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1. Current Endodontic Treatment

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Introduction

Current treatment procedures in endodontic therapy have undergone almost a complete changeover in the last ten years. Teeth that were formerly considered to be hopeless now may be saved on a very high percentage basis using procedures and instruments recently introduced. The net result is that teeth that were extracted not long ago now have an outstanding chance for being retained. The following pages will deal with many of these innovations and illustrate how they may be used for best results. Even so, the author wishes to inform the reader that some of the views listed here are not universally accepted by endodontic experts and there are many outstanding textbooks and publications which can be consulted for excellent discussions of alternative methods of therapy. The areas of discussion will be accompanied by appropriate case radiographs, some with extensive follow-up views (Figs. 1 and 2), to illustrate the methods of treatment and healing.

Figure 1

Figure 2 - 27 Years Post- Op

The subjects to be discussed will be the following: (1) use of tapered, rotary instruments, (2) recent studies of canal configuration, and (3) areas of future interest.

Use of Tapered, Rotary Instruments - Initiation of Changes in File Design

Prior to 1958, there were many varying types of intracanal instruments for use in endodontics. Although all were designed with the intent to clean and widen the canal, differences in instrument length, width, degree of taper, and rake angle were constant. Then, John Ingle decided to introduce a standardized system for the use of canal preparation, with a clear demarcation in the relationship of the instruments⁽¹⁾. Ingle then was Departmental Chairman at Washington University Dental School and later became Dean at University of Southern California School of Dentistry. His contributions vaulted endodontics into a new era where canal preparation became easier to perform and results were more predictable.

There were many differing file shapes available at that time. Long-handled instruments, useful only on maxillary anterior teeth (where much of the endodontic treatment of that time was performed), were grasped by the entire hand and conducive for use with reaming action. There were numerous file systems available with short handles, for use with the fingers, but there were variations in length, width, taper, and length of flutes among them. The instruments were designated #1 through #12, but, for instance, a #3 Union Broach might be totally different than a #3 Kerr file in flute length, width, and/or taper. Even files by the same manufacturer could be different from year to year. Ingle’s system changed this. He standardized the files to have similar total lengths, length of flutes, degree of taper, and widths, and also assigned a lettering/numbering system to use for identification of these sites on the new instruments. The International Standardization Organization (ISO) had control over file dimensions, but immediately recognized the improvement offered and accepted Ingle’s principles.

Ingle standardized the total length of the files to 21 mm, 25 mm, or 31 mm in length, eliminating the long-handled instruments. Of these lengths, 16 mm of the apical portion was fluted to provide for dentin removal. The tip area of the file where the first rake-angle was formed was called “diameter 1”, or D₁ and the flutes’ end site (16 mm up the shaft) was called “diameter 2”, or D₂. At that time, flute length commonly ranged from 15 mm to 25 mm. He fixed the width of the files to widen from the first rake- angle (at D₁) to the rake-angle at D2 at a rate of approximately 1 mm of length to enlarge the file width by .02 mm.

Several alterations were made within the next few years. Some manufacturers had their flute level ground between a length of as little as 15 mm to as long as 19 mm. The D₁ and D₂ indicators were changed, D₁ (the first rake-angle) to D₀. The D₂subscript was changed to indicate the exact length of the flutes, which usually was 16 mm, but could be anywhere from 15 mm (referred to as D₁₅) to 19 mm (referred to as D₁₉) up the shaft (Fig.3). In all cases, the flute width still increased by .02 mm of width per one mm of length—still a .02 taper.

Figure 3

These changes were welcomed by virtually all dentists involved in endodontic therapy, even those who treated a minimal number of teeth. This innovation was especially valuable when reordering files because files purchased at varying times would still fit into the standardized system. Previously, older files, that were still usable, had to be discarded to maintain tip and taper degree relationships.

