronal path of the canal to give the post straight-line access and greater depth.  Because tooth structure has been needlessly removed on the furcal side of the canal, the newly prepared straight-line coronal access has resulted in a canal space that may be significantly wider than it needed to be (Figure 3).  If a cast post is to be placed, it will have a wider taper that makes it that much more of a potentially destructive wedge (Figure 4). 
 

FIGURE 3: A picture of the canal space after tooth structure has been removed from the outer wall, producing a coronal canal space with a wide taper.	FIGURE 4: A wide tapered cast post with a wide coronal cement interface.

If a parallel prefabricated post is placed, a lack of coronal fit will result in a large cement interface that has lower retention and is more prone to breakdown (Figure 5).  If the practitioner chooses not to straighten the coronal canal path, the length of the post will be shorter, concentrating more functional stresses over a smaller area and the post will not be in line with the long axis of the tooth, resulting in more functional stresses being distributed laterally within the root containing the post.
    The EZ-Fill SafeSider, on the other hand, create straight-line access early in the instrumentation process.  Coronally, dentin is selectively removed from the outer wall, turning C-shaped canals into J-shaped canals (Figure 6). 
 

FIGURE 5: A parallel prefabricated post with a wide coronal cement interface.	FIGURE 6: The C-shaped canal opened up early on with a No. 2 Peeso to attain straight-line access, showing where the dentin is selectively removed.

Not only is a J-shaped canal far easier to instrument, because you are dealing with one curve and not two, but the canal shape needs little modification to accept a conservative cast or prefabricated parallel post.  The cast post will have a thinner taper, making it less of a wedge (Figure 7).  The parallel prefabricated post should fit fairly accurately within the prepared canal space along its entire length, assuring greater retention and a longer-lasting cement interface (Figure 8).  Straight-line access assures that the dentist can create adequate length as well as post placement in line with the long axis of the root.
 

FIGURE 7: A thinner tapered cast post in the canal with a thin coronal cement interface.	FIGURE 8: A parallel prefabricated post with a thin coronal cement interface.

    This discussion is an example of taking a concept that was first enunciated by engine-driven NiTi proponents, namely that greater tapered shapes are superior to those produced by conventional endodontic techniques, and achieving those results in a far simpler, more efficient, and more economical way. 
    As usual, we welcome discussion on any points that you may agree or disagree with and we will be happy to publish the insights gained from those discussions.  I personally prefer discussion by phone at (212) 582-8161, but if e-mail is to your liking, by all means use it.