Cone fit can be challenging. Ideally, cones should contact only the apical 3-4 mm of the prepared canal space. Achieving this intimate contact in the apical third provides what has classically been known as “tugback.” Tugback is the resistance of the cone to gentle coronal displacement. As a properly fit cone is inserted into the canal, it should feel like it is locking into or seating into the prepared canal space. In other words, there should be some tugback of the cone as it fits into the final 3-4 mm of the canal. In essence, the cone should exhibit a definite resistance to coronal displacement. Once the cone is removed from the canal, it should be free of any kinks and defects, and should not appear stretched or deformed in any way. Instead of trying to match a sized cone to a particular file of a given taper and tip size, it is much more efficient and practical to learn to masterfully trim cones. With practice, it is relatively simple, and should present a modest learning curve.
Consider the following example. If the master apical preparation is a .06-tapered size 40, and if a matching gutta percha or RealSeal (SybronEndo, Orange Calif.) point does not slip in place passively and with tugback - you may wish to take a 35 tip-size cone and remove a millimeter from the tip. After doing this, see if the resulting cone fit is clinically acceptable. Alternatively, it might be possible to take a medium or a fine-medium cone and trim the cone millimeter by millimeter until the resulting cone fit is clinically acceptable. I only use chloroform to custom-fit cones in very unusual clinical cases that are beyond the scope of a general dental practice.
For those dentists performing bonded obturation, the .06-tapered, 20 tip size (.06/20) RealSeal cone by SybronEndo is an “all-purpose” cone that can be trimmed to meet the vast majority of clinical situations. For those using gutta percha, the .06/20 gutta percha cone also can act as a universal cone. Other cones can serve the same function. But this is the tip size and taper that, once trimmed, has been the most functional for me.
In some cases, trying to fit a master cone to a prepared canal space can prove vexing even for an experienced clinician. Cone fit problems most often result from underpreparation of canal spaces. It is relatively simple to fit cones into canals that are adequately prepared. Relying upon a given size master cone to fit or correlate exactly with a matching size rotary nickel titanium file is problematic. Manufacturing variances occur, so cones often will range between various rotary file sizes and vice versa.
Cone fit challenges are not indications for warm carrier-based obturation products. As an aside, having re-treated many failed warm carrier-based obturations, I would not want such a product placed in my tooth. A discussion of such products is beyond the scope of this particular column. While warm carrier-based products are valid clinical techniques, they can - in my opinion - be difficult to remove in some instances. Besides their high cost relative to the alternatives, removal of warm carrier-based products also provides an unnecessary obstacle to re-treatment.
It has been said that a canal is ready to fill when the clinician can fit a cone. This is true in some instances. It would matter greatly based upon what diameter the canal is prepared. In other words, the initial size of the canal ultimately would indicate what the final prepared canal size should be. A significant body of evidence demonstrates unequivocally that creation of a larger apical diameter than usually achieved is consistent with cleaner canals. Creation of the ideal prepared canal space, to the correct taper and tip size, can go a long way toward making cone fits simple and easily obtained. Apical preparation size will be discussed at length in future columns.
I welcome your questions and feedback.
Dr. Richard Mounce is in private endodontic practice in Portland, Ore. Dr. Mounce is the author of a comprehensive DVD on cleansing, shaping, and packing the root canal system for the general practitioner. The material also is available as audio CDs and a Web cast pay-per-view. He lectures worldwide and is a widely published author. For more information, contact Dr. Mounce via e-mail at comfort@MounceEndo.com. Visit his Web site at www.MounceEndo.com.