Written by Barry L. Musikant, DMD, and Young Bui, DDS Monday, 09 April 2012 08:38
As partners in the largest endodontic practice in New York City, we feel it is important to discuss the adaptability and flexibility of a system of instrumentation and obturation that produces well-shaped canals effectively cleansed without resorting to rotary Ni-Ti that in our hands has led to procedural stress that is best avoided. In like manner, we emphasize room temperature obturation techniques that offer the practitioner the ability to create a 3-dimensional seal within a wide range of safety. The fact that we advocate these positions is the main reason that we are optimistic about the future of endodontics. Once the basics are well understood, it becomes readily understandable why using instruments designed as relieved reamers and confined to short arcs of motion are far safer and more predictably utilized than those used in perpetual rotation. We will present a number of cases that most practitioners would agree produce significant stress to a rotary Ni-Ti instrument, but are safely prepared when used with the short arc of rotation that reciprocation affords.
EXAMPLES OF BROKEN INSTRUMENTS
In review of our first case, let us look at a molar with a fractured Ni-Ti segment in one of the mesial canals (Figure 1).
One line of reasoning is that the majority of these cases will do just fine. The molar represented in Figure 1 was performed by an endodontist 4 years ago. According to conventional wisdom, there is little to be concerned about here. Yet, 4 years later, the case degenerated into its present state (Figure 2).
|Figure 1. Radiograph of a molar with a fractured Ni-Ti segment in one of the mesial canals.||Figure 2. Radiograph of a mandibular molar with a fractured segment in the distal root.|
|Figure 3. Radiograph, again, of a mandibular molar. Note: It was impossible to remove this segment since it was so tightly bound in the apical third of the canal.||Figure 4. Radiograph of the final results, which exhibits not only excellent shaping and obturation of the mesial roots, but a puff of cement in proximity to the tip of the separated segment.|
|Figure 5. Photograph comparing the differences between a K-file and a relieved reamer. Note the increased number of horizontally oriented flutes on the K-file versus the reduced number of vertically oriented flutes on the relieved reamer.|
Conventional wisdom is not always consistent, and the best way to deal with these types of problems is to avoid them in the first place. In fact, knowing that rotary Ni-Ti instruments can separate and then induce a periapical infection1,2 produces both short-term and long-term stress to the practitioner, reinforcing the desire to seek ways to avoid the problem before it occurs.
To flesh out the problem a bit more, please observe the next radiograph (Figure 3) of a mandibular molar with a fractured segment in the distal root.
It was impossible to remove this segment since it was so tightly bound in the apical third of the canal. However, inspection of the final result (Figure 4) shows not only excellent shaping and obturation of the mesial roots, but a puff of cement in proximity to the tip of the separated segment.2,3
Using instruments limited to a short arc of motion provides the dentist with several advantages,4 the most obvious ones being the virtual elimination of torsional stress and cyclic fatigue,3 the 2 factors that lead to rotary Ni-Ti separation.5 This has been such a consistent problem for rotary Ni-Ti that the industry continues to introduce newer systems, each supposedly having an incrementally increased resistance to fracture. The problem cannot be switched off like a lightbulb, because fatigue and torsional stress6 are indigenous to the weakness of all metals including Ni-Ti when used in rotation. In that regard, some rotary systems are now incorporating what is best described as hybrid rotation or hybrid reciprocation, depending upon your point of view. By that, we mean that for each motion of 150° there is a compensating motion of 30° in the opposite direction. Reciprocation is occurring, but there is still a net increase of 120° per 2 oscillations. With a frequency of 600 cycles/minute, that still amounts to 200 full rotations per minute, producing a similar degree of rotations that have been in existence since their introduction. As we know, rotations around a curve lead to cyclic fatigue7 and an increasing instance of separation as the tip size and taper of the instruments increase and the curvature of the canal becomes more acute.