Need for Early Enlargement of the Orifice Area

The standardized use of .02 for taper degree opened up an entirely new horizon for endodontic instruments. It had been assumed for many years that most teeth treated endodontically were widest at the orifice and then tapered down to a much narrower width near the apex. This might be true in teeth that required treatment due to trauma or from incorrect dentin removal during restorative procedures. However, Leeb⁽²⁾ wrote an eye-opening paper indicating the error of such views (Fig. 4). He showed that teeth that were heavily restored or had extensive decay (which included most teeth requiring treatment) had very narrow orifices that widened to a larger diameter in midroot and then narrowed again toward the apex.

Figure 4. Schematic representation of Leeb’s study. Dentists have an erroneous idea of the shape of canals in heavily restored teeth (center, mesial canal enlarged at right). It has been assumed that the canal is widest near the orifice and then tapers down to the apex. In truth (left), the canal is narrow at the orifice due to the deposition of reparative dentin in response to the restoration. Away from the orifice, the canal widens in midroot and then tapers down to the apex. (From: Weine FS: Endodontic Therapy, 6th ed. Elsevier Publishing, with permission)

Since all endodontic files had some degree of taper, this finding meant that in many teeth the small orifice area could cause premature binding of the instruments and result in much less apical enlargement than was anticipated. Whereas many clinicians believed that the narrowness of the canal preventing the penetration of the entire canal was from the apical area, it was obvious that the coronal site was largely responsible. This transferred the need for early enlargement of the canal in the orifice area in order to gain more effective cleaning towards the apex.

With this in mind, changes in the files had to be made so that the orifice area could be widened quickly and safely without any damage to the apical portion. This was accomplished by the introduction of rotary engine driven files of tapers greater than the standard .02 taper developed by Ingle. The areas at the upper portion of flutes were made to be wider so that the orifice could be enlarged early in the preparation. At this time, the apical portion (minimally enlarged) merely provided a pathway for directing the instrument but did little, if any, enlargement.

The “Zip” and the “Elbow” and Their Significance in Curved Canal Preparation

For many years, it was assumed that canal preparation in anterior teeth and posterior teeth were very similar. Of course, posterior teeth almost always had more canals, many of which were quite curved as compared to the straighter anterior teeth, but no severe alteration was drawn in the facets of treatment. Then, in a landmark study by Weine, Kelly and Lio in 1975⁽³⁾, the difficulties encountered in treating teeth of greater curvature were elucidated and new words were added to the endodontic glossary of terms.

In their analysis, Weine, et al. determined that in curved canals the files did not cut dentin equally in all directions, but instead took more off from the inside portion of the curve than the outer side except nearer to the apex where the opposite occurred. This meant that the desirable canal long-section view of a gradual taper from the orifice to the apex was not obtainable in curved canals by using the techniques then in vogue. Because of the files working to the inner portion from the orifice and then to the outer portion near the apex, the tapered funnel really was never present when a sharply curved canal was prepared, but the shape in long-section was really more of an hourglass. From the orifice it was wide and tapered to the center of the curve, but then widened again (Fig. 5).

Figure 5

The authors of the study attached names to the two most significant changes in canal shape. The narrowest area was called the “elbow,” which is a plumbing term at the turning site of plumbing equipment which may fill with debris and cause blockage in the system. The area at the very apex of the preparation was called the “zip,” because this site was wider than expected and ragged. The “elbow” generally occurred in the middle of the curve in a canal with an apical curvature of 30 degrees or greater and usually is noted from 2 to 5 mm from the tip of the preparation. “Zipping” occurs from that point to the apex (Fig.5). The article went on to suggest that modification of the flutes should be performed and use of files of greater flexibility be used to lessen the zipping problem.

Many endodontists refused to accept these findings and believed that their preparation methods were useable in any canal configuration. However, within a few years most endodontists were compelled to agree with the study and began to make changes in their preparation techniques, which also included their choice of files. Newer files were evaluated which were more flexible than those in common use and several styles were found to be useable and able to minimize the elbow and zipping. However, most dentists felt that flute removal was a tedious and, perhaps, unnecessary procedure. But, the best was yet to come!

