Understanding Endodontics Mechanics: The Key to Real Progress

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I CAN’T HELP but notice the introduction of new endodontic instrumentation systems, the most recent emphasizing single-instrument shaping. I read the articles written by advocates who support this approach, and I note in the background a push for a greater use of pulpotomies because endodontics requires too much skill and time. That approach is so much more challenging that the increased success rate is not worth the increased effort. Pulpotomies are a natural extension of single-instrument endodontic instrumentation. If one instrument can replace several while shaping a canal to a degree significantly less than that achieved by traditional endodontic standards, and success rates don’t plummet, why not skip all canal shaping and place a fixative paste in the pulp chamber and call it a day?

Well, one reason we don’t skip canal shaping is that, despite the increased sound and fury from single-instrument advocates, most of us have seen the failures that come from missed canals or undertreated canals, treatments that are associated with poor obturation results. The patient’s symptoms don’t improve until the canals are enlarged to a point at which they are far more completely cleansed or canals that were missed are found, properly prepared, and then obturated. So, we have conflicting pressures on us. On the one hand, the overwhelming increased expenses we experience from the use of rotary NiTi systems coupled to the fear of separation make a single system that minimizes rotation quite attractive from the point of rotary NiTi’s weaknesses. On the other hand, the reduced cleansing by single-instrument systems, as noted in the literature, sets up conditions that logically will lead to a higher failure rate. This predictably higher failure rate is directly related to poorer cleansing procedures, particularly in the bucco-lingual planes of highly oval canals. Furthermore, the benefits of reduced instrumentation, lower costs, and a reduced incidence of separation are experienced immediately. Failure often takes several years to take form and produce clinical symptoms. The manufacturers can count on that delay, hawking the immediate benefits that are unquestionably tangible economically for the dentist, while the negatives will arrive at a far later date.

We can buy into the concept of buying time for the patient; we can let ourselves be the willing victims of marketing; or we can take a more scientific point of view that assesses the factors that lead to the most predictable and highest success rates. I think that any list of important factors should include the following:

Debridement: As full debridement of the canal system as possible while maintaining as much of the remaining tooth structure as possible.

Disinfection: As thorough disinfection of the intricate canal system via irrigants as possible. At this point it is important to note the direct relationship between canal shaping and effective irrigation. The latter is a function of the former.

Safety: The greatest ability to eliminate iatrogenic events, including separated instruments and distorted canals that may be the result of perforation, ledging, or transportation.

Sealing: The ability to seal the canal systems to prevent the ingress of bacteria, or the ability to fix any bacteria that were not removed and killed at the time of instrumentation or irrigation.

Restoration: Proper restoration of the treated tooth so that it can successfully function for many more years. An important corollary would be to remove no more tooth structure than necessary when first gaining access to the canals. We will have a stronger restoration if more remaining tooth structure is present to support it.

When we provide a list such as the one above, we are really stating the results that we would like to achieve while avoiding the side effects, extra results that are not beneficial to the success of treatment. Having clear insight into exactly what we want to accomplish should be a great motivator in determining what instrument designs and implementations are most likely to produce the results we want. For example, do you want to use a K-file with a watch-winding motion to do the initial canal shaping? From a mechanical point of view, the horizontally-oriented flutes along the length of the shaft will tend to engage and disengage from the canal walls when a watch-winding motion is employed. Engagement and disengagement cut grooves into the dentin, but do not remove any dentin until the pull stroke is employed. This is so because the horizontally-oriented blades on a K-file will shave dentin away only when the motion employed is at right angles to that of the cutting blades of the instrument. One could use the K-files in this manner using a twist-and-pull motion to shave dentin away from the canal walls. However, continuous use of the pull stroke with instruments designed as K-files will selectively instrument to the outside walls of curved canals, causing transportations. Furthermore, the horizontal blades along the length of the shank of the K-file (Figure 1) tend to impact debris apically, producing unwanted blockages to the apex. Recapturing this lost length often results in further distortions to the outer wall.

