- Most canals are oval, much thinner in the mesio-distal plane than they are in the bucco-lingual dimension. The presence of thin isthmuses connecting wider orifices provides spaces where pulp tissue and bacteria reside and unless widened to some degree (30/02) will not be of sufficient width to overcome the surface tension of the irrigants used to digest the tissue and kill the bacteria.
- Rotary NiTi more or less limited to producing a conical shape in the presence of highly oval isthmus-like anatomy has the added problem of impacting debris along the length of adjoining thin isthmuses creating a more impenetrable barrier to the irrigants.
- The same factors that lead to instrument separation of greater tapered rotary NiTi, namely torsional stress and cyclic fatigue that create and elaborate micro-cracks in the rotating instruments, are also responsible for the production of dentinal micro-cracks. These dentinal defects have been documented in many studies and underline the impact of Newton’s third law of motion, stating: For every action, there is an equal and opposite reaction. The statement means that in every interaction, there is a pair of forces acting on the two interacting objects. The size of the forces on the first object equals the size of the force on the second object. Given dentin’s similarity in fatigue characteristics to common metals, it should come as no surprise that the defects found in the metal finds its counterpart in the dentin when both are subjected to the same stresses.
- The two similar types of defect are not recognized as being a common reaction to rotary NiTi, continuous or interrupted, simply because dentinal defects are impossible to observe clinically until they coalesce and propagate into a full vertical fracture while a separated instruments is impossible not to see. The lack of symmetrical observation does not reduce the impact of dentinal defects any more than they do of the highly observable outcome of the metal defects resulting from rotary instrumentation.
- Conical shaping removes excessive tooth structure in the mesio-distal plane where the roots are not only most often thin, but are accompanied by concavities most prevalently on the furcal sides of their roots, but present on many external surfaces as well. At the same time, conical shaping does not touch the buccal and lingual walls in highly oval canals leaving debris, bacteria and infected dentin behind.
- Despite the many improvements in the utilization of rotary NiTi, the concern for separation, while reduced with some of the new systems and heat treated metals is still a strong determinant in their use, encouraging straight-line access, crown down preparation and centered preparation taking care not to exert increased stresses by avoiding their use in strong lateral motions. The result is a conical shape, likely producing an attractive two-dimensional picture of a filled canal mesio-distally that too often has left the buccal and lingual extensions inadequately attended to.
By learning these principles the attendants gain some insight into the steps that rotary NiTi requires for the protection of the instruments all of which come at the expense of the structural integrity and effective cleansing of the roots in three-dimensions.
Once the problems of the rotary approach to endodontic shaping is understood it is far easier to address the benefits attained by using thin 02 tapered relieved reamers limited to a 30-45º arc of motion. These advantages include:
- The instruments are virtually free of separation. This is a key point. Unless a dentist is extremely confident that an instrument will remain intact, he/she will use these instruments in the most conservative fashion, namely staying centered and leaving the tissue, bacteria and debris that remain in the buccal and lingual extensions after the use of rotary NiTi. In contrast, knowing that instruments limited to a 30-45º arc of motion virtually eliminate breakage, the dentist can confidently use them vigorously in what is often the far wider bucco-lingual plane removing the debris and creating a space that is large enough for effective irrigation in three-dimensions. It should be noted that the instruments used in this manner can create a space far larger than itself when the canal anatomy calls for it.
- The virtual elimination of instrument separation applies to the thinnest stainless steel reamers used including the 06/02 tapered reamer. At this dimension, the instruments are highly flexible and will adapt to any curve that they encounter when negotiating apically. The 30-45º arc of motion is generated in a reciprocating handpiece that oscillates at 3000-4000 cycles per minute allowing a canal that was tight to a 06/02 tapered reamer to quickly create a space that the subsequent 08/02 tapered reamer will rapidly negotiate.
- Such engine-driven reciprocation eliminates hand fatigue from the start. The thinnest instruments may be used in the reciprocating handpiece because the small amplitude of motion reduces both torsional stress and cyclic fatigue to having an insignificant impact. The glide path to a 20 is made within a minute or two and then fully prepared in most cases to a 30/04 using two more instruments again employed in a 30-45º reciprocating handpiece.
- Following Newtons’s third law, not only are the instruments avoiding metal micro-cracks, but the dentin itself is free of dentinal defects, a fact that was documented in the studies of the self-adjusting file (SAF) from Israel. The SAF oscillates vertically at 5000 times per minute with an amplitude of motion of 0.4 mm. A size 40 tipped 02 tapered reamer oscillating at 3000-4000 cycles per minute in a 30º reciprocating handpiece will scribe an arc of motion of 0.104 mm or roughly one quarter that of the SAF that has been shown unequivocally not to produce dentinal defects. It is reasonable to conclude that 30-45º reciprocation, where the largest instrument rarely exceeds a 30, is safe for both tooth and the shaping instrument. This is the instrumentation system that is applied from the beginning to the f most pulpal anatomy were that of a maxillary central incisor endodontics would be far less complicated. Alas, that is not the situation and for us to make progress, we must develop and use instruments that address the limitation of rotary endodontics in ways that let us attack the wider bucco-lingual plane, while preserving tooth structure in the mesio-distal plane and further maintaining the integrity of the instruments and the teeth being instrumented. Rotary NiTi falls far short of that goal. The use of relieved reamers in 30-45º reciprocation doesn’t.
By correlating a more accurate view of anatomy, with mechanical instrumentation, we have a far better appreciation for instrument design, application and function that leads to solutions that are an improvement for the patient and the dentists performing the procedures.