Written by Barry L. Musikant, DMD Thursday, 31 March 2005 19:00
The path that endodontics has taken to arrive where it is today is a study in the evolution of common sense. With perfection as a goal, the interrelationship between this objective and the means to reach it has been tempered by many dead ends, cul-de-sacs, closed loops, and occasional forays of progress.
For most of us practicing today, we started our journey in the land of 0.02 tapered stainless steel files. These instruments were used manually to create 0.02 tapered spaces of wider diameters than the original canal space and in so doing mechanically cleanse the canal of debris. Irrigation fluids were introduced to digest organic debris left behind as well as flush out debris that did not exit with the instruments. These techniques did not produce shapes that allowed for the predictable obturation of canals by either lateral or vertical condensation. Yet, the attempt to obturate a canal fully with lateral and vertical condensation led to overfills, voids, postoperative pain, and fractured roots. Taking stainless steel files of greater diameter around curved roots also led to hand fatigue and various forms of canal distortion.
Even with much practice, performing endodontics limited to these 2 procedures produced less than ideal results. In fact, the present concept of the ideal had yet to be defined. Greater tapered shapes had not yet become an ideal, and consequently, the means to get there had not been developed. At some point, it became increasingly apparent that the use of stainless steel files enlarging canals to a larger version of a 0.02 tapered canal was not sufficient to clean, irrigate, or obturate the canal adequately.
Common sense directs us to understand that there is a relationship between shaping and filling. If the canal space was larger, then it would most likely be more thoroughly debrided during the shaping procedure. If the canal shaping included a greater taper than 0.02 mm/mm, then the gutta-percha points placed into the canal and subject to both vertical and lateral condensation could also have a greater taper, providing greater resistance form and being far less likely to be driven over the apex. If the canals had a greater taper, then they would also be much easier to irrigate. Tissue-dissolving solutions would be more likely to come into closer approximation with the dentinal tubules, which would also be more open once the smear layer was removed with EDTA.
A dichotomy exists as to what constitutes safe instrumentation. On the one hand, safe end-cutting peeso reamers have been condemned because of the alleged risk of perforation and ledging. On the other hand, aggressive rotary Ni-Ti reamers with end-cutting tips have been advocated for their efficiency in shaping. Some advocate non-end-cutting instruments with rounded tips to negotiate curved canals better without the chance of perforation. Yet, an instrument that does not cut apically can impact debris apically, making negotiation to the apex problematic. Typically, apical resistance leads the dentist to apply more apical pressure with the present instrument or requires recapitulation with smaller instruments that may also create a canal where one did not exist prior to the application of pressure.
On the other hand, end-cutting reamers tend to pierce tissue rather than impact it. Along with a flute design that engages the dentinal walls far less than files, the end-cutting reamers give the dentist an increased tactile awareness, helping to avoid an apical perforation if and when a wall is hit. Rather than applying more apical pressure to overcome a blockage, the wall will be recognized for what it is. A small bend at the apical end of the instrument will most often allow the dentist to negotiate any sharp apical bends that may exist while remaining within the confines of the natural anatomy of the canal. Peeso reamers limited to straightening the coronal curve and deepening the flare to that point, when used with a mild, nonlingering pecking motion, is a technique that is easily taught and mastered and constitutes no threat to the integrity of the root. A canal can only be straightened by removing tooth structure from the outer wall, the one away from the furcation. In so doing, the risk of strip perforation with the No. 2 peeso is eliminated.