Introduction of Nickel-Titanium Instruments

Another important innovation pertained to the metal used in the instruments. Originally made from carbon steel blanks, at this time most endodontic files were being made from stainless-steel blanks and thus were less susceptible to corrosion. Stainless-steel was at least mildly resistant to breakage and could be cleaned and resterilized after use. Then, a newer metal was introduced into endodontic treatment after previous use in other areas of grinding. This metal was Nickel-Titanium, which had been used in industry since 1960 and was at first referred to as Nitinol—“ni” for nickel, “ti” for titanium and “ol” for the Naval Ordinance Laboratory in Maryland, where it was first used. The initial dental use was orthodontic wiring and shortened to be called “NiTi.” Endodontic files of this material have increased flexibility due to a low modulus of elasticity and therefore retain its general shape even when strained within the canal. Too much strain can still be responsible for breakage, but requires a much higher level of stress than does carbon or stainless-steel. While the Ni-Ti instruments are more expensive than stainless-steel, they retain their shape much longer, a phenomenon known as pseudoelasticity⁽⁴⁾. The metal has different properties when stressed, but returns to the original shape when the stress is relieved. There is a limit to the amount of stress that NiTi files can resist and this is an important consideration when the file is used in conjunction with mechanical handpieces, as compared to use by hand⁽⁵⁾.

The metal may go through any of three transformations, to detwinned martensite, to austenite, or to twinned martinsite via heating and cooling⁽⁶⁾. It appears that the phase of transformation between austenite and martinsite is where the greatest elasticity occurs and thus is the best time to work under some stress.

To obtain even more advantage to these newer file systems, they were modified to be used in conjunction with mechanical handpieces. Mechanical handpieces had been introduced in endodontic treatment many years earlier for use with carbon steel as well as stainless-steel. When used in straight canals, which were by far the minority found in the teeth being treated, they seemed to work quite well. However, in the more curved canals many files fractured within the tooth and posed many difficulties in attempts to retrieve them. When used in conjunction with the NiTi instruments, this was much less of a problem. Thus a very useful series of instruments that could complete routine canal preparation in much less time than hand filing and allow for considerable increase in orifice size to promote more effective canal cleaning and filling were then available.

Immediately after their introduction, the NiTi systems became quite popular. In fact, they probably were used more quickly than any of the endodontic systems for the past 50 years. For many manufacers introduced varying systems, combining the use of NiTi instruments and .04 tapers. Some of the systems still have wide usage (Fig. 6a and b) whereas others came into the market and in a short period of time were withdrawn (Fig. 6c).

Figure 6A	Figure 6B
Figure 6C

The combination of NiTi files with mechanical handpieces as rotary instruments delivering an extraordinary type of flexibility alone was greatly responsible for diminishing zipping. However, file separation did occur if these new instruments were stressed to a great level and this caused a serious problem. Cantatore, an excellent endodontist practicing in Rome, with great skill in the removal of separated instruments in the apical third of the canal (where a huge majority of separations will occur), has stated that even with much time, effort, experience and auxiliary instruments, only approximately one-third of the files separated at this level could be retrieved⁽⁷⁾.

With more dependable instruments and the knowledge that the mechanical handpieces could give a better shape to the canal preparation, another innovation was introduced. This was the making of some of the files to double the width that Ingle had promoted and with even greater than double—to triple or quadruple them (Fig. 7). This meant that the orifice area could be widened more quickly and the areas toward the apex also could be made wider. Personally, I am not an advocate of those files which are much wider, such as the .08 tapers or larger, and I rarely even use the .06 instrument. I fear that these wider files have a greater chance of getting stressed in narrow, curved canals and thus lead to possible separation. I much prefer using only the .04 tapered instruments, even though they may require some additional time for completion of the preparation, but this is more than balanced by the greater safety afforded to the narrower files. In fact, my preference is to use only the .04 tapered instruments, and then never even use them past the elbow, which virtually assures very low chance for separation, and completing the apical portion (past the elbow) with hand files, usually the most flexible obtainable. This aspect of treatment has been followed by me in virtually every complex case that I have had in the last 10 years without any separation whatsoever.