Yet K-files continue to be used despite the clear illogic of their design and utilization, and despite the insights of the endodontic giant, Dr. Herb Schilder, who wrote many articles on the benefits of reamers, instruments that are designed with a significantly lower number of predominantly vertically oriented flutes. In fact, the benefits derived from Dr. Schilder’s insights can be further enhanced by placing a relieved flat along a reamer’s entire working length. We now have an instrument that engages far less along length, shaves dentin away far more efficiently, and is more flexible. Together, those improvements produce an instrument that delivers far superior tactile perception to the dentist. With increased tactile perception, the dentist is now using an instrument that can differentiate between a solid wall and a tight canal, thus telling the dentist when he must remove the instrument from the canal, bend it at the tip in an attempt to manually negotiate around any impediment that may exist, and then negotiate to the apex either manually or using a thirty-degree reciprocating handpiece (Figure 2).

Mention of the thirty-degree reciprocating handpiece brings up the topic of the proper method of utilization. A properly designed instrument can still be ineffectual or can even induce damages if it is utilized in the wrong way. Rotary NiTi has clearly had problems of separation as a result of rotation. The twin factors of torsional stress and cyclic fatigue are both present when utilizing the NiTi instruments in rotation, and both either separately or together can induce instrument breakage. While this problem has been somewhat modified with newer NiTi instruments that are less vulnerable to breakage, the problem still exists and induces caution among rotary NiTi users, rewarding those who shape the canals most conservatively with a lower incidence of separation. This is a beneficial sequence of events from the vantage point of reduced breakage, but a negative because the canals are not cleansed, irrigated, or obturated as well as they could be. This brings us full circle to the point made earlier about incorporating a single-use system used in a modified reciprocating motion to shape canals. The dentist gains by reduced costs and fewer separations. The patient loses because of poorer shaping, irrigation, and obturation.

An alternative far safer than rotation is thirty-degree engine-driven reciprocation, using a handpiece that effectively eliminates torsional stress and cyclic fatigue as factors to be concerned about. Used with K-files, horizontal reciprocation would not work. It works only with instruments that incorporate vertically-oriented flutes. These instruments are called K-reamers. When the K-reamers incorporate a flat along their working length (Figure 3), they then become what are known as relieved reamers, and they work most effectively, routinely opening even significantly curved canals to a 35, 1 mm back to a 40 with a 25/06 overlaid tapered reamer finally employed. It is rare that a dentist will shape a curved mesio-buccal canal of a molar to a 35. We all know the reason why. Yet this is what we routinely do with the relieved reamers utilized either manually or in the thirty-degree reciprocating handpiece.

We want to shape canals without distortion. We want the instruments to stay intact, and we want enough flexibility in the system so that it can be used in all sorts of challenging situations (Figure 4). This is most possible with a system that addresses dentists’ concerns from the outset, so that they can then build the confidence necessary to shape a canal system without compromise. In short, if the instrumentation follows the proper rules of design and utilization, we will get the results we want without worrying about iatrogenic side effects—because we will know that bad things are not going to happen.

Please keep in mind that I conduct free 2–3 hour one-on-one workshops in my office for those who are interested in learning more. For more information or to set up a mutually convenient time for the workshop, call (212) 582-8161. You receive 2 CE credits for taking this course, making any expenses you incur in reaching me a legitimate tax deduction.

April - June 2012

Figure 1
FIGURE 1: A K-file. Note that K-files have an increased number of horizontally oriented flutes.
Figure 2
FIGURE 2: A reciprocating handpiece with a relieved reamer.
Figure 3
FIGURE 3: A relieved reamer. Note that relieved reamers have a patented flat side and fewer, more vertically oriented, flutes.
Figure 4
FIGURE 4: An .08 reamer negotiating to the apex in an abruptly curved J-shaped canal.