Whatever the rotary Ni-Ti system used, we can agree that greater tapered shapes are an advantage over 0.02 mm/mm shapes. Rotary Ni-Ti has attained these shapes for the most part using reduction-geared handpieces to shape the canals. They generally rotate at approximately 150 to 300 rpm, although some newer systems recommend higher rates of rotation. Rotary Ni-Ti systems have the potential to produce greater tapered shapes fairly efficiently. Their usage, however, becomes an increasing concern as the canal anatomy includes greater and more abrupt curves. Curves produce greater resistance to the apical negotiation of the instruments, producing torsional stress as well as cyclic fatigue. To reduce the degree of torsional stress and some cyclic fatigue, rotary Ni-Ti instruments are used in a crown-down fashion, thereby opening up the more coronal aspects of the canal first before greater depth is encountered. There is some attempt made to straighten the coronal curve by leaning against the outer wall of the preparations. This attempt must be a limited one because of the weakness of Ni-Ti in general and rotary Ni-Ti in particular. Straightening the coronal curve before shaping the apical curve subjects the rotary Ni-Ti instruments to far less torsional stress and cyclic fatigue. However, the apical curve alone (if severe) offers increased risk of separation due to cyclic fatigue, even if torsional stresses are largely eliminated.
When measured against the disadvantages of traditional manual techniques, rotary Ni-Ti offers a series of pluses that have convinced many dentists of their overall superiority. Where traditional technique creates 0.02 mm/mm spaces, rotary Ni-Ti creates spaces that most often have a taper of at least 0.04, producing twice the resistance form for the vertically compacted or thermoplastically adapted gutta-
percha. Greater tapered spaces are irrigated more efficiently. A smooth, continuous taper looks far better than the inconsistent results that 0.02 mm/mm tapers often produce. The greater the taper, the greater the hydraulics on the cement with greater chances of filling lateral canals with either cement or gutta-percha when thermoplastic techniques are employed.
The next step in the goal of simplifying excellence in endo-dontics was to see if these great-er tapered shapes could be attained by means that were safer than rotary Ni-Ti and where the instruments could be used many times rather than the very few uses dictated by rotary Ni-Ti. Where rotary Ni-Ti requires crown-down canal preparation, stainless steel reamers can safely be used in step-back fashion. The proper use of the No. 2 peeso and No. 2 Gates Glidden supply all the crown-down preparation that is necessary.
Rotary Ni-Ti represents progress when compared to the traditional ways stainless steel files were used. However, the transition from stainless steel files to stainless steel reamers immediately halved the engagement that stainless steel instruments encounter with the dentinal walls when negotiating to the apex. The traditional K-file is a twisted, square piece of metal wire that results in continuous 4-point contact. The K-file also has 24 flutes in its 16 mm of working length. The K-reamer is a twisted, triangular piece of wire that results in continuous 3-point contact. The K-reamer has 16 flutes along its 16 mm of working length. The greater the number of flutes within the 16 mm of working length, the more horizontally oriented the flutes are and the less efficiently they remove dentin when used in a rotary or reciprocating motion. In a time and motion study,1 we found that reamers negotiate to the apices of canals more efficiently than files based on this reduced en-gagement, which directly leads to reduced resistance. Other studies have shown that reamers reproduce the multiplanar architecture of canals with far more accuracy than files.2
|Figure 1. The SafeSider reamer powered by the reciprocating Endo-Express engine.||Figure 2. With the creation of a flat along the working length of the reamer, 2 columns of chisels are created, with one cutting in the clockwise motion of the SafeSider reamer and the other cutting in the counterclockwise motion of the SafeSider reamer.|
|Figure 3. This slide demonstrates the 45º reciprocating motion and machining action of both the chisel-like edges as well as the more vertically oriented flutes on the SafeSider reamers.|
While K-reamers clearly demonstrated greater ease of apical negotiation based on reduced engagement, it became clear that the resistance to apical negotiation could be further reduced by placing a flat along the entire working length of the reamers (SafeSider reamer, Essential Dental Systems) (Figure 1). The flat reduces the engagement of the reamer to the dentinal walls to a continuous 2-point contact. Additionally, the flat creates a space for debris, preventing the typical clogging that may occur with traditional files used in a rotary fashion, whether they are made of stainless steel or Ni-Ti. Perhaps most intriguing, where the relieved flat meets the flutes, a chisel is created (Figure 2). In effect, the flat creates 2 columns of chisels that run along the instrument's entire working length. An instrument of this design works best in a horizontal reciprocating handpiece, (Figure 3) where one column of chisels works in the clockwise direction and the other works in the counterclockwise direction. It should also be noted that an instrument with a flat along its entire working length will be more flexible than an instrument of comparable size that is not relieved.