Figure 7

Other Excellent Files for Use in Curved Canals

Several files were developed in order to gain safely the wideness of the canal orifice, as per Leeb’s instructions. The file that I find to be most advantageous for this important step is the “SX” file (Fig. 8). Instead of the file having a taper of .04 or larger it has a taper that is quite wide at D₁₆, but then decreases much more than the routine files to the apex and is a shorter file—19 mm in length. When placed into the canal orifice, only the set of flutes highest on the shaft will cut, with the tip portion of the file merely indicating the direction of the canal. To be extra-sure that the file will not separate nor cut into the sides of the canal, I often cut the apical 2 mm off the file prior to use. The resultant instrument is shorter and, thus, easier to insert and the safety is increased.

Figure 8

The importance of the use of SX files, particularly in molar teeth, is demonstrated in Figure 9. It shows how the instrument may be used to place pressure on the outer surfaces of the molar canals to allow the exterior triangles to be reduced and result in decreasing the angle of entry—a very desirable condition. There are other mechanical handpieces that are used in endodontic treatment—the Gates-Glidden drills and the Peeso reamer. However, they are centering files and when pressed against the outside of curved canals may result in instrument breakage. The manufacturers of these instruments acknowledge this but state that the broken portions of the instrument are not difficult to remove. This may be true, but it never is fun to have to remove a separated instrument.

Figure 9

The most difficult canal in the dental arch to prepare is the mesio-buccal of the maxillary 2^nd molar. This tooth has a distal inclination and the files must be inserted from the disto-lingual direction. This is an extremely difficult file placement. As will be discussed later in this report, there is a high incidence of two canals in this root. By early use of the SX file and pressure on the mesial aspect of the orifice with the SX, a much straighter access is obtained to the root. Pressure to the mesio-lingual wall will allow for examination for the mesio-lingual canal.

No-No’s When Using NiTi Files

As excellent as the NiTi files are, the instruction booklet that may come with them may be incomplete or inaccurate. Also, some of the speakers who discuss them at dental meetings may give erroneous information. I have used these instruments for over ten years with outstanding results, but I have modified and added information to make their use better and safer. My advice for safety follows.

When using the NiTi files, the operator must distribute the rotation of the stress throughout the canal and, therefore, must NOT stop preparation in this area. To accomplish this best, the file rotation must be started prior to the insertion into the canal and use of light pressures to obtain deeper insertion. If the file starts to rotate more slowly as it goes down the canal, it is best to withdraw the file, still rotating, and go back to smaller sizes nearer the orifice.

The handpieces that are typically used with the NiTi files are made with controls that are different

than those used for routine preparation for operative dentistry, that is cavity, crown, or onlay preparation. The best handpieces for use with these files are those with torque control electric engines that allow for 300 to 450 rotation per minute. The readout for the machines are indicated on the control box and the operator is free to utilize the desired speed. I prefer something in the neighborhood of 350 to 400 revolutions per minute (Fig. 10).

Figure 10

As stated earlier, the best safety for rotary instruments involves using them ONLY to the center of the apical curvature and using hand instruments for the remaining distance. Therefore, place the marker on the NiTi file to indicate no more deeply than the center of the curve and do NOT allow for the marker to go past this length. In most mandibular molars, this calculation is easily made by placing the file along the long axis of the tooth on the radiograph. In maxillary molars and bicuspids with unusually placed curvatures, a working-length file of small diameter (no wider than a #15 file) may be placed and measured to indicate the position of safety.

When starting out to use the NiTi instruments, the operator should select easier cases—even anterior teeth that do not ordinarily require complex methods of therapy, merely to adjust to the “feel” of the files within the canal. Once these are mastered, it is best to use extracted teeth or plastic blocks (available from companies selling these NiTi products) and examine the result to be certain that mastery has been obtained.