Up to this point, we are emphasizing the safest method of instrument utilization: short arcs of motion either manually or in a 30° reciprocating handpiece. Of at least equal importance is the design of the instruments. Used predominantly with a horizontal watch winding motion, the blades remove dentin most effectively when the cutting edges of the flutes are more or less at right angles to the plane of motion. To understand this concept, think of shaving one's face with a razor blade. It works quite well because the cutting edge is at a right angle to the plane of motion.8 That is why the blade is on a T; if the blade were in the same plane as the plane of motion, we would be slicing into our face, removing nothing but blood, which we all agree is not our goal. Yet, when we use K-file,9 we are using instruments whose flutes are more or less in the same plane as the plane of motion only to pull up after engagement to shave away the dentin. While this second application of motion removes dentin, it does so at the expense of straightening the outer wall of curved canals on the upstroke while the horizontal flutes along length tend to impact dentin upon the downstroke.
Please note the difference in the angles of flutes along the shank of a K-file file compared to the angle of the flutes along the length of the relieved reamer. It is the vertical orientation of the flutes along the length of the relieved reamer that account for their great efficiency when used in shaping canals (Figure 5).
When we use relieved reamers,10 instruments with a flat along their entire working length and vertically oriented flutes with the customary watch-winding stroke, we are immediately shaving dentin away, freeing up the instrument for further apical negotiation while reducing the engagement along length, the exact opposite that occurs when using an instrument designed as a file. In other words, the relieved reamer is the proper design to maximize the penetration of the instrument to length with the least resistance encountered during the process.
To sum up where we are at the moment, we are establishing the fact that K-files represent an incorrect instrument design that nullifies the benefit of being used with the correct motion. Rotary Ni-Ti, an instrument correctly designed as a reamer, is unfortunately utilized incorrectly. Relieved reamers, on the other hand, represent both the correct design and correct utilization of instruments to accomplish the task of cleansing and shaping canals. Ultimately the proof is in the pudding, and we display a series of cases done with receiprocating relieved reamers, virtually eliminating any concern for instrument separation and canal distortion that is associated with rotary Ni-Ti.11
CASE EXAMPLE 1
A case like the one shown in Figures 6 and 7 had to have each relieved reamer pre-bent to negotiate the curve of the canal, but once pre-bent and manually negotiated around the curves yet not to the apex, the reamers were reattached to the reciprocating handpiece and taken to length. At no point, neither manually nor in the reciprocating handpiece, were the instruments subject to any of the stresses that rotary Ni-Ti places on their instruments. This fact virtually eliminates torsional stress and cyclic fatigue and the incidence of separation that comes with it. The net result is a case that was difficult, but free of the anxiety that comes from doing cases with systems that we know are vulnerable to breakage.12
|Figure 6. Radiograph of a case performed with relieved reamers that were pre-bent.||Figure 7. Another radiograph of a case completed with relieved reamers, pre-bent for instrumentation.|
CASE EXAMPLE 2
On the other hand, the maxillary molar in Figure 8 required no pre-bending because the curve was one with a long radius, a gradual one although quite curved in the aggregate. It was shaped from the straight position within a few minutes.
|Figure 8. Radiograph of a maxillary molar that required no pre-bending because the curve was one with a long radius, a gradual one, although quite curved in the aggregate.|
One has to appreciate that relieved reamers are far more flexible than K-files and have a high ability to record curvatures as they maneuver through them. A high tactile perception gives the dentist the ability to differentiate between hitting a solid wall and being in a tight canal. Being able to make this distinction tells the dentist if and when he or she must pre-bend the instrument. As long as the instrument can give the dentist this vital information, the potential for distortion is eliminated and curves such as the one above are within the capabilities of the dentist using the system.
