When confronted with an angled access to a canal orifice, is attaining straight-line access good for the tooth, the instrument that will negotiate the canal or both? At one time, what was considered good for the instrument equated with what was good for the tooth. That symmetry is no longer taken for granted. Certainly, straight-line access along with preflaring reduces the stresses a greater tapered rotary instrument will encounter in turn reducing the incidence of instrument separation. That is good for the instrument and lowering the chances of leaving a segment within the canals is good for the tooth. It is my contention that any system that has a significant potential to leave a portion of the instrument in the canal is one that should not be employed in the first place. Creating straight-line access and preflaring is only acceptable because we are limiting ourselves to systems that require these preconditions. If we use systems that don’t require straight-line access and preflaring as prerequisites for safe instrumentation, there would be no need to unnecessarily remove the tooth structure associated with these procedures.
Here we come to the insight required for more conservative and more effective root canal instrumentation. What is good for the instruments is not good for the tooth when employing greater tapered rotary instrumentation. And this insight is true if we limit our instrumentation to the two dimensional mesio-distal view we see on we see on x-rays. When we think in terms of the bucco-lingual third-dimension, the problem of shaping now includes inadequate cleansing. The imposition of a conical shape excludes the tissue that resides in the buccal and lingual extensions of oval canals. The limiting factor is once again the greater tapered instruments that are more prone to separation when used in a brushing motion. In a sense this limitation is a plus. We already know that greater tapered instrumentation removes more tooth structure than necessary in the mesio-distal plane. A system that can extend this greater tapered shape the entire mesio-distal length of the canal will weaken the root significantly more. In light of the increased awareness of vertical fracture as a possibility after root canal therapy, increasing the amount of dentin removed is a direction we don’t want to travel.
I’ve written extensively on these built in short comings of rotary NiTi whether used in continuous or interrupted rotation. Rather than full rotations short arcs of motion not exceeding 45º oscillations leave instruments intact even when oscillating at 4000 cycles per minute. Separation is not an issue. Consequently, straight-line access and preflaring is not a requirement. 02 tapered relieved reamers even in sizes as small an an 06 tip are virtually invulnerable to breakage when the arc of motion is so limited. With breakage removed as a concern, the thinnest instruments can be applied with vigor against the thin, but elongated bucco-lingual configurations of pulpal tissue. The results include more thorough three dimensional debridement while preserving far more tooth structure in the mesio-distal plane. Roots remain stronger, a thicker layer of dentin remains on the furcal sides of roots and the effects of irrigation are extended to all aspects of the canals.
For those who consider the convenience of rotary NiTi a major advance over the traditional use of K-files, please recall that the glide path, a prerequisite prior to the use of rotary NiTi is generally a fatiguing manual task. Using a reciprocating handpiece oscillating at 30-45º at 3000-4000 cycles per minute eliminates hand fatigue from the start and significantly speeds up the entire shaping process. As we begin to appreciate the critical research associated with the use of greater tapered rotary instrumentation, it is becoming increasingly obvious that more conservative approaches represent a progressive alternative that ultimately produces less stress in the instruments, the teeth they are shaping and the dentists employing them.