Step-By-Step Use of .04 Files in Curved Canals

In order to allow the reader to follow the step-by-step use of the instruments described, I am utilizing three cases of expanded complexity that will serve to demonstrate the techniques.

CASE ONE: Mandibular second molar with recurrent decay, acting as posterior abutment of 5-unit fixed bridge, with adjacent tooth replaced by an implant in place for over one year. The involved tooth had a periapical lesion at the apex that was draining along the distal portion of the distal root, a condition thought never to be treatable some 25 years earlier. The preparation of the mesiolingual canal is given, and the radiographs and file use are presented in Figs. 11 and 12.

The curvature of the mesiolingual canal (Fig. 11, b) is approximately 50 degrees. A size #10 file of a flexible file system is passed to the working length and the tooth is radiographed to verify length, which was calculated to be 19 mm. If an apex locator is being used along with the radiographs to calculate working length, it too may be used at this time. The distance to the center of the curve is approximately 4 to 5 mm from the apex.

Use the *SX file in the mechanical handpiece to widen the orifice area to allow for ease of placement of the files which will be inserted to the apex.

Figure 11A	Figure 11B

1. Place the #10 file to 19 mm and using rasping action by hand widen the apical portion of the canal until the file is loose.
2. Clip 1 mm from the size #10 file (making it a size #12) and widen the apical portion of the canal.
3. Set the stop on the smallest rotary file at 14 to 15 mm (distance to the center of the curve) and widen the canal. Do not worry if the file does not go to the stop, but BE CERTAIN that it does not go BEYOND the stop.
4. Use the #12 file to the full working length.
5. Set the stop on the second smallest rotary file at 14 to 15 mm and widen the canal.
6. Repeat step 5.
7. Set the stop on the third smallest rotary file at 14 to 15 mm and widen the canal.
8. Repeat step 5.
9. At this point, if you took a radiograph with the size #12 file a file in the ML canal, the angle of the canal is now 30 degrees, a much easier preparation than a curvature of 50 degrees (Fig. 11, c).

The two mesial canals merge and the ML is made the master canal with the mesiobuccal merging several mm from the apex and the preparations of the three canals are completed. Be certain to keep the canals heavily irrigated with sodium hypochlorite or similar irrigant during the completion of the preparation.

The distal canal is very straight and may be prepared easily after the mesials. In the 30-month post-operative film, complete healing of the lesion is seen. (Fig 11G)

Figure 11C	Figure 11D
Figure 11E	Figure 11F
Figure 11G
* In the case shown, I was only rarely using the SX files and was not sufficiently understanding their excellent use. Now I would use them more aggressively, after each file to the full working length. The .04 tapered files were used as indicated. No tapered, rotary instrument was used past the site of the elbow—only hand instrumentation.

Figure 12

CASE TWO: Mandibular first molar with periapical and lateral lesion associated with the mesial root and drainage into the furcation. Preparation of the mesial and distal canals is provided. I was using the SX files actively at this time.

The working length of the mesial canals was calculated to be 21 mm and the distal was 20.5. The apex locator may be used at this time to verify the length. The curvatures on the mesial canals were approximately 35 degrees, close to the root tips. The curvature of the distal, which was wide bucco- lingually, was approximately 45 degrees, also close to the root tip. The preparation of the three canals is presented in Figs. 13 and 14.

Use the SX file first in the mechanical handpiece to widen the orifice area to allow for ease of placement of files which will be inserted to the apex.

