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Written by Kenneth A. Koch, DMD, and Dennis Brave, DDS Sunday, 01 September 2002 00:00
If you are performing a lot of root canals, we want to let you in on a secret that endodontists know, but no one talks about. Here it is: a continuously tapered 0.06 preparation. If you can understand the rationale of a continuously tapered 0.06 preparation and perform it in a consistent manner, you will be stunned by how quickly endodontics can become predictable and a profit center in your practice.
Why do endodontists prefer a continuously tapered 0.06 preparation? Actually, there are a number of reasons. Two of the major benefits of the 0.06 tapered preparation are a dramatic reduction in postoperative sensitivity for patients, and the ability to perform more cases in one visit. This combination leads to increased patient satisfaction, and will be a practice builder.
When performing a continuously tapered 0.06 preparation, the larger taper removes that tooth structure in the coronal part of the canal that has a tendency to bind instruments. Consequently, the removal of this tooth structure results in a dramatic increase in proprioceptive ability. Therefore, one benefit of this technique is more tactile awareness. Additionally, the continuous 0.06 taper allows your irrigation agent (full-strength sodium hypochlorite) to work in a more efficient manner. How effective is your irrigation agent when you can hardly screw a No. 25/0.02 hand file to length? Not very effective! But with a 0.06 preparation, you are getting your irrigation agent into the root canal system right from the start.
You should think of the root canal as a three-dimensional system with webs, fins, and anastomoses. The only way to effectively clean these areas is with an irrigation agent. Your irrigation agent has the ability to work much more effectively in a tapered 0.06 preparation as compared to a 0.02 or 0.04 preparation. Canals that are preflared with gates glidden burs (and a 0.02 or 0.04 taper) are not as effectively irrigated as a 0.06 preparation. Gates glidden burs make a parallel preparation in the coronal part of the canal. A greater taper enhances the hydraulics of irrigation. Additionally, a continuously tapered 0.06 preparation allows the use of an ultrasonic tip to generate acoustic streaming more effectively and achieve the cleanest possible canal.
Another aspect of the 0.06 preparation that contributes to patient satisfaction is the reduction of extruded debris. Quite often when performing a root canal with hand files, we push debris out past the end of the tooth. We are, in a sense, inoculating the periapical tissues. What is the sequela of this? Increased postoperative sensitivity, if not frank pain and swelling. However, by using rotary files that, by design, pull debris coronally rather than push it apically, we can further reduce the amount of extruded material.
Endodontists do not like to break files. It really does ruin your day. This is why we use 0.06 taper rotaries. Rotary files with a continuous 0.06 taper are exponentially stronger than 0.04 tapers. This is because of the extra metal behind the blade. Additional benefits of this technique come after we have created a fully tapered 0.06 preparation, namely, the primary cone fit and ease of obturation. Instead of guesswork and frustration with bent cones, the 0.06 preparation makes the primary cone fit a “no brainer.” The key is the continuously tapered 0.06 preparation.
The ability to create consistent, predictable shapes is where rotary instrumentation has really changed endodontics. When performing a 0.06 preparation, 95% of the time you will need only one of two sizes of gutta-percha points. These are fine-medium and medium nonstandardized cones. If you are unfamiliar with nonstandardized cones, another option is to use tapered cones such as the 0.06 taper cones by SybronEndo. Charles Schwed and Diadent both have cones that are ISO sized (25, 30, etc) and come in different tapers such as 0.04 and 0.06. These are also excellent cones and, in fact, may be easier to use for some clinicians.
When we do live demonstrations at our Real World Endo courses, the participants are always stunned at the ease of the cone fit. It is not magic! It is all a result of the 0.06 preparation. There is also more good news concerning obturation of a continuously tapered 0.06 preparation. We can confidently state that whatever obturation method you use (lateral condensation, vertical condensation, System B, etc), they all work better with a 0.06 preparation! Even the solid core obturator systems, such as Thermafil (DENTSPLY Maillefer), work better with a 0.06 preparation. This is because you no longer have to “verify” the size. Verification is not really needed because of the taper of the plastic carrier. If you finish with a No. 25 0.06 taper, you simply fill with a No. 25 Thermafil. The hydraulics of heated gutta-percha also work better with the 0.06 preparation.
There is even more good news. Because it is faster and more effective, a fully tapered 0.06 preparation allows you to perform more single-visit endodontics. This is not only very profitable, it is a huge practice builder. The time savings, of course, translates to greater production and net profit. It should also be acknowledged that there is a tremendous savings in consumables such as patient setup supplies (napkins, anesthetic solution, suction tips, etc).
