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ePaper created 2016-04-27, 11:39:51 | version 1.38.0
clinical opinion Endo Tribune U.S. Edition | April 6, 2016 4 By Barry L. Musikant, DMD With the increasing awareness that greater tapered rotary instruments can induce dentinal micro-cracks,1-2 which in turn can lead to vertical fractures and tooth loss over time, a growing number of endodontists are preparing canals more conservatively, with smaller apical preparations and reduced tapers along length. Where apical preparations of a .35–.40 mm were once the norm, today those preps are being reduced to .25 mm or less, with tapers no greater than 0.04 mm/ mm. The result is the preservation of tooth structure in the mesio-distal plane with the additional bonus of needing fewer instruments, lowering costs and reducing the time needed for instrumen- tation. It should also be stated that lesser tapered instruments are more flexible and more resistant to cyclic fatigue that, if excessive, leads to instrument separa- tion.3 From the perspective of iatrogenic events, the implementation of conser- vative endodontics as defined above re- duces instrument breakage and leaves a tooth less prone to vertical fracture. On the other hand, we must not forget that pulpal tissue is most often configured as a highly oval body with buccal and lin- gual isthmus-like extensions. When preparing canals with greater tapered rotary NiTi instruments, these buccal and lingual extensions are left untouched and, worse, are blocked off from the irrigants by a smear layer that coats their entry points (Fig. 1). From the point of tissue removal, thinner less ta- pered preparations are removing less tis- sue and leaving a space where a smaller volume of irrigant will be applied. Stated simply, rotary shaping produces round conical preparations. Reduce the tip size and taper of the shaping instru- ments and we still produce a conical preparation, only smaller. Dentin is pre- served, but more pulpal tissue and the bacteria they support are left untouched. We are left with a bit of a dilemma. Use greater tapered instruments and we will remove more tissue but increase the likelihood of producing dentinal micro- cracks4 while definitively weakening the remaining tooth structure and increas- ing our costs and the time needed for the shaping procedure. Switch to lesser- tapered preparations and the canals are not adequately cleansed particularly in the bucco-lingual plane. What appears to be a set of choices, each of which have their strengths and weaknesses, does not give us the tools we need to attain our goals of optimum cleansing with minimal tooth removal. Since its implementation more than 25 years ago, those who use rotary in- struments have learned that care must be taken to minimize the incidence of instrument separation. That learning curve includes: 1. The creation of a glide path that al- lows the unimpeded progress of the ro- tary instrument to the apex. 2. Crown-down shaping that reduces the engagement of the instrument along the full canal length. 3. Straight-line access. 4. Single usage. 5. The application of minimal apical pressure using a light pecking motion when negotiating apically. 6. Staying centered with minimal lat- eral brushing. 7. The possible adoption of interrupted rotations (a version of reciprocation that includes full rotations) to reduce torsion- al stress and cyclic fatigue. 8. The implementation of newer heat- treated alloys that reduce without elimi- nating instrument separation. A learning curve that must incorporate so many prerequisites conditions a den- tist using a rotary system to be cautious when shaping canals. The result is the creation of conically shaped canals, be- cause deviating laterally from that cen- tered approach increases the chances of instrument separation. The fact that a good deal of tissue and bacteria exist in lateral spaces is under- appreciated in the quest for procedural safety. It is also a fact that the mesio- distal periapical X-ray of a tooth fully ob- turated from stem to stern without any obvious distortions is considered proof of a job well done, despite the dearth of information on the shaping and obtura- tion done in the bucco-lingual plane. Until we develop techniques that can remove tissue and bacteria without any instruments touching the walls of the canals and then obturating them com- pletely, we will remain dependent on metal instruments physically shaping the canals. To resolve the present dilem- ma, we must primarily have an approach that is virtually immune to instrument separation. Instruments break for two reasons: ex- cessive torsional stress and cyclic fatigue, both resulting from varying degrees of rotation. If we limit the amplitude of mo- tion to 30 degrees to 45 degrees, we limit the arc of motion to either a 1/12 or 1/8 of a single full rotation, a reduction so sig- nificant that neither torsional stress (pro- duced by an instrument locking apically) nor cyclic fatigue (produced by rotations around a curve) becomes a destructive factor. The fact that an instrument is now vir- tually free of breakage means the dentist no longer has to employ the precautions that were necessary when using instru- ments that undergo complete rotations. Straight-line access and crown down preparations are no longer required, preserving more tooth structure in the mesio-distal plane while significantly re- ducing the incidence of dentinal micro- cracks. Not only is more tooth structure preserved but the integrity of the re- maining dentin is not degraded. Studies have repeatedly shown that small ampli- tudes of motion are far less likely to in- duce dentinal damage. Of most importance, with a 30 degrees to 45 degrees arc of motion, we are now free to work our thinnest 02 tapered stainless-steel relieved reamers vigor- ously against the buccal and lingual ex- tensions. These are the most appropriate instruments because they will encounter the least resistance when being worked buccally and lingually. Highly flexible in thin dimensions, stainless-steel relieved reamers main- tain a sharp cutting edge, shaving den- tin away as they rapidly oscillate in the 30 degrees to 45 degrees reciprocating handpiece at 3,000 to 4,000 cycles per minute. Unlike rotary NiTi continuous or interrupted, we now have a means to shape canals that reflect their original anatomy in larger form, something that can only be done predictably because the dentist knows the instruments will stay intact. Rotary NiTi for some has proven itself to be more efficient in shaping canals compared to the traditional hand tech- niques, and the idea of abandoning such an approach may bring back memories of hand fatigue and reduced rates of ef- ficiency. It is a reasonable question to ask whether or not the implementation of 30 degrees to 45 degrees engine-driven reciprocation will impact their expecta- tions of efficiency. The best way to answer that is for the dentist to realize that after the first reamer is negotiated to the apex manu- ally, all instruments after that are placed in the reciprocating handpiece oscillat- ing at 3,000 to 4,000 cycles per minute. Short arcs of motion utilized at high fre- quency allow a sequence of instruments to rapidly negotiate to the apex. In fact, the entire glide path preparation, a task often taking a good deal of time prior to the use of rotary NiTi, is now done quick- ly and efficiently. To better understand the innate advan- tages of the use of relieved reamers with short arcs of motion at high frequency, one must appreciate the fact that thin in- struments can widen canals beyond their own dimensions. In the thin mesio-distal plane, there is minimal need for that, but in what is often the far wider bucco- lingual plane, a thin, rapidly oscillating reamer can extend the canal preparation several times its own size, extending the Endodontic trends that do and don’t make sense Here at the AAE To learn more about tapered stainless-steel relieved reamers, stop by the Essential Dental Systems (EDS) booth, No. 823. Fig. 1: CT scan showing treated canals (red) and untreated isthmus (green). Photo/Provided by Barry L. Musikant, DMD, and used with permission from rootcanalanatomy.blogspot.com. ” See TRENDS, page 6