Curved canals are plain sailing
Instrumentation of curved canals is like a mountain climber who seeks to scale a significant mountain. If the preparation is carried out correctly and, once at the top of the mountain appropriate care is taken to get off the summit, success is the predictable outcome. Therefore, the process needs to be broken down into a number of very small steps to make it successful.
All canals are curved to one degree or another, canals that might appear radiographically straight have some degree of curvature if only in a buccal to lingual direction. This said, even if canals are curved in a mesial to distal direction, they are also curved in a buccal to lingual dimension as well. Clinicians should not consider the process of treating curved canals any differently from straight ones.
The level of focus, concern and diligence in carrying out the process of instrumenting the root canal will not differ simply because one canal might appear straight on an x-ray and another might appear severely curved. The techniques, principles, and care each event will entail, should be essentially the same. However, there will be special emphasis on certain elements of the process in curved anatomy that can help increase the efficiency of canal preparation and reduce the risk of morbidity.
The sequence of rotary nickel titanium (RNT) file use, irrigation technique and access used is important. But, the single greatest technique for the management of curved canals, in the empirical opinion of the author, is to spend significant time with hand K files at all stages of the process, including the assurance of patency, glide path creation, and recapitulation during instrumentation. As long as the canal path is kept open and unblocked by debris, RNT instruments can be used successfully to make canal paths larger and instrumented successfully.
The correct sequence involves using the files from larger tapers and tip sizes to smaller ones. The touch is gentle and controlled, minimising engagement to 1-2 mm of canal wall at any given time. The hand file use that is key to management of curved canals is designed primary to prevent the build up of debris in canals, which can cause iatrogenic events.
Debris can be prevented from accumulating by using a combination of irrigation with canal exploration by hand files, assurance of patency at all stages of the instrumentation process, and creation of a glide path in any given canal third, before the clinician begins to use RNT files.
In a curved canal it may be necessary to irrigate after every RNT file insertion. It may also be needed to recapitulate after these irrigations and before the next RNT file insertion. Especially in pig tailed multiplanar curvatures at the end of some challenging canals, it may take significant time to clear the canal path with hand files before ever placing a rotary file.
It may also take significant effort, even after modest rotary use, to irrigate canals to remove debris and make certain that the canal path is still open. In addition, assuring patency and recapitulating in such situations requires a precurved hand K file, and will pass through the minor constriction of the apical foramen, ideally no more than 1-2 mm.
It is important to ensure that the minor constriction of the apical foramen is not altered in any way and that any debris that might block the canal path is removed. This is so that at any given time, the subsequent RNT instruments will not pack debris into the narrowing cross sectional diameters of the root.
For irrigation, the author uses 5.25% sodium hypochlorite, 2% Chlorhexidine (Vista Dental Products, Racine, WI, USA) and SmearClear (SybronEndo, Orange, CA, USA). This final irrigant is used as a final rinse to set the stage for bonded obturation through removal of the smear layer.
In summary, making certain that created debris is removed and that canal patency is assured through the use of small hand files is, in the empirical opinion of the author, the two most important techniques to remember while negotiating curved canals.