Figure 13A	Figure 13B
Figure 13C	Figure 13D

Mesial canals:

1. Insert the size #10 file to 21 mm and using rasping action by hand widen the apical portion of the each canal until the file is loose.
2. Clip 1 mm from the size #10 file (making it a size #12) and widen the apical portion of the canal.
3. Use the SX file in the mechanical handpiece after cutting off 1 mm at the tip and widen the orifice portion of the canal, slightly deeper than it had been in the first use. The clipped portion of the tip keeps the file at 18 mm or less, not long enough to do any preparation at the tip.
4. Set the stop on the smallest rotary file at 19 mm (distance to the center of the curve) and widen the canal. Do not worry if the file does not go to the stop, but BE CERTAIN that it does not go BEYOND the stop.
5. Use the #12 file to the full working length.
6. Set the stop on the second smallest rotary file at 19 mm and widen the canal.
7. Repeat step 5.
8. Set the stop on the third smallest rotary file at 19 mm and widen the canal.
9. Repeat step 5.
10. At this point, the size #20 should go the full working length without any problem. The canal curvatures should now be approximately 25 degrees, much easier than 35 degrees. The canals should be widened to size #30 easily.

Figure 14

Distal canal: This canal is wide bucco-lingually, so the SX file used should make an oval shape in the orifice instead of round as was done on the mesials. Use the SX in this manner now to start the preparation.

1. Place the initial size (size #10 or #15) into the disto-buccal and disto-lingual portions of the canal using rasping action and widen the apical portion of the canal. Set the stop at 20.5 and use rasping action at that length.
2. The SX file length is normally 19 mm, just about the length to the middle of the curve, so it is safe to place it into the canal and allow it to go to the full length and no damage to the apex will occur.
3. Set the stop on the second or third smallest rotary file at 18 mm and widen apical portion of the canal. Do not worry if the file does not go to the stop, but BE CERTAIN that it does not go BEYOND the stop.
4. Use the SX file to widen the occlusal portion of the canal.
5. Use the next size hand file and widen the canal at 20.5 mm.
6. Repeat step 4.
7. Go up one more size with the next larger rotary instrument and file no farther than 18 mm.
8. Repeat step 4 then step 7.
9. Go up one more size with the next larger rotary instrument and file no farther than 18 mm.
10. At this point, the canal curvature should be approximately 30 degrees, much easier than 45 degrees. Little more preparation is required at the apex and some further use of the SX may be employed.

As before, use the irrigant and lubricant constantly and ensure that the path to the apex remains clear by using a smaller-sized file (size #12) at any time.

CASE THREE: Maxillary second bicuspid with double curves of almost 90 degrees each. Large periapical lesion, slightly behind the implant, often making the apical area difficult to see. The working length of the tooth (with the crown removed) was 17 mm. NOTE: This case is presented to illustrate the excellent possibilities afforded by the .04 tapered and SX instruments. This case is VERY difficult and I tried treating similar teeth without these burs with limited success. The final results here were very gratifying but not always predictable. They should ONLY be attempted after a considerable number of teeth has been successfully treated.

The SX file was used short of the first curve to widen the orifice. Second bicuspids are wide buccolingually, but much narrower mesio-distally, a condition not often realized. Therefore, the SX file is used to the buccal and lingual initially to give more access to the canal portion in the curved area. The size #8 file was used to the apex, with minimal clips made and heavy use of irrigant and lubricant. Then the size #10 was used to the apex (Fig. 15, a). The SX file was placed more deeply into the canal, but kept short of the initial curve. A small amount of the tip of the size #10 was removed, making it a size #11 and passed to the apex. Much irrigant and lubricant are used. Continue gradually working the hand files and SX files toward the apex, using minimal clip on the hand files and the SX files short of the first curve. When the apical portion has reached size #25, it is safe to attempt to fill the canal (Fig. 15, b). The four-year postoperative film indicates excellent healing (Fig. 15, e).

Figure 15A	Figure 15B
Figure 15C	Figure 15D
Figure 15E

Conclusion

The use of tapered, rotary instrument of sizes .04 taper plus files to open the canal orifices quickly and safely is a huge step for endodontic treatment even on very complicated cases. It is anticipated that more file systems will be introduced in the future and it is up to the individual dentist to assess these newer products and determine their usefulness. Just because something is new does not mean that it is better. Use extracted teeth and plastic blocks to verify the abilities of the newer products before initiating treatment based on advertisements or poorly designed studies.