To produce a tapered 0.06 canal preparation, our instrumentation of choice is the recently introduced K3 file system by SybronEndo. There are many excellent file systems available, with varying design features. In endodontics, the specific instrumentation system used dictates certain aspects of clinical technique, based on the design of the system. Therefore, it is important to understand the design features associated with the K3 file system as a precursor to discussing our clinical technique for producing a tapered 0.06 canal preparation.
Six important components in the K3 file design are: a positive rake angle, variable helical flutes, variable pitch, improved radial land design, a noncutting tip, and an easy access handle. Additionally, it should be noted that the K3 is available in both a 0.04 and a 0.06 taper, with full working lengths of 16 mm. Let’s examine each of these features more closely.
Positive Rake Angle
|Figure 1. Various rake angles.|
An instrument’s cutting efficiency is dependent upon the overall rake angle of the instrument’s cutting blades. A negative or a “substantially neutral” rake angle results in a scraping rather than a cutting action. The ideal rake angle is slightly positive but not overly positive. An overly positive rake angle will result in digging and gouging of the dentin (Figure 1). This can lead to instrument separation. The K3 features a slightly positive rake angle, which results in optimum cutting efficiency.
Variable Helical Flute Angles
|Figure 2. Rotary files with variable helical angles.|
As a rotary file works in a canal, the dentinal debris needs to be removed in an efficient manner. Files with a consistent helical flute angle allow debris to accumulate, particularly in the coronal part of the file. Additionally, files with a consistent helical angle have a tendency to suck you down into the canal. In the K3, the helical angle increases from the tip to the handle. The result of this design is more channeling for debris removal, as well as better control. More control is achieved as a result of not being pulled into the canal (Figure 2).
|Figure 3. Variable pitch reduces “suck down.”|
Pitch is the number of threads or spirals per unit length. Files with a consistent pitch have a tendency to suck you down into the canal. This is particularly important in rotary instrumentation when using files of a constant taper. The K3 file sequence has purposely been designed with constant tapers, but each file utilizes a variable pitch and helical angles (Figure 3). The result is a dramatic reduction in the sense of getting “sucked down into” the canal. This is very significant, especially when performing a fully tapered 0.06 preparation.
|Figure 4. Radial lands help keep a file centered. The K3 has two recessed lands and a full third land.|
The concept of radial lands is critical. It is the combination of a noncutting tip and radial lands that keeps a file centered in the canal. We find the concept of radial lands most reassuring in rotary endodontics. Another feature of radial lands is blade support. Most rotary files derive their strength from the mass of material in the core. However, peripheral strength can be added to the file by extending the width of the radial land. The K3 land design combines core and peripheral strength. The file is three fluted with three lands. However, two of the K3 lands are broad and recessed, while the third one is a narrow full land. The relieved portion of the recessed lands minimizes resistance while the extended width maximizes strength (Figure 4), a combination that increases resistance to separation.
|Figure 5. Noncutting tips are safety oriented.|
Tips are either cutting tips or noncutting tips (Figure 5). Some files claim to have “modified cutting” tips or “partially active” tips. This is like being “a little bit pregnant.” Either you are, or you are not. Cutting tips do have an indication in endodontics, but these are limited and should be used only in the hands of an experienced clinician. The K3 employs a noncutting tip, which enhances the safety of this file.
The K3 file has a handle that is 3 mm shorter than other full-shank 0.06 tapered rotary files. This, along with the access contra-angle, provides almost 5 mm of additional access. The result is easier access to posterior teeth.
Having reviewed the engineering features of the K3, what are the clinical ramifications of such a design? The best way to comprehend this is to understand how the K3 works in a tooth. In particular, how do the tip design, radial lands, rake angles, and pitch affect the performance of this file?
As stated previously, most dentists are best served using a rotary file with a noncutting tip. We feel very strongly about that statement. While some experienced clinicians may be able to use cutting tips, we feel they are too aggressive for the majority of dentists. There are two serious concerns with a cutting tip. The first is if you accidentally go long (past the end of the tooth). Going long with a noncutting tip will create a concentric circle at the end of the root. These are easily filled with a nonstandardized cone. However, if you go long with a cutting tip, upon retraction of the file, you generally will create an elliptical tear. This is very difficult to repair and obturate. The second concern is the very real possibility of transportation with a cutting tip on a nonlanded file. Because of this, we support the design concept of radial lands on rotary files. Either full or recessed lands will ensure that the file stays centered in the canal. The combination of a noncutting tip and radial lands offers the greatest safety margin and will prevent transportation of the root canal. The K3 combines both full and recessed lands. The result is a file that stays centered in the canal but has less frictional resistance than a fully landed file.
