A few days ago I posted a video on the endodontic shaping of highly oval canals that is unlike any other video that I know of in terms of conservative, but thorough instrumentation. I say this because, heretofore, thorough shaping has been defined as a greater tapered preparation that not only cleanses the canal in the thinner mesio-distal dimensions of oval canals, but is wide enough to include a major portion of the broader bucco-lingual anatomy. The downside to this approach includes excessive removal of dentin in the mesio-distal plane and at least some inadequacy in reaching the furthest extensions of pulp tissue in the bucco-lingual plane. Greater tapered NiTi instruments are used with the least chance of separation when they stay centered within the canal and are used in a crown-down fashion. These insights have been well established over the 25 years that have passed since the introduction of greater tapered rotary NiTi instrumentation.
The video, in sharp contrast to the techniques employed with greater tapered NiTi instrumentation demonstrates the following:
1. Instruments of lesser taper, mainly 02 tapered and only one instrument with an 04 taper will safely and accurately enlarge the canal space beyond their own dimensions if the amplitude of motion is limited to a 30º to 45º arc and the oscillations reciprocate at the rate of 3000-4000 cycles per minute. Short arcs of motion are necessary to consistently prevent instrument separation
2. The capability to widen beyond the dimensions of the instrument is important when shaping oval isthmus-like canal anatomy that is often quite wide in the bucco-lingual plane. At the same time we will be preserving more dentin in the thinner mesio-distal plane
3. Somewhat thicker tipped 02 tapered relieved reamers (SafeSiders and Tango Endo) will faithfully follow a glide path that has been first established with thin highly flexible 02 tapered reamers in both the mesio-distal and bucco-lingual planes. The result is most often a far greater taper in the bucco-lingual plane and a small taper in the mesio-distal plane preserving dentin where it is most crucial.
4. As the pulpal space is widened, the irrigants become increasingly effective even in the thinner mesio-distal dimension. This can be clearly seen in the video.
5. The bidirectional spiral cement applicator coats the entire common space that exists between the disto-buccal and disto-lingual canals with sealer and the prefitted gutta percha points are then coated liberally with more epoxy cement and placed to the apex in each canal. The result is a thoroughly obturated canal space via a combination of 2 single points and a highly flowable epoxy resin cement that can safely flood the canal space when applied with the bidirectional spiral.
6. Observing the final fill from the distal and furcal side of the roots we notice that dentin in the thin mesio-distal plane has been preserved, avoiding excessive thinning of the root on its furcal side where concavities are most often present.
7. The net result is more thorough shaping in the bucco-lingual plane and the preservation of dentin in what is from the start a thin root in the mesio-distal plane.
There is an increasing understanding on the part of many endodontists that the greater tapered preparations are weakening roots. At a minimum there is no proof that they have improved success rates, the only real rationale for their use. We see this insight by viewing increasingly conservative access and canal preparations that have become a source of discussion in recent articles and commentary on various dental forums. My take at this stage is that rotary NiTi even when used more conservatively with lesser-tapered instruments is still the wrong way to go. As long as full rotations are generated, whether continuous or interrupted, they will be a source of instrument separation and consequently, a disincentive to use them with vigor in the wider buccal and lingual plane leaving tissue behind as it presently does. There is also a large body of research documenting rotary instrumentation as a source of dentinal micro-cracks. With lesser tapered instrumentation, crown-down preparations must be modified increasing the amount of engagement the thinner instruments will encounter, a factor that increases the incidence of separation and places more stress on the canal walls.
As this video demonstrates, a great deal of instrumentation freedom is released when the amplitudes of motion are kept small, no more than 7 minutes on the face of a clock. Once the dentist is confident that the instruments he/she is using will remain intact, they will be applied with vigor wherever the anatomy tells us tissue is residing and that is something that does not presently exist in the world of rotary NiTi whether used with greater or lesser-tapered instruments. At 3000-4000 cycles per minute the reciprocating handpiece is activating the irrigants for effective removal and digestion of tissue remnants. There is nothing passive about this form of irrigation. The rapidly oscillating instruments are actively touching all the walls of the canal driving the irrigants up against the walls so the action is active and intimate.