Enhancing Predictability in the Mechanics of Endodontics

?The mechanical steps we take in endodontics include access, irrigation, instrumentation along with irrigation and obturation. We could also include post placement in those situations where inadequate coronal dentin is remaining. Increasing predictability includes increasing the chances of keeping the instruments intact, increasing the amount of pulp tissue and bacteria removed while retaining as much tooth structure as possible consistent with that goal, sealing the canals in three-dimensions with a cement that is highly resistant to dissolution while leaving a path for retreatment via removable gutta percha.?

Typically, the most popular item to talk about is instrument separation, but that only becomes an issue for the most part when rotary NiTi instrumentation is used. The initial issue after access is finding and negotiating the canals to the apex. This stage of treatment often termed glide path creation, is to produce a smooth pathway for the rotary NiTi instruments to follow. While this is an important function it is not the only function. It is also important to cleanse the canal space in three-dimensions taking note that many canal configurations are highly elliptical incorporating thin isthmuses as well as narrow buccal and lingual extensions. If the thinner stainless steel instruments don’t address these off-centered pulpal configurations, in addition to creating a patent pathway to the apex for rotary, it is unlikely that the rotary instruments most safely used with centered shaping will address them at this stage of the cleansing process.

Given the initial needs of three-dimensional irrigation and cleansing, the manual use of K-files is fatiguing and time consuming. It would take a lot of effort to extend the manual cleansing operation in a three-dimensional manner. Consequently, while a patent pathway is predictably created, three-dimensional shaping is often not included as a goal. Both phases of initial canal preparation are far more likely to be attained if the instruments are used in a 30o handpiece oscillating at 3000-4000 cycles per minute. Like manual, oscillation is confined to a short arc of motion that virtually removes torsional stress and cyclic fatigue virtually negating any possibility of instrument separation.?

The 30o handpiece generates horizontal motion at right angles to the long axis of the tooth. We know that blades shave dentin away from the canal walls when they are more or less at right angles to the plane of motion. To take advantage of the handpiece and enhance canal instrumentation, reamers with flutes twice as vertical as those on a file, are used instead. They have the added advantages of being more flexible, encountering reduced resistance, shaving dentin away with the first clockwise stroke and minimizing the potential for apical debris impaction when used with an up and down motion compared to a file design. Being virtually immune to breakage, the thinnest reamers (06/02) can be vigorously applied to narrow pulpal anatomy. Common sense tells us that thinner reamers are more likely to penetrate narrow configurations activating the irrigants flooding these spaces with the high frequency handpiece as a source of energy. Removing both pulp tissue and bacteria with minimal loss of tooth structure is a basic goal of endodontics and is routinely accomplished with stainless steel reamers in the 30o oscillating handpiece.

The reamers can be further enhanced by incorporating a flat along their working length making them more flexible, encountering even less resistance, creating two vertical shaving blades that shave dentin away as the handpiece oscillates in the clockwise and counterclockwise direction. The flat does not increase their chances of separation when used in the oscillating handpiece. Three-dimensionally cleansed canals that preserve more tooth structure are likely to produce more predictable results.

Relieved stainless steel reamers up to size 40/02 coupled to the 30o oscillating handpiece may be used for the creation of the three-dimensional glide path generally no larger than a 20/02 or they may be extended through a 35 or 40 to shape the entire canal space when rotary is not employed. They may also be used as an intervening safety valve if and when rotary is encountering what is determined to be excessive resistance that could lead to instrument separation. Oscillating relieved reamers gives the dentist a great deal of adaptability.??

In a previous post, I showed an obturated middle mesial in the mesial root of a lower molar that was prepared to the same dimensions as the mb and ml. It looked quite good on x-ray, but from the preparation it appears that there was a lack of recognition that unlike the mb and ml canals that are in wider more robust sections of the root, the mm canal travels through a length of root that has a well-defined concavity along its furcal surface. A canal preparation of much smaller dimensions would have been safer and in accordance with the original canal anatomy in this zone that would have also been much thinner reflecting the external anatomy of the root.?

The general idea would be in all phases of endodontic shaping to remove a uniform amount of dentin circumferentially. Following this dictate does not lead to over preparation of what were originally narrow pulpal configurations. If an equal amount of dentin is removed from all the walls, a canal that had a highly elliptical configuration to start with would have that same configuration in larger form after instrumentation. The sonic activation of the irrigants in close proximity to the thin rapidly oscillating reamers has been shown to remove the smear layer throughtout the length of the canal opening up the dentinal tubles for better penetration of the sealer when obturation is performed.?

Talking about ways to enhance endodontic predictability in somewhat reverse form, let’s say a bit about access. This is where the color of dentin can be a strong clue to where the underlying pulp tissue is as well as the canal orifices. Where the pulp chambers are large, the color of dentin is barely an issue with the easy popping in of a round bur after about 7 mm of penetration. However, as the canals become more calcified, popping in is no longer a possibility. We must proceed with caution making sure we are going in the right direction and making doubly sure that we don’t go so deeply that we perforate the floor of the pulp chamber. Dentin is always darker when pulp tissue lies beneath. This becomes far more noticeable when observing the floor with loops or preferably a microscope. You never want to go where the dentin is becoming lighter and lighter. That will lead to a lateral perforation. This type of perforation is most likely to occur when attempting to gain access to a maxillary canine. Line the bur up incorrectly and what you believe is centered accessing is often either mesially or distally oriented away from the center. In these situations, take a step back, examine the floor you have created, look for a portion of dentin even along a lateral wall that is darker (assuming an amalgam is not present). Take an x-ray and see if the location of that darker dentin aligns more closely with where the canal should be than the original access path you have made. If it does follow the darker dentin and you are far more likely to locate the canal orifice. I find the use of Munce burs quite valuable when encountering situations like this one.

It’s a funny thing about hints for improved endodontic mechanics. What are first hints simply become part of a repetitive protocol that makes the procedures more predictable. They become an integral part of the procedures setting us up for future hints that will help us even more. The process never stops.

Regards, Barry