CASE EXAMPLE 3
The case illustrated in Figure 9 shows another ability of the relieved reamers: negotiating s-shaped anatomy without causing distortions. This is best accomplished by maintaining patency 0.5 mm beyond the constriction or the measurement we attain on our apex locator. By extending the length of instrumentation through a 25 0.5 mm into the apical foramen, we prevent the buildup of debris that so easily occurs at the constriction, preventing the loss of length and the distortions that may occur when the dentist attempts to regain that lost length. The impacted debris literally pushes the head of the instrument to the outer wall of a curved canal. Length will be regained, but the original canal shape is transported to the outer wall. By preventing the buildup of debris, there is no loss of length throughout the instrumentation sequence eliminating the potential for distortion.13 The s-shaped apical configuration is maintained because the 0.5 mm overextension into, but not beyond, the apical foramen through a 25 maintained patency throughout the shaping procedure.
|Figure 9. Radiograph of another case instrumented with relieved reamers, negotiating s-shaped anatomy without causing distortions. This is best accomplished by maintaining patency 0.5 mm beyond the constriction or the measurement we attain on our apex locator.|
CASE EXAMPLE 4
The 2 x-rays shown in Figure 10 point out another humbling fact: making a correct diagnosis from the start. I believe few would expect the dramatic healing that occurred within a year. More than likely, this tooth would have been a candidate for extraction followed by the placement of an implant. Yet, the tooth was diagnosed with a completely nonvital pulp, and as it turned out, a good case could be made that elimination of the source of the endodontic infection would lead to healing. The dramatic return of bone exemplifies the benefits that may derive from a correct initial diagnosis. The shaping was accomplished with relieved reamers, using a combination of 6% NaOCl and 17% EDTA as irrigants. After final shaping, the canals were further irrigated using 2% chlorhexidine14 and then activated using the reciprocating handpiece oscillating at between 3,000 and 4,000 cycles per minute. We routinely shape the canals to a minimum of 35, 40 one-mm back with a 25/06 overlaid taper. At times, the distal and palatal canals of molars are shaped to a significantly wider apical preparation.
|Figure 10. Radiograph of a case preoperatively, and then postoperatively at one year.|
CASE EXAMPLE 5
Imagine the reduction in stress when dentists know they can shape canals such as the case below knowing that separation has been removed from the equation, that the challenge of shaping the canals without distortion still remains, but is not further complicated by the fear of leaving a portion of the instrument in the canal system and all that that may imply. In fact, the instruments are designed to maximize tactile perception, giving the dentist the ability to clearly know what the tip of the instrument is encountering. It will be either a solid wall, in which case there will be no immediate tug-back, or the tip will be engaged in a tight canal, producing immediately recognized tug-back. The former situation calls for removal of the instrument, pre-bending it at the tip and manually negotiating around the impediment with reattachment to the handpiece while the instrument is in the canal at the newly negotiated depth. The latter is calling for continued apical negotiation. It is the instrument's design and utilization (both manually and in the reciprocating handpiece) that gives the dentist this valuable feedback and keeps him or her in control regardless of the anatomy encountered (Figures 11 and 12).
|Figure 11. Radiograph of a complex case completed with relieved reamers in a reciprocating handpiece.||Figure 12. Radiograph of a complex case completed with relieved reamers in a reciprocating handpiece.|
The advantages of a relieved reamer compared to a K-file start with the fact that the reamer's flutes are fewer in number and more vertically oriented along the length of the shank. Fewer flutes lead directly to less engagement along length with the vertically oriented flutes effectively shaving dentin away from the canal walls. The flat incorporated along its length produces 2 vertical columns of chisels that also shave dentin away in both the clockwise and counterclockwise direction while also reducing the cross-sectional area of the instrument, making it more flexible. These 3 factors—greater flexibility, less engagement, and more effective removal of dentin—create a superior tactile perception that gives the dentist precise information on what the tip of the instrument is encountering. The flat also produces an asymmetric instrument that gives the dentist the ability to differentiate between a round and oval canal, telling the practitioner when to widen the canals further. No symmetric design can give the dentist this feedback.
CASE EXAMPLE 6
The final case, just recently completed, is yet another example of the control we have in shaping abruptly curved canals. Even without a radiograph, we would know by our tactile perception that we have encountered an impediment; in this case, a right angled bend. We took the appropriate steps and pre-bent the instrument, negotiated around the curve and once around reattached the reamers to the reciprocating handpiece. Due to the extreme nature of the curve, we elected to shape the canal to a 30 rather than a 35, but were still able to apply the 25/06 overlaid taper to the canal shape, allowing it to be effectively obturated. Once again, we want to emphasize that while these canals are challenges, at no point is the concern for a separated instrument something we have to deal with. The 30° reciprocating system in conjunction with primarily stainless steel relieved reamers renders the possibility of separation to virtually zero percent (Figures 13 to 15).
|Figure 13. Radiograph of mandibular first molar with a distinct right angle curve in the distal root.||Figure 14. Radiograph showing the trial fit of the points, taking particular note of the obturation of the distal canal.|
|Figure 15. Radiograph of the final fill, accomplished once again with the relieved reamers used both manually and in the reciprocating handpiece.|
We could go on and on showing many more cases, but the ones we have shown hopefully demonstrate the ability of the relieved reamers to shape canals with challenging anatomy. Needless to say, they will work with great efficiency when simpler anatomy is encountered.
The take-home lesson is that the emphasis should be placed on an understanding of appropriate instrument design and utilization. Either one alone is a bridge to nowhere. Together, they become a powerful tool demonstrated by their ease of use, invulnerability to breakage, and a dramatic reduction in costs.
- Xia WW, Zhu YQ, Wang XY. Six cases report of differential diagnosis of periapical diseases. Int J Oral Sci. 2011;3:153-159.
- Faramarzi F, Fakhri H, Javaheri HH. Endodontic treatment of a mandibular first molar with three mesial canals and broken instrument removal. Aust Endod J. 2010;36:39-41.
- Shen Y, Haapasalo M, Cheung GS, et al. Defects in nickel-titanium instruments after clinical use. Part 1: Relationship between observed imperfections and factors leading to such defects in a cohort study. J Endod. 2009;35:129-132.
- Yu DC, Tam A, Schilder H. Patency and envelope of motion—two essential procedures for cleaning and shaping the root canal systems. Gen Dent. 2009;57:616-621.
- Inan U, Aydin C, Demirkaya K. Cyclic fatigue resistance of new and used Mtwo rotary nickel-titanium instruments in two different radii of curvature. Aust Endod J. 2011;37:105-108.
- Park SY, Cheung GS, Yum J, et al. Dynamic torsional resistance of nickel-titanium rotary instruments. J Endod. 2010;36:1200-1204.
- Wan J, Rasimick BJ, Musikant BL, et al. A comparison of cyclic fatigue resistance in reciprocating and rotary nickel-titanium instruments. Aust Endod J. 2011;37:122-127.
- Wan J, Rasimick BJ, Musikant BL, et al. Cutting efficiency of 3 different instrument designs used in reciprocation. Oral Surg Oral Med Oral Pathol Oral Radiol Endod. 2010;109:e82-e85.
- D'Souza JE, Walton RE, Maixner D. Cross-sectional configuration of endodontic files compared with manufacturers' design. J Endod. 1995;21:599-602.
- Musikant BL, Cohen BI, Deutsch AS. Comparison instrumentation time of conventional reamers and files versus a new, noninterrupted, flat-sided design. J Endod. 2004;30:107-109.
- Stewart JT, Lafkowitz S, Appelbaum K, et al. Distortion and breakage of Liberator, EndoSequence, and ProFile systems in severely curved roots of molars. J Endod. 2010;36:729-731.
- Bahcall JK, Carp S, Miner M, et al. The causes, prevention, and clinical management of broken endodontic rotary files. Dent Today. 2005;24:74,76,78-80.
- Flanders DH. Endodontic patency. How to get it. How to keep it. Why it is so important. N Y State Dent J. 2002;68:30-32.
- Rasimick BJ, Nekich M, Hladek MM, et al. Interaction between chlorhexidine digluconate and EDTA. J Endod. 2008;34:1521-1523.
Disclosure: Dr. Muiskant is the co-owner and president of Essential Dental Systems.
Disclosure: Dr. Bui reports no disclosures.
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