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In Search of a Working Length

A common question among dental professionals throughout the years has been, “Is my working length acceptable?” One usually answers this question by measuring the length from the radiographic apex of the tooth to the end of a file temporarily residing in it. If this distance is less than the prescribed length (commonly 1 mm), the file is said to be “long;” if it is greater, it is said to be “short.” The problem with this time-honored approach is that nobody asks why it is so important that the working length be precisely correct, or even why there should be a working length at all. If these issues were understood, one could move on to ask why the distance should be 1 mm, and of course, why the same distance is supposedly appropriate for all teeth. The answers to these questions are intrinsic to the very principles of endodontics.

Although at times they can be difficult to put into practice, endodontic principles are easy to understand. There are two goals in endodontics: cleanse and shape the canal to remove or neutralize bacteria and their byproducts, and obturate the canal to prevent reinfection and perhaps to “entomb” those bacteria that were unable to be removed.1 These two goals should effectively guide one’s measurement of working length.

 

Determining the Working Length

The principles behind achieving the ideal working length for successful endodontic treatment have been scrutinized for decades (Figure 1).2-8 Astute readers can attest to the number of studies conducted in pursuit of the ideal working length.9-14 Most of these and other similar studies, however, are only centered on the point to which one should fill the canal, and not the point to which one should instrument it.

Structures and bio-organisms removed from the root canal are more important than the filling materials used to restore the canal system. So what does this mean for the working length? Simply put, if the goals of endodontic therapy are fulfilled in every step of the process, the working length must achieve the biologic goal of ridding the canal of bacteria. This implies that the root canal must be cleansed and shaped for its entire length, not just to within some distance of the apex. If bacteria average 1 µm in length, imagine how many can fit into the uninstrumented 1 mm left at the end of the canal. The concern is that the radiographic apex may not coincide with the exit of the foramen. Thus, if one were to instrument to the radiographic apex, he or she may be instrumenting outside the tooth. This fear, however, must be put into perspective.

To begin, there are two messages that should be derived from all the histological studies of the root apex. The first is that there exists a great deal of variety in how and where the canal terminates, and the second is that the only way to determine where the canal for a particular tooth terminates is to remove the tooth and examine it microscopically. That is the reality to which clinicians must accommodate their techniques. From a clinical standpoint, the only facts to work from are the measurements determined by the radiograph and apex locator. It is not uncommon, however, to see results that differ, where a radiograph suggests that the apex is 19 mm from the cuspal landmark, for example, and the apex locator says the apex is at 16 mm. Is the foramen actually exiting 3 mm from the radiographic apex or is there perhaps a lateral canal or a perforation? Was such a discrepancy caused by an amalgam restoration or an electrolyte in the canal that compromised the apex locator’s readings? Is the apex locator responding to a lateral canal, or multiple apical foraminae? While current apex locators have been able to produce readings with accuracy as high as 95%, there is no current clinical method to determine the precise location of the foramen with 100% certainty.15

If a tooth is instrumented to the radiographic apex, one can be sure that all the bacteria and their byproducts have been removed from the canal. If a tooth is instrumented too long, the worst that could happen is a minute hole may be made in the medulary bone at the root apex. When an intraosseous anesthetic technique is used with an intraosseous anesthesia delivery system making a hole into the bone does not result in a lifetime of inflammation and intractable pain. There is no reason to assume that this damage should cause any more than a temporary inflammatory response. Possible destruction of the apical constriction may occur, but according to the histological studies, an apical constriction is often not present.16 If it is, it may be found anywhere from 0.2 mm to 3.8 mm from the apex.17 Additionally, some 74.7% of teeth with periapical disease resulting from pulpal infection have undergone inflammatory resorption of their apices that has destroyed any constriction that may have existed.18

If, however, a tooth is not instrumented to the radiographic apex and the canal does, in fact, terminate there, bacteria may be left behind that will perpetuate the periapical disease indefinitely. Such a fill to that length might get an “A” from a dental school professor, but it will come back at recall with an even larger periapical lesion than before. The clinician would then be asking how such a good-looking fill could fail (Figure 2).

Possible negative results should always be anticipated as well. While gutta-percha is relatively inert, many sealers in common use, particularly those using eugenol, can produce long-lasting periapical inflammation. Clearly, it is advisable not to extrude materials into the periapical space if it can be prevented. Teeth with overextended fills have been shown time after time to have the poorest prognosis.19 Some extrusion, however, may be the inevitable result of using a patent apex technique. The answer is not to leave a millimeter of bacteria and necrotic debris at the apex to prevent this extrusion, but to use a biocompatible sealer instead.

 

Cleansing and Shaping the Canal

The most logical way to achieve the biological goal of cleansing and shaping the canal completely and to remove all bacteria while avoiding extrusion of gutta-percha and/or sealer into the periapical area is to use two working lengths. The first length might be termed the anatomic length. This length would correspond to the radiographic length of the tooth. A small, prebent K-file should be gently advanced through the apex to that length and verified radiographically. Gentle hand instrumentation in a watch-winding, balanced forces-type technique should then be administered for two or three sizes. Vertical filing should be avoided, as it is possible to zip the apex. At this point, the area should be clean. Approximately 1 mm should be subtracted from this length and used as the working length. All future instrumentation, hand or rotary, and obturation should then be accomplished to this level.

The key is to realize that endodontic therapy consists of two parts: instrumentation and obturation. Their goals may be the same, but the techniques necessary to accomplish them are not. The working length should not be handled any differently.

 

*Chairman, Department of Endodontics, Virginia Commonwealth University, Richmond, VA.

 

References

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  16. Wu MK, Wesselink PR, Walton RE. Apical terminus location of root canal treatment procedures. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2000;89(1):99-1031.
  17. Gutierrez JH, Aguayo P. Apical foraminal openings in human teeth. Number and location. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 1995;79(6):769-777.
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