This relieved reamer design works so efficiently that even canals with substantial apical curves can be enlarged up to a 40 using 0.02 mm/mm tapered stainless steel without significant concern for canal distortion. In fact, the only need for Ni-Ti is to create tapers greater than 0.02 mm/mm in canals with apical curvatures. Here, too, the Ni-Ti instruments are relieved like the stainless steel ones. And like the stainless steel relieved reamers, they are used in the reciprocating handpiece. By switching the engine from a rotary one to a reciprocating one, none of the instruments used are subject to significant torsional stresses, nor are they subject to cyclic fatigue, a major problem when using rotary driven systems.
Since fatigue is no longer a concern, the need for frequent replacement to prevent separations no longer exists. In fact, the only valid reason for replacement is when the instrument becomes overly dull, which takes at least 8 uses. It is a sign of progress that the downside of not replacing an instrument is limited to a decrease in efficiency, and not fracture. What this means for the stress level of the average practitioner is incalculable. The initial low cost of the instruments and their ability to be used multiple times reduce the cost of these instruments on a per-use basis dramatically.
When separation is no longer a concern, a virtuous cycle takes effect. The dentist tackles more challenging cases, because separation is not a potential unfortunate consequence. Tackling harder cases gives the dentist more experience, which leads to greater confidence, which allows him or her to tackle still more difficult teeth. Compared to rotary Ni-Ti, the dentist works within a much wider window of success. Reciprocation with relieved reamers offers the following advantages over rotary Ni-Ti:
• The instruments are subject to far less torsional stresses.
• The instruments are subject to far less cyclic fatigue.
• Applying excessive apical pressure is far less critical.
• Lingering apically too long is far less critical.
• Negotiating curved canals does not increase the chances of separation.
• The tips of the reamers may be bent to negotiate abrupt curves.
• Less tooth structure is removed from the inner wall of the canals.
• Tactile perception is maintained throughout the procedure.
• The instruments can be used multiple times without separation concerns.
• Hand fatigue is eliminated while maintaining instrument integrity.
• Cost is dramatically re-duced on a per-use basis.
• Most importantly, the confidence level of the practitioner is greatly increased.
|Figure 4. An example of the excellent canal preparations that are routinely produced with the SafeSiders in the Endo-Express and the consequent excellent obturations that follow.||Figure 5. Note the lack of distortion using the SafeSider sequence of instrumentation with the Endo-Express reciprocating handpiece.|
|Figure 6. S-shaped canals are also routinely shaped with the SafeSiders sequence with virtually no concern for instrumentation separation.|
|Figure 7. Routine SafeSider shaping allows the EZ-Fill obturation to predictably produce a 3-D fill. Here you see an example of a single-cone, room temperature fill with an EZ-Fill epoxy-resin interface.||Figure 8. An example of a lower molar fill after the SafeSiders have shaped the canals to a 35 apical preparation and a 0.08 mm/mm taper. Note that a 0.08 mm/mm taper does not thin out the coronal dentin excessively, and an immediate post hole can be routinely placed if desired.|
There is nothing gimmicky about using these instruments in the prescribed manner. Steps cannot be skipped, and each instrument must accomplish its task before the next one in the sequence is used. However, if these easily learned steps are followed, then excellent, predictable, and efficiently achieved results are within the grasp of all who use the system (Figures 4 to 8).
1. Jerome CE, Hanlon RJ Jr. Identifying multiplanar root canal curvatures using stainless-steel instruments. J Endod. 2003;29:356-358.
2. Musikant BL, Cohen BI, Deutsch AS. Comparison instrumentation time of conventional reamers and files versus a new, non-interrupted, flat-sided design. J Endod. 2004;30:107-109.
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