Rake angles are also important, and affect the cutting efficiency of the instrument. Positive rake angles will cut more efficiently than neutral rake angles, which plane the inside of the canal. A negative rake angle is least aggressive, but the cutting efficiency of a file can also be affected by the blank design and taper. The K3 offers increased cutting efficiency (positive rake angle) and combines this with a strong safety component (radial lands and a noncutting tip).
We have covered design features of the K3, but how is it used? In other words, what are the directions for use? Before we answer this question, a few observations are in order concerning flexibility and cutting efficiency. The K3 when first viewed appears less flexible, but flexibility is not the real issue. The real issue is how well a file follows the path of the canal. Whether or not a file follows the true path of a canal is a result of the design features of that file, not merely a matter of flexibility. Additionally, the cutting efficiency of the K3 is such that very often the flutes are full after only two advances to resistance points. At this point the file needs to be wiped clean or replaced with the next successive file. Simply put, instrumentation with the K3 is often a matter of arithmetic: 4 + 2 = four files, two strokes each. Many canals will require only four files. However, there are certain canal challenges that may require as many as six files. Consequently, we would like to offer the clinician the following sequences. These steps are common to all K3 sequences:
|Figure 6. K3 basic sequence.|
(1) Confirm coronal patency.
(2) Determine projected apical size.
(3) Begin crown down.
(4) Establish working length.
(5) Complete crown down.
(6) Obturate the canal (Figure 6).
(1) Confirm coronal patency with a No. 10 stainless steel hand file. The file only needs to go one third to one half the projected working length. If a canal is patent in the coronal third, it usually will be open to the apex. Too many dentists make the mistake of trying to force a hand file to length before coronal flaring.
|Figure 7. Conversion chart for a 0.02 taper to a 0.06 taper.|
(2) Determine projected apical size based on the diagnostic radiograph and the fit of the No. 10 stainless steel hand file. This is not a binding contract. Canal size is generally small, medium, or large. (See the conversion chart in Figure 7.) The projected apical size is determined initially to help us choose our first rotary file.
(3) Begin crown down with a file that is three sizes larger than your projected apical size, eg, if projected apical size is a No. 25, begin crown down with a No. 40 K3.
(4) Establish working length with a No. 10 hand file and an apex locator. You determine working length after the second rotary file to take advantage of the crown down. At this point, the file is no longer binding in the coronal half of the canal.
(5) Complete rotary preparation in a crown down fashion taking the files to resistance. Resistance is when the file no longer progresses apically in an easy manner. The first rotary file to length, with resistance, completes the preparation.
(6) Obturate the canal with the technique of your choice. A fully tapered 0.06 preparation is ideal for lateral condensation, the System B, Obtura, or Thermafil.
VARIABLE TAPER SEQUENCE
Another sequence that is very effective is the variable taper sequence. The concept of variable tapers is to minimize the engagement of the file against the canal wall. The classic example has been to think of one styrofoam cup placed into another. You only need to have two different tapers to reap the benefits of this technique. It is particularly helpful to the practitioner who wants to “transition” to a 0.06 preparation. Consequently, here is our recommended sequence using K3 variable tapers:
•No. 35/0.06 to resistance.
•No. 30/0.04 to resistance. Determine final working length after the second rotary file.
•No. 25/0.06 to resistance.
•No. 20/0.04 to working length.
•No. 20/0.06 to working length.
Notice that we have “looped back” with a No. 20/0.06 K3 to working length. If possible, we try to finish all canals with a 0.06 preparation. Run the files at 300 to 350 rpm.
The third sequence is one we have always used with 0.06 taper files. We refer to this technique as “real world” because this is how endodontists prepare narrow canals:
•No. 25/0.06 to resistance.
•No. 30/0.06 to resistance. Determine final working length after the second rotary file.
•No. 25/0.06 to resistance.
•No. 20/0.06 to resistance.
•No. 15/0.06 to working length.
•No. 25/0.06 to working length.
Run the files at 300 to 350 rpm.
As always we begin by confirming coronal patency. This is extremely important when treating narrow canals. We start the crown down preparation with a No. 25/0.06 taper K3. We take this file to resistance. Generally this goes down the canal about 15 mm. We then follow this with the No. 30/0.06 K3. This file will generally go 1 to 2 mm less. However, after two files, we have successfully preflared the coronal half of the canal. We now determine our final working length with a No. 10 stainless steel file and an apex locator. If you wish, you can create a glide path at this point with a No. 10 or 15 hand file. Following length determination, we return to our original No. 25/0.06 K3. As usual we work this file to resistance. But instead of just going to 15 mm, it now tracks down to about 18 mm. This is what is really sweet about this technique. Following this, we take the No. 20/0.06 to resistance. Quite often the No. 20 will reach the final working length. If not, continue the crown down with a No. 15/0.06 and generally this will get you to your final working length. If you must crown down all the way to the No. 15 to reach working length, we recommend that you loop back with a No. 20/0.06 and take this to length. We do not advocate finishing the preparation with a size smaller than a No. 20.
We feel this is a great technique. However, it is not limited to narrow canals. It will work on all canals. Once you understand what resis-tance is, this sequence will work in a consistent, predictable manner.
Rotary files, when run at an increased speed, will plasticize gutta-percha and remove it from the canal like an auger. This technique is so effective that occassionally you will see a lateral canal filled by the plasticized gutta-percha. This is very similar to a “compactor.” However, the best way to understand re-treatment with rotary files is to follow this sequence:
Begin with a No. 30/0.06 taper K3 and run the file at an increased speed between 1 k to 2.5 k. We recommend 1250 rpm. Take this initial file to two thirds of the canal depth or just prior to any curve. Now place a solvent into the prepared channel. We recommend chloroform, but if you have a problem with chloroform, you can use xylol (xylene). Following the placement of the solvent, go back in with the No. 30/0.06 K3, but this time at the regular speed of 350 rpm. We now take this to length. To make this technique even easier, use a Touch ‘n Heat (or System B) at the beginning to plasticize the coronal 4 to 5 mm of gutta-percha. The rotaries will then go through the gutta-percha like it is butter. This is an unbelievable way to perform re-treatment. At our Real World Endo courses, we not only have participants fill teeth, we have them remove the gutta-percha with rotary files.
This is an example of a 90o hook on a mesial root. You might think an extremely flexible file would be needed to shape this canal. Actually, this was done with a 0.06 taper K3. As previously mentioned, the real issue is how well a file follows the path of a canal, not its flexibility (Figure 8).
Hybrid techniques have become popular and this case is a hybrid of the SX and S1 from the ProTaper series combined with the K3. Excellent coronal flaring is combined with precise apical control (Figure 9).
Even in challenging canals, a 0.06 taper preparation is possible (Figures 10a and 10b).
S-shape canals are routinely seen on maxillary premolars. Quite often they are also seen on mandibular molars. These cases can present specific challenges to rotary files. This case was totally instrumented with 0.06 taper K3 files (Figures 11 and 12).
Retreatment with a 0.06 taper preparation allowed for better irrigation and superior hydraulics during the fill (Figures 13 through 15).
|Figure 8. An almost 90o curve instrumented with a 0.06 taper.||Figure 9. An example of a “hybrid technique.” Photo courtesy of Dr. Ali Nasseh.|
|Figure 10a. Pre-op x-ray. Photo courtesy of Dr. John Patterson.||Figure 10b. Post-op x-ray showing 0.06 preparation. Photo courtesy of Dr. John Patterson.|
|Figure 11. Pre-op x-ray of mandibular second molar. Photo courtesy of Dr. Ali Nasseh.||Figure 12. Double back canal prepared and filled with a 0.06 taper. Photo courtesy of Dr. Ali Nasseh.|
|Figure 13. Failing 0.02 taper hand preparation.||Figure 14. Re-treatment completed with a 0.06 taper preparation.|
|Figure 15. Recall shows excellent periapical healing.|
Dr. Koch is the founder and past director of the program in postdoctoral endodontics at the Harvard School of Dental Medicine. In addition to maintaining a private practice limited to endodontics, he has written numerous articles on endodontics and maintains a faculty position at Harvard.
Dr. Brave is a diplomate of the American Board of Endodontics and is a member of the College of Diplomates. In endodontic practice for 27 years, he has lectured extensively throughout North America and holds patents, including the VisiFrame. Formerly an associate clinical professor at the University of Pennsylvania, Dr. Brave currently holds a staff position at The Johns Hopkins Hospital.
Disclosure: Real World Endodontics is an independent company which receives material support from Sybron Dental